PASCO ME-8932 User manual

© 1998 PASCO scientific $7.50
Instruction Manual and
Experiment Guide for the
PASCO scientific
Model ME-8930
012-06734A
09/98
SMARTTIMER
Plane4

Smart Timer 012–06734A

01206734A SmartTimer
Table of Contents
Section Page
i
Table of Contents..........................................................................................................................i
Copyright, Warranty, and Equipment Return ................................................................................ii
Introduction ................................................................................................................................. 1
Equipment................................................................................................................................... 3
Operating the Smart Timer .......................................................................................................... 4
Smart Timer Modes of Operation ................................................................................................ 4
Quick Cross-Reference for Suggested Activities and Smart Timer Modes.............................. 5
Time Modes....................................................................................................................... 6
Speed Measurement Modes ................................................................................................. 7
Table 1: Summary of Timing Modes................................................................................ 89
AccelerationMeasurementModes ..................................................................................... 10
Count Modes .................................................................................................................... 10
Test Mode ........................................................................................................................ 11
Timing Diagrams ................................................................................................................11
LOCK Switch: ................................................................................................................... 11
Caring for the Smart Timer ........................................................................................................ 12
Specifications .............................................................................................................................12
AccessoryOptions...................................................................................................................... 12
Troubleshooting ........................................................................................................................ 14
ExperimentOne: Accelerationof Gravity ................................................................................... 15
Experiment Two: Newtons Second Law ................................................................................... 17
Experiment Three: Conservation of Momentum In Collisions ....................................................19
Experiment Four: Conservation of Angular Momentum ............................................................. 25
OtherSuggestedExperiments .....................................................................................................31
TechnicalSupport ......................................................................................................... back cover

ii
Copyright Notice
The PASCO scientific 012-06734A Smart Timer
manual is copyrighted and all rights reserved. However,
permissionisgrantedtonon-profiteducational
institutions for reproduction of any part of the manual
providing the reproductions are used only for their
laboratories and are not sold for profit. Reproduction
under any other circumstances, without the written
consentofPASCOscientific,isprohibited.
Limited Warranty
PASCO scientific warrants the product to be free from
defects in materials and workmanship for a period of
one year from the date of shipment to the customer.
PASCO will repair or replace at its option any part of
the product which is deemed to be defective in
material or workmanship. The warranty does not
cover damage to the product caused by abuse or
improper use. Determination of whether a product
failure is the result of a manufacturing defect or
improper use by the customer shall be made solely by
PASCO scientific. Responsibility for the return of
equipment for warranty repair belongs to the
customer. Equipment must be properly packed to
prevent damage and shipped postage or freight
prepaid. (Damage caused by improper packing of the
equipment for return shipment will not be covered by
the warranty.) Shipping costs for returning the
equipment after repair will be paid by PASCO
scientific.
Copyright, Warranty, and Equipment Return
PleaseFeel free to duplicate this manual
subject to the copyright restrictions below.
Equipment Return
Should the product have to be returned to PASCO
scientific for any reason, notify PASCO scientific by
letter, phone, or fax BEFORE returning the product.
Upon notification, the return authorization and
shipping instructions will be promptly issued.
ä NOTE: NO EQUIPMENT WILL BE
ACCEPTED FOR RETURN WITHOUT AN
AUTHORIZATION FROM PASCO.
When returning equipment for repair, the units must
be packed properly. Carriers will not accept
responsibility for damage caused by improper
packing. To be certain the unit will not be damaged in
shipment, observe the following rules:
➀The packing carton must be strong enough for the
item shipped.
➁Make certain there are at least two inches of packing
material between any point on the apparatus and the
inside walls of the carton.
➂Make certain that the packing material cannot shift in
the box or become compressed, allowing the
instrument come in contact with the packing carton.
Address: PASCO scientific
10101 Foothills Blvd.
Roseville, CA 95747-7100
Phone: [phone]
FAX: [phone]
email: [email protected]
web: www.pasco.com

012–06734A Smart Timer
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Introduction
The PASCO ME-8930 Smart Timer is an accurate, versatile digital timer and
measurement system for the student laboratory. The Smart Timer offers 0.1 ms timing
resolution and an easy-to-use memory function. The Smart Timer measures several types
of events detected with PASCO’s digital sensors, including speed and acceleration using
standard photogates. The PASCO ME-9387 Smart Pulley, the ME-9207B Free Fall
Adapter, the ME-6810 Time-of-Flight Accessory, or the ME-9259A Laser Switch also
work with the Smart Timer. The Smart Timer counts radiation emission events detected
with the SN-7927 G-M Tube/Power Supply or the SE-7997 G-M Tube. The Smart Timer
features two input channels and a 2-line, 16-character alphanumeric liquid crystal display
that indicates the operating mode and experimental results.
Features:
The Smart Timer’s memory function makes it easy to time events that happen in rapid
succession, such as a Dynamics Cart passing twice through the photogate, once before
and then again after a collision. The 0.1 ms resolution is especially useful in some
experiments, such as measuring velocity or acceleration during free fall. The Smart
Timer can calculate velocity before and after a collision between two carts using a single
timer. With the new fence design and sensing logic, parallax errors are eliminated and
timing accuracy is improved considerably over existing timing options. The Smart
Timer’s internal microcontroller also eliminates the problem of incorrect readings due to
multiple passes through the same photogate by “understanding” the type of measurement
selected and ignoring multiple passes.
The Smart Timer has many different options for timing and calculating values based on
inputs from a variety of sensors. The graphics on the keypad aid in the selection of the
appropriate timing mode. Options include One or Two Gates, Fence, and Pendulum
modes. These modes allow you to measure the speed of an object as it passes through the
photogate or between two photogates, or to measure the period of a pendulum. The
stopwatch mode lets you use the timer as an electronic stopwatch.
The Smart Timer can measure speed and acceleration for both linear and rotational
motion experiments. Alternatively, the time can be measured directly, and the speed and
acceleration can then be calculated by the student. The speed and acceleration
measurement features can be enabled or disabled using a switch inside the Smart Timer.
Use With Photogates:
The Smart Timer is optimized for use with PASCO’s ME-9204B Accessory Photogate
(available separately). These narrow-beam infrared photogates plug directly into the
Smart Timer and are used to provide the timing signals. An LED in one arm of the
photogate emits a narrow infrared beam. As long as the beam strikes the detector in the
opposite arm of the photogate, the signal to the timer indicates that the beam is
unblocked. When an object blocks the beam so it doesn’t strike the detector, the signal to
the timer changes.

Smart Timer 012–06734A
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In One Gate Mode, a single photogate lets you measure the time, velocity, or acceleration
of a fence as it passes through the photogate. Two photogates are used for collision
experiments using one or two carts or for experiments where the velocity of a cart must be
measured at two different points. In Two Gate Mode, two photogates are used and the
time to travel between the two can be measured. This mode can also be used to measure
time of flight using the ME-6810 Time-of-Flight Accessory.
Use with the Smart Pulley:
For rotary motion studies, the ME-9387 Smart Pulley (available separately) is ideal. It
combines a photogate with a pulley, and when used with the Smart Timer, allows direct
measurement of angular speed and acceleration. When used with a string connected to a
glide or Dynamics Cart, the Smart Timer and Smart Pulley can be used to determine
linear acceleration (cm/s2) as well.
➤Note: The use of pulleys with different diameters and/or different number of spokes
than the Smart Pulley or the ME-9450 Super Pulley will give incorrect results in the speed
and acceleration calculations.
Power Options:
The Photogate Timer can be powered using the included 9VDC adapter. It will also run
on 4-AA size, 1.5 volt alkaline batteries which will provide over 100 hours of operation in
typical use.
Experiments:
Four copy-ready experiments and 9 additional suggested experiments are included in this
manual, showing a variety of ways to use the Smart Timer. The equipment requirements
vary for different experiments. For many of the experiments, you will need a Dynamics
Track and Dynamics Carts. Some experiments also require two ME-9204B Accessory
Photogates or the ME-9387 Smart Pulley. Check the equipment requirements listed at the
beginning of each experiment.

012–06734A Smart Timer
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Plane4
Equipment
Included:
Smart Timer
Smart Timer Picket Fences (2)
9 VDC adapter liquid crystal display
touchpad keys
mode illustrations
on/off switch
(side panel)
battery compartment
(bottom panel)
input channel 2
input channel 1
connector for 9VDC adapter
(side panel))
9 VDC adapter
Figure 1
Includedequipment Smart Timer Picket Fences
Smart Timer Picket Fence
Smart Timer Picket Fence
5 cm fence
1 cm fence 1 cm flag

Smart Timer 012–06734A
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Plane4
Operating the Smart Timer
1. Plug the ¼-inch phone plug from the photogate into the
Smart Timer’s input channel 1 or 2 (see Figure 2). For all
experiments using a single photogate or pulley, either of the
two available jacks may be used interchangeably. For all
other modes see the individual descriptions below.
2. Plug the 9 VDC power adapter into the small receptacle on
the side of the timer and into a standard 110 VAC, 60 Hz
wall outlet.
3. Position the photogate head so the object to be timed will
pass through the arms of the photogate, blocking the
photogate beam. Loosen the clamp screw if you want to
change the angle or height of the photogate, then tighten it
securely.
4. Slide the power switch to the ON position. The Smart
Timer will “beep” and show PASCO scientific on the
display. From this point, the three-step setup of the Smart
Timer is easy:
1. Press the Select Measurement key until the
desired measurement type is displayed on the top
line of the display. Note that the menu rolls over to
the beginning after the last type is selected.
2. Press the Select Mode key until the measurement
mode is displayed after the measurement type.
You cannot begin a measurement until both the
type and mode have been selected.
3. Once a complete measurement has been selected,
press Start/Stop to begin. You will hear a “beep”,
and a asterisk (*) will appear on the second line of
the display. In most modes, the (*) indicates that
the Smart Timer is now waiting for an event to
occur, like a fence passing through a photogate.
5. If an event occurs, the Smart Timer beeps again, displays a
result, and the (*) disappears. Pressing Start/Stop before an
event occurs will remove the (*) and allow you to change
the measurement type.
Smart Timer Modes of Operation
The Smart Timer has 18 modes of operation organized into five
groups: Time, Speed, Acceleration, Counts, and Test. A
Figure 2
Connecting the photogate to the Smart
Timer
ä Important Setup Note:
The Smart Timer incorporates a feature to
increase the battery lifetime. The photogate
turns on only when Start/Stop is pressed
to start an experiment. Photogate power is
turned off when the measurement is
complete or the operator presses Start/
Stop. The exception is the Test mode in
which photogate power is turned on as
soon as the display reads Test:Gates and is
not turned off until Select Measurement is
pressed again. Setup for the experiment is
often best accomplished in Test:Gates
mode.
input channel 2
input channel 1
➤Note: Smart Timers shipped to certain
locations are supplied with a transformer
for 220/240 VAC, 50 Hz power.

012–06734A Smart Timer
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Smart Timer 012–06734A
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summary of the mode suggested for a given experimental activity can be found on page 5
(Quick Cross-Reference for Suggested Activities and Smart Timer Modes). Refer to the
timing diagrams in Table 1 (pages 8 and 9) for a detailed look at how the Smart Timers
times events on its input(s) and an explanation of how speed and acceleration calculations
are performed internally.
The following are detailed descriptions of the Smart Timer’s modes of operation.
Time Modes
One Gate: In One Gate mode, timing begins when the beam is first blocked and
continues until the beam is blocked again. This mode can be used to measure the speed of
an object as it passes through the photogate. If an object of length L blocks the photogate
for a time t, the average velocity of the object as it passed through the photogate was L/t.
Fence: In Fence mode, the timer measures the time between successive interruptions of
the photogate. Timing begins when the beam is first blocked and continues until the
beam is unblocked and then blocked again. The Smart Timer can remember ten such
interruptions and will allow the user to scroll through the times using either the Select
Measurement or the Select Mode keys. Pressing the Start/Stop once will allow another
measurement type to be selected. Pressing it twice begins a new Fence Mode
measurement. Note that once a measurement has begun with an initial block of the
photogate, the Smart Timer will continue to time until ten interruptions are counted before
stopping the measurement and displaying the result. Pressing Start/Stop will stop the
measurement, and any recorded times will be displayed.
Two Gates: In this mode, the Smart Timer measures the time between blocking two
photogates. This mode is useful for not only air tracks and Dynamics Tracks but also with
the ME-6810 Time-of-Flight Accessory. In this mode, you must plug the photogate you
expect to encounter first into input channel 1, and the second photogate into input
channel 2.
Pendulum: In Pendulum mode, the timer measures the period of one complete
oscillation. Timing begins as the pendulum first cuts through the beam. The timer
ignores the next interruption, which corresponds to the pendulum swinging back in the
opposite direction. Timing stops at the beginning of the third interruption, as the
pendulum completes one full oscillation. Press the Start/Stop key again to begin a new
timing cycle.
Stopwatch: The Manual mode is actually a dual-use function. It provides a means of
manually timing events (like a using a Stopwatch) by pressing the Start/Stop key. It also
allows timing of events using the ME-9207B Free-Fall Adapter and the ME-9259A Laser
Switch, which function via a block/unblock sequence.
Using the Stopwatch: Enter Stopwatch mode and press the Start/Stop key. The
Smart Timer will beep and a “*” will appear on the second line of the LCD. Press
the Start/Stop key again to start the timer. Press the Start/Stop key to stop timing
and display the elapsed time. Press the Start/Stop key again. The old result is
➤Note: The
picket fence
supplied with the
Smart Timer is
designed to
increase timing
accuracy when
used with
photogates. The
fence has three
sections: the 1 cm
flag, the 5 cm
fence, and the 1
cm fence; one of
these must be
aligned with the
photogate light
path before the
experiment can
proceed.

012–06734A Smart Timer
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cleared and the “*” reappears. This RESET-START-STOP sequence is repeated for
each new elapsed time. Whenever the “*” is not showing, the mode may be
changed.
Using the Alternate Timing Function: Connect an appropriate accessory to input
channel #1 or #2. Enter Stopwatch mode and press the Start/Stop key. The Smart
Timer will beep and a “*” will appear on the second line of the LCD. At this time
the accessory will be powered. By blocking and unblocking the beam in the case of
the Laser Switch, or by dropping the steel ball in the case of the Free Fall Adapter,
the elapsed time will be measured. The Smart Timer will resolve 100 microseconds
in the alternate timing mode.
➤Notes about the Stopwatch Mode:
1. Although it is possible to use older style fences in the alternate timing function to
obtain photogate beam block times, the Smart Timer will provide much higher
accuracy when used with the included fences and the standard timing modes.
2. Two photogates cannot be plugged into the Smart Timer when you are using a
photogate to start and stop the timer. A single photogate can be plugged into channel
#1 or #2 to start and stop the timer. But if two photogates are plugged in, when one
gate is blocked, the other gate immediately is counted as an unblock, and the timer
will display 0.0001 seconds, regardless of the length of time the first photogate is
blocked.
3. You cannot start the timing with the Start/Stop key and end it with a photogate
block or vice versa.
4. If a photogate is plugged in and blocked when you try to use the Start/Stop key as a
stopwatch, the Smart Timer will be timing the photogate and waiting for the
photogate to become unblocked. So when you push the Start/Stop key, the asterisk
disappears and when you push the Start/Stop key again, the asterisk reappears. No
time is displayed until the photogate is unblocked.
Speed Modes
One Gate: In this mode a 1 cm flag passes through the photogate. The Smart Timer
measures the time and calculates the average speed in cm/s.
Collision: In this mode either one or two carts and one or two photogates can be used for
a collision experiment. Once Start/Stop is pressed, the Smart Timer waits for the first
collision and begins timing. The Smart Timer stops timing when two carts have passed
through their respective photogates twice. Timing can always be stopped manually by
pressing Start/Stop and the Smart Timer will display speed(s) based on the information it
has (you will need to press Start/Stop for single cart collisions). The display will present
the results in the following format:
1: xx.x,yy.y
2: xx.x,yy.y
The first number represents the input jack and the following two numbers indicate the
initial speed (xx.x) and final speed (yy.y), respectively.
➤Hint:
For easier
alignment of the
laser with the
Laser Switch, use
the Test:Gates
mode.
➤ Note:
The stopwatch
function is not a
precision timing
mode and is
therefore most
useful for events
that are longer
than one second.
The accuracy of
the stopwatch is
+/-10
milliseconds.

One Gate
Fence
Two Gates
Pendulum
Stop Watch
One Gate (cm/s)
Collision (cm/s)
Pulley (rad/s)
Pulley (rev/s)
Table 1. Summary of Smart Timer Modes
TimeSpeed
Type Modes Key Sequence* Timing Diagram** Calculation Algorithm

One Gate
Linear Pulley
Angular Pulley
Two Gates
30 seconds
60 seconds
5 minutes
Manual
One Gate
Acceleration
Counts
Test
* The key sequences shown
are valid in the initial or
power-on situation only.

Smart Timer 012–06734A
10
Pulley (rad/s): The Smart Timer will measure the speed of a pulley passing through a
photogate in units of radians/second. One measurement will be taken each time the Start/
Stop switch is pressed. The Smart Timer cannot differentiate between clockwise and
counterclockwise directions. Note that, as in many other modes, if a “*” shows in the first
character position of the second line, the Smart Timer is actively waiting for an external
timing event to occur. If the “*” is not showing, the Timer measurement mode may be
changed. This mode has a minimum speed requirement. The photogate must be blocked
twice within two seconds to obtain a valid reading, translating to a minimum speed of
0.31 rad/s.
Pulley (rev/s): Besides displaying in different units, this mode uses the display differently
than in the radians/second measurement. The display provides a real-time measurement of
the speed of the pulley by updating once per second. Once a speed is displayed, press the
Start/Stop key to freeze the display. The Smart Timer indicates that the measurement is
frozen by displaying a “!” in the first column. Press Start/Stop to erase the “!” and start
collecting new measurements. Any time the first column is blank or has a “!” displayed,
the type of measurement being done can be changed by pressing the Select
Measurement or Select Mode keys. To move to a different measurement, press Start/
Stop again. Like the radians/second measurement, there is a two-second maximum time
for two photogate blocks, translating to a minimum speed of 0.05 rev/s. Also note that
accuracy decreases rapidly as pulley speeds increase above 600 rpm. At 600 rpm,
accuracy is 1%.
Acceleration Modes
One Gate: The Smart Timer uses the time measurement between two equally spaced (5
cm) block/unblock/block sequences to calculate average acceleration. In activities using
the Smart Timer Picket Fence, the Picket Fence must be positioned so that the photogate
blocks only the 3-bar segment (5cm fence) of the Picket Fence. Note that the Smart Timer
is able to discern between acceleration (positive number) and deceleration (negative
number).
Linear Pulley: In this mode, the Smart Timer converts rotary motion of a PASCO pulley
to an equivalent linear acceleration in cm/s2.
Angular Pulley: In this mode, the Smart Timer converts rotary motion of a PASCO
pulley to an equivalent angular acceleration in rad/s2.
Two Gates: When two photogates are placed an arbitrary distance apart, the average
acceleration between the two can be calculated. In this mode, the inputs used are not
arbitrary. The first photogate to be encountered must be connected to input #1 and the
second to input #2.
Count Modes:
30 Seconds: The timed 30-second count mode will count high-to-low voltage transitions
on either input and display them on the second line of the liquid crystal display. After the
counting period is over, the Smart Timer will beep once, remove the power to the external
device, and freeze the display. Pressing Start/Stop erases the old count and begins a new

012–06734A Smart Timer
11
timing interval. If you wish to stop the count during a timing interval, press Start/
Stop. The display will freeze the current count and the “*” will disappear from the
first column. At this time you may select a new measurement or start a new timing
interval. The maximum counting rate for any of the counting modes is 5,000 counts
per second and the maximum count is 9,999,999.
60 Seconds: Other than timing the count interval for 60 seconds, the 60-second count
mode is the same as the 30-second mode.
5 Minutes: Other than timing the count interval for 5 minutes, the 5-minute count mode
is the same as the 30-second mode
Manual: Manual mode will count high-to-low voltage transitions on either input and
display them on the second line of the liquid crystal display. There is no time limit for
counting, however the upper limit on the number of total counts is still 9,999,999. Each
count will be accompanied by a short beep. Used with a PASCO SN-7927 G-M Tube/
Power Supply, this mode is useful for group demonstrations to show the random nature
of atomic disintegration and the inverse-square relationship between number of
disintegration’s detected and distance from the radioactive source.
Test Mode
Gates: In the Test:Gates measurement, the external measuring accessory is powered as
long as the top line of the display reads Test:Gates. This mode is useful for experiment
setup or for testing accessory photogates, G-M tubes, or other Smart Timer accessories.
Pressing the Select Measurement key will exit the test mode and remove power to the
external device. The display graphics depict a blocked photogate as a vertical line and an
unblocked photogate as an arrow.
Timing Diagrams
The timing diagrams on pages 8 and 9 show the interval, t, that is measured in each
timing mode. In each diagram, an elevated line corresponds to the photogate being
blocked, and a depressed line corresponds to the photogate being unblocked. The
calculation modes assume the use of a fence of fixed width (1 cm or 5 cm) or a pulley
having a diameter (groove to groove) of 4.8 cm and 10 spokes, such as the Super Pulley
or Smart Pulley.
LOCK Switch
The internal LOCK switch provides a way to temporarily lock
out speed and acceleration modes. To access the LOCK
switch, turn the power off, and remove the bottom half of the
Smart Timer case as if you were going to replace the batteries.
Look along the lower edge of the printed circuit board, and
note the LOCK switch button at the edge (Figure 3). The circuit
board also has the words “LOCK” and “UNLOCK” printed
along the same edge. Moving the switch to the LOCK position
will cause the display to read MODE UNAVAILABLE
whenever speed or acceleration modes are selected.
LOCK switch
circuit board
Figure 3
Location of the LOCK switch on the circuit
board.

Smart Timer 012–06734A
12
Caring for the Smart Timer
• Do not use a pointed object (such as a pen) to press the keypad buttons. Wrap the
Smart Timer separately when transporting it with other items. The transparent
covering over the keypad can be creased by a fingernail or other sharp object.
• Clean the keypad with a soft cloth and mild detergent, avoiding hard rubbing of the
transparent window.
• Do not leave the Smart Timer exposed to direct sunlight except for brief periods.
Strong ultraviolet light can damage the Smart Timer display.
• Remove the batteries prior to storage for more than one month. Batteries can leak
and damage the internal circuitry, especially if the batteries are old.
• For best results, use alkaline batteries. Rechargeable NiCad batteries may be used,
but the operating time between charges is much shorter than the lifetime of alkaline
batteries.
Specifications
Resolution: The basic timing resolution of the Smart Timer is 100 microseconds in all
modes except Stopwatch, which is 10 ms.
Calculated Values: Calculated values are displayed to one or two decimal places with
typical accuracy being +/- 1 in the least significant digit. For extremely high speeds (such
as might be generated by hand spinning a Super Pulley), accuracy is degraded for
calculated parameters because of the very short timing intervals involved.
Maximum Output Power: The Smart Timer allows many different accessories to be
used as inputs. The total 5-volt load (both inputs added together) that can be
accommodated is 180 mA, maximum.
Accessory Options
The following PASCO accessories are available to help extend the utility of the Smart
Timer. See the current PASCO catalog for more information.
Accessory Photogate (ME-9204B): The stereo phone plug of the Accessory Photogate
plugs into either of the phone jacks on the side of the Smart Timer, giving you the option
of two identical photogates operating from a single timer. (Some of the experiments in
this manual require the use of a Smart Timer with two Accessory Photogates.)

012–06734A Smart Timer
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Time-of-Flight Accessory (ME-6810): The Timer-of-Flight Accessory facilitates the
accurate measurement of the flight time of a ball launched by a PASCO projectile
launcher. See page 33 (Time of Flight and Initial Velocity and Figure 9.1) for details of
the setup.
Free Fall Adapter (ME-9207B): The Free Fall Adapter facilitates easy and accurate
measurements of the acceleration of gravity. It comes with everything you need,
including two steel balls (of different size and mass), a release mechanism, and a receptor
pad. The release mechanism and the receptor pad automatically trigger the Smart Timer,
so you get more accurate measurements of the free fall time of the steel ball. See page 34
(Determining the acceleration due to gravity (g) with the Free Fall Adapter and Figure
10.1) for details of the setup.
Laser Switch (ME-9259A): This highly collimated photodetector is identical to a
photogate, except that a laser (available separately) is used as the light source. With the
Laser Switch, the motion of objects that are too big to fit through a standard photogate
can be measured. Thus, you can measure the period of a bowling ball pendulum or the
velocity of a car, for example.
G-M Tube/Power Supply (SN-7927): The G-M Tube Power Supply is a Geiger-Muller
Probe that senses beta, gamma, and alpha radiation. See page 34 (Counting radiation
with the G-M Tube/Power Supply and Figure 11.1) for details of the setup.

Smart Timer 012–06734A
14

012–06734A Smart Timer
15
Experiment One: Acceleration Due to Gravity
EQUIPMENT AND MATERIALS REQUIRED
 Smart Timer (ME-8930)  Smart Timer Picket Fence
 Photogate (ME-9498A)
Purpose
The purpose is to determine the acceleration due to the Earth’s gravity.
Theory
The accepted value for the acceleration due to gravity on the Earth’s surface is 9.8 m/s2.
With the Smart Timer, the acceleration due to Earth’s gravity can be quickly determined
experimentally. The acceleration may be calculated from measurements of distance and
time, or it can be measured directly.
To calculate the acceleration from time measurements, the following formula must be used:
Note that Dt is not t2, but is 1/2 t2(see Figure 1.1).
Figure 1.1
Explanation of why ½t2is used in the formula for calcu-
lating g
V
1
V
2
∆t
t
1
t
2
∆t=12
12t1+12 12(t2–t1)
=12
12t1+12 12t2–12 12t1
∆t=12
12t2
Procedure
PARTA—Determining the acceleration from time and
distance measurements
1. Mount the photogate on a stand, or hold the Photogate
steady so it is parallel to the floor, as shown in
Figure 1.2. Figure 1.2
Experiment setup
Smart Timer
Picket Fence
Photogate
Picket Fence must
be dropped at a
90º angle.
To Smart Timer
channel 1 or 2
The 5 cm fence should
block the photogate
beam.
g=
∆
v
∆ t=v2–v1
12 12t2
where
v1=5cm
t1and v2=5cm t2–t1

Smart Timer 012–06734A
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Questions
1. How do the two methods for determining the acceleration on a body due to Earth’s
gravity compare?
2. How do the experimental measurements compare to accepted values?
2. Insert the plug of the photogate into channel 1 or 2 of the Smart Timer, and set up the
Smart Timer to measure Time, Fence.
3. Hold the Smart Timer Picket Fence in a position so it will drop vertically through the
photogate and so the 5 cm fence will block the photogate beam as the fence drops
through the photogate.
Note: Three conditions must be met for greatest accuracy:
1. The Picket Fence must be dropped at a 90º angle to the photogate beam in such a
waythat it does notrotate onthe waydown. One methodto improvethe dropis to
hold the edge of the Picket Fence with a clothspin or binder clip, and drop the
fence by squeezing the clothespin or clip.
2. The Picket Fence must be dropped so the 5 cm marks cut the photogate beam.
3. The Picket Fence must pass close to the LED that emits the photogate beam.
4. Press and drop the fence.
5. Record t1and t2, and calculate the acceleration in meters/second2.
PART B—Determining the acceleration directly.
1. Repeat steps 1 – 4 in PartA with the following modification: Set the Smart Timer to
measure Acceleration: One Gate. Repeat several times and calculate the average
acceleration (g).