PHYS 1401 – General
Physics I
Free Fall
Motion
Leader:
___________________ Recorder:
__________________________
Skeptic:
___________________ Encourager:
________________________
Materials
Laptop Picket
Fence
LabPro Foam
rubber
Photogate with ring stand meter stick
Masking tape
Introduction
In this
lab we will examine free fall from several different experimental points of
view. One particular goal is to study
the graph of position vs. time for uniformly accelerated motion. Free fall is an excellent example of uniformly
accelerated motion in one-dimension. It
is a type of motion which we encounter all the time, and thus we have a lot of
experience with it. Consequently, the
study of free fall gives us a good opportunity to test our understanding of
uniformly accelerated motion.
Part I –
Graphs of Uniformly Accelerated Motion
Procedure
In this
procedure we will make use of a "picket fence". A picket fence is a piece of plastic with
regularly spaced light and dark stripes.
We will drop the fence through a photogate connected to the
computer. The photogate will record the
time intervals between the passing of successive stripes.
1. Set-up
Plug the
photogate into the DIG/Sonic 1 input of the LabPro. Clamp the photogate at the edge of the table so
that you can drop the picket fence through it without hitting anything. Place the piece of foam rubber directly
below the photogate so that the picket fence will land on it when dropped
through the photogate.
Make sure
that the LabPro is connected to the computer and turned on. Start the LoggerPro software by double clicking
on the icon which looks like the caliper jaws on the task bar. Click on the open folder, double click on the
folder labeled "Probes & Sensors" then double click on the folder
titled "Photogates". Double
click on the file called "Motion Timer Picket Fence.cmbl".
2. Data
Acquisition
When you
hit the Collect button, the computer will wait until the photogate is first
blocked to start collecting data. Hold
the picket fence vertically above the photogate and drop the picket fence so
that it falls through the photogate vertically.
The computer will display graphs of distance vs. time, velocity vs. time
and, acceleration vs. time.
Data Analysis
1. Examine
the graph of distance versus time displayed by your computer. Note that your computer has been set up so
that down is chosen as the positive direction.
Q1) What is
the shape of the graph of distance versus time?
Q2) What
about the shape of the graph tells you that the motion is accelerated? Hint:
What would the graph look like if you moved at constant velocity or
stood still?
2. Click on
the graph of Distance vs. time. Click
and drag the mouse so that you put a box around the data points. Click on the Analyze Menu, and then choose
Curve Fit … (Alternatively you can
click on the Curve Fit button 
). Fit a quadratic function to the data by
choosing “at^2 + bt + c (Quadratic)”, then clicking on the Try Fit button. Click OK to accept the fit.
Q3) How can
you use the information given to you by the fit to determine the acceleration
due to gravity?
Q4) What values
of acceleration and initial velocity are given by your graph? Don’t forget units.
Q5) Explain
why the initial velocity might not be 0.
3. Examine
the graph of velocity versus time displayed by your computer.
Q6) What is
the shape of the graph of velocity versus time?
Q7) What
about the shape of the graph tells you that the motion is accelerated? Hint:
What would the graph look like if you moved at constant velocity or
stood still?
4. Click on
the graph of velocity vs. time. Click
and drag the mouse so that the data points of the v versus t graph are
selected. Click on the linear regression
button on the toolbar 
. This will use automatically find the best fit
line to your data.
Q8) What physical
quantities do the slope and y-intercept give you?
Q9) How well
does the slope compare to the expected value?
Compute the percent difference between your slope and the expected value. For this case you can define the percent
difference as 
.
5. Examine
the graph of acceleration versus time.
Q10) What is
the shape of the graph of average acceleration versus time?
Q11) Does
this shape seem to suggest that the average acceleration remained constant
during the fall? Explain.
6. Determine
a mean value by first clicking and dragging the mouse over the data. Then click on the statistics button on the
tool bar
.
Q12) How does
this mean value for the acceleration compare to your expected value? Compute
the percent difference between your constant and the expected value.
Part II –
Measuring Reaction Time
In this
part, we will apply the ideas of free fall to make a simple measurement of each
student's reaction time. The idea is
that one person drops a meter stick and another catches it. From the distance the meter stick fell, the
reaction time can be estimated.
Procedure:
Q13) In the
space below, in a few sentences describe a procedure for measuring the reaction
time. Be sure to describe what
assumptions you are making, and how you will analyze the distance the stick
fell to determine the reaction time of the person.
Q14) Carry
out your procedure for each person in your group. Construct a properly labeled data table in
the space below which includes the person’s name, the distance the stick fell,
and the reaction time that you determined from the distance the stick
fell. Be careful about units.
Q15) Explain
how you can modify your procedure to test whether a person has a different
reaction time if they see the stick fall and if they hear the stick fall.
Q16) Carry
out the new procedure you described in Q13).
Construct a properly labeled data table in the space below which
includes the person’s name, the distance the stick fell for each trial, and the
reaction time that you determined from the distance the stick fell for each
trial. Be careful about units.
Q17) Draw a
conclusion about whether the people in your group reacted more quickly to a
visual or aural stimulus.
Q18) How
might you improve the apparatus so that it gave a more reliable measurement?
Q19) An old
trick is to have one person hold a dollar and another hold their fingers open
at the middle of a dollar. The person
holding the dollar lets it go and if the other person can catch it they get to
keep it. Based on this experiment,
comment on whether or not the catcher is likely to get to keep the dollar. (Feel free to try it.)