Physical Science Activity
Video Point Analysis for free fall
VideoPoint is a software program (Lenox Software Associates) which
takes digital movies of physics motions and allows the student to
click the locations of a moving object in successive frames in order to
plot graphs of the motion.
Open and preview the movie.
- Launch Video point (on the start button of the physics lab
- Open the movie clip. (location is
The movie was photographed earlier and digitized by the
(Use one object)
- Play the movie with the play controls (forward, reverse, single
- Place the movie at the beginning of the ball release.
- If the movie shows the drop of 2 objects of different masses, do
masses fall at the same rate or does the more massive object fall
Predict the graph of position vs. time.
- After watching the clip of free fall, predict a graph of the
of the ball vs. time. You may consult with your partners.
- Show your prediction to the instructor.
Turn-on the graph.
- Choose the graph icon.
- Choose time (default) for the horizontal axis.
- Choose S1, Position, y for the vertical axis. (This will
height of the ball as a function of time
Click to get data points.
- Choose one of the balls to analyze (golf ball or bowling ball)
- Choose a portion of the ball (top or bottom) which can be seen in
of the free fall.
- With the mouse, point and click on the ball part which you
The frame automatically advances with each click. You should see
the points plotted in the graph when you click the points.
- Click until the ball disappears or the ball hits the ground/floor.
- If the graph is not smooth, you should re-do the clicking with
care. Simplest to re-open the movie.
- Compare your graph with the prediction.
- Explain your graph based on motion and acceleration due to
- Your summary should sketch the computer-drawn graphs and give a
Scale the data.
The data in pixel position needs to be scaled in units of length.
Hence the ruler is part of the picture in order to determine the scale.
- Click on the ruler icon.
- Click on both ends of the meter stick in the photo and the
converts pixel position to meters.
- Be sure to click accurately at the ends of the ruler, or your
be in error.
Predict a graph of velocity vs. time.
- Imagine a speedometer attached to the falling ball.
the speed changes as the ball falls.
- Sketch a graph of the prediction of the speed of the falling ball
Consult with your partners and turn-in the predictions.
Have the computer draw the velocity-time graph.
- Open another graph, but select velocity (y) for the vertical
(velocity y means the velocity in the vertical direction.)
- Examine the graph and compare with your prediction.
- Explain the graph and sketch the graph for your summary.
Calculate the acceleration due to gravity (difficult)
The acceleration is a measure of how fast the velocity changes.
a car, the acceleration may be given by "0 - 60 mi/hr in 15 sec".
This means that the acceleration can be calculated as 60 mi/(hr* 15sec)
= 4 mi(hr*sec). In physics we usually measure acceleration by
The acceleration is also thought of as the "change in
in time". You can calculate this from your velocity graph.
After you calculate the acceleration in meter/(sec2),
your value with the standard textbook value.
Be sure to save your analysis files on your network directory.
You may copy the graphs into your report document, but you can only
access Videopoint from the physics lab computers due to licensing
Repeat the analysis for the other dropped ball and person.
Examine the position vs time and the velocity vs time and calculate the
acceleration for both the golf ball and the falling person if the
person-drop was obtained.
Answer the following question:
The bowling ball weighs about 160 times more than the golf ball.
That is, the gravitational pull on the bowling ball is 160 times
greater than the gravitational pull on the golf ball. With this
vast different in the weights, explain why the two masses fall the way
they do. See Ch. 2 of Hewitt.
Clearly state your conclusions and the significance of your results.
Are the accelerations of the two objects the same or different?
If they are different is that difference significant? If the
same, explain why using the bases of forces, acceleration, and inertia.