## Physical Science Activity

VideoPoint is a software program (Lenox Software Associates) which takes digital movies of physics motions and allows the student to easily 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 computers)
• Open the movie clip.  (location is G:\classes\physics\videos).  The movie was photographed earlier and digitized by the instructor.  (Use one object)
• Play the movie with the play controls (forward, reverse, single frame, slew...)
• Place the movie at the beginning of the ball release.
• If the movie shows the drop of 2 objects of different masses, do the two masses fall at the same rate or does the more massive object fall faster or slower?

### Predict the graph of position vs. time.

• After watching the clip of free fall, predict a graph of the vertical position 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 plot the 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 all frames of the free fall.
• With the mouse, point and click on the ball part which you select.  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 greater care.  Simplest to re-open the movie.
• Compare your graph with the prediction.
• Explain your graph based on motion and acceleration due to gravity.
• Your summary should sketch the computer-drawn graphs and give a satisfactory explanation.

### 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 program automatically converts pixel position to meters.
• Be sure to click accurately at the ends of the ruler, or your scale will be in error.

### Predict a graph of velocity vs. time.

• Imagine a speedometer attached to the falling ball.  Describe how the speed changes as the ball falls.
• Sketch a graph of the prediction of the speed of the falling ball vs. time.  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 axis.  (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.  In 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 (meter/sec)/sec or meter/(sec2).

The acceleration is also thought of as the "change in velocity"/"change in time".  You can calculate this from your velocity graph.

After you calculate the acceleration in meter/(sec2), compare 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 issues.

### 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.