Video Activity - Freely Falling Objects
Part I
Open the video of the dropped ball on the laptops, or Log on to http://paer.rutgers.edu/pt3/, scroll down, click “Motion”, then “Types of Motion”, then “Motion of a falling object”.
Watch the movie a few times before doing anything else. Remember, we are trying to describe the motion of an object under the influence of gravity.
The speed of filming is 30 frames per second. Use the arrow keys to watch it frame by frame. The ruler shows centimeters. (When the ruler is ‘upside down’, we in essence are calling the down direction positive. This changes between videos. Be flexible!)
1. Make a data table of position vs. time of the falling ball. Be as accurate as possible.
2. Add a third column which will be your calculated average velocity between each 1/30th second. Remember, average velocity is Δx / Δt.
3. Plot an x vs. t, and v vs. t graph for your data. Use the EXCEL program to do this. EXCEL help sheets are attached. Determine the slope of the v vs. t graph by using a best fit line. Be sure to report your answer in the proper units.
Sample Data Table:
t position (cm) v calculated (cm/s)
0 72 cm v = Δx / Δt
v= 3 cm / (1/30th sec) = 90 cm/sec
1/30th 75 cm
v=4 cm / (1/30th sec)
= 120 cm/sec
2/30th 79 cm v=5 cm / (1/30th sec) = 150 cm/sec
3/30th 84 cm
Questions Part I:
a) What can you say about the amount of distance the ball in the movie traveled in each successive time interval?
b) Was the velocity of the ball constant? Look at your v vs. t graph. What property of it leads you to conclude that the velocity was changing?
c) Was the acceleration constant? Describe how you made this determination. What average value (in cm/s2) did you calculate for the acceleration?
Part II
Click ‘next experiment’ at the bottom right of the page. This will be ‘A Vertical Throw’.
Watch the movie and answer the following questions before proceeding.
1) Does the ball in this experiment have an initial velocity? Is it upward or downward?
2) How do you think the initial velocity of the ball will affect its acceleration?
Repeat all steps from Part I above for the entire motion of the ball, once it leaves the thrower’s hand.
Questions Part II:
a) What was the direction of the velocity of the ball when on its upward trajectory? What was it when it was on its downward fall?
b) Was the velocity constant over the entire movie? Explain.
c) Based on your interpretation of your data, what was the value and direction for the acceleration of the ball when it was going up?
d) What were they when the ball was going down?
e) What was the velocity of the ball at the instant it reached its peak?
f) What do you think the acceleration was then? Discuss how you arrived at your conclusion.
Part III
Repeat the experiment for the second movie on the page Galileo’s Pisa experiment / Test of Aristotelian motion. This is the one where the ball is thrown down. (The first one is just a dropped ball again.)
In addition to the Lab Overview, hand in all data tables, and all graphs and complete answers to all the questions, as detailed as you can make them, for your lab grade.