«Kinematics: The Gravity Lab Teacher Version (Grade level: 4 – 7) *** Experiment with Audacity to be sure you know how to do what’s needed for the ...»
5. Look at the graph of sound that you recorded and find the peaks where the microphone heard the two marbles hitting and where the first marble hit the foil on the floor. Zoom in and/or play back the recording if you aren't sure which peaks are the right ones.
**TEACHER NOTE** There will generally be two peaks at the beginning (first the marbles hitting then the top marble hitting the table) and two peaks at the end (each marble hitting the ground). The first peak is the one to use in both cases.
6. Find the time interval between when the marble was struck and when it hit the floor.
**TEACHER NOTE** For a typical height table (70 cm), the time interval should be about 0.4 sec.
7. Now repeat the experiment using a ping-pong ball at the edge of the table.
**TEACHER NOTE** The time interval should be similar to the one above.
8. (OPTIONAL) If you have time, repeat steps 6 & 7 once more so that you have two measurements of the time interval for both the marble and the ping-pong ball.
**TEACHER NOTE** If there is time, the extra trial is useful for comparing the difference between marble and ping-ping ball to the variation between two trials of the marble and two trials of the pingpong ball. This is a good place for a discussion of what constitutes “significant” difference (difference between marble and ping-pong ball should be comparable to difference between trials) Q8. Was there a significant difference in the time of falling for the ping-pong ball versus the marble?
Students should not see a significant difference.
**TEACHER NOTE** If they did not have time to do more than one trial, and it looks like one ball fell faster than the other, encourage them to look at the result of another group to see whether they got the same ball falling faster, and how much the times differed between the groups even when comparing marble to marble.
Q9. Does the time of falling depend on the mass of the object? No it doesn’t.
Q10. Which traveled a greater distance overall in the time it took to reach the ground, the marble or the ping-pong ball? What does that mean about the horizontal speed of each ball as it rolled off the table?
Ping-pong ball will travel further. It has a higher horizontal speed as it rolls off the table.
Q11. If you shoot a bullet out of a gun and at the same time drop an apple (starting from the same height), which will reach the ground first?
They will both hit the ground at the same time, just like the ping-pong ball and marble did.
The fact that the bullet has a fast horizontal speed does not affect how fast it falls.
Q12. If gravity makes all objects fall equally fast, then why does a sheet of paper fall slower than a pebble? Why does a skydiver with a parachute fall slower than one without?
(both reasons are the same) Near the surface of the planet, gravity is not the only force that affects how fast things fall.
There is also air resistance from the atmosphere, which slows down a falling object (it acts like the brakes of a car). The bigger the surface of the object viewed from below, the more air it has to push out of its way, and the more air resistance it experiences. A parachute feels a lot more air resistance than just the skydiver alone.
**TEACHER NOTE** The analogous resistance to moving water out of the way is the reason that things fall slower