# Day 22: Comparing Gravitational Pulls Between a Tennis Ball and the Earth

College-Prep Physics: Today we established that gravitational attraction was caused by mass. But first I asked if 2 tennis balls were gravitationally attracted to each other. Then we watched this:

Now  we’ve established that all objects pull on all other objects. So does a tennis ball pull on the Earth?

“Yes.”

But how does the tennis ball’s pull compare to the Earth’s pull?

“It’s less. Not wait, may be it’s the same? But the earth doesn’t move up, so it’s gotta be less.”

Then I had 2 students stand on one side of the room to represent the less massive tennis ball and 3 students stand on the other side to represent the more massive earth. Each student represented a “particle” of equal size and mass, but one side had 2 particles (tennis ball) and the other side had 3 (earth).

So how does the pull between any two particles compare?

“The same”

And every particle pulls on every other particle, right?

“Yep.”

So then I ran strings connecting each student to each other student. Then we counted up the strings. 6 strings pulling on the 2 particle object (tennis ball) and 6 strings pulling on the 3 particle object (earth). THE FORCES ARE THE SAME!

Ah, but the effects of the forces have on the objects are different. The earth feels 6 forces on 3 particles (or 2 forces/particle) while the tennis ball feels 6 forces on 2 particles (or 3 forces/particle). Or think about it in terms of a tug of war between the two student teams: Both feel the same force, but the 3 person team is harder to move than the 2 person team for the same force. This correctly matched students intuition that the Earth would move less than the tennis ball.

Again, thanks to Preconceptions in Mechanics. I just made the demo more grandiose by using students and string instead of nails and rubber bands.

PS: In another class I was running low on time and I didn’t use string. Instead I had the students point at each other with hands and feet. Again, 6 forces = 6 forces. Worked just as well, IMO.

UPDATE: I used small wind-up (NOT retractable) metric tape instead of string. Worked much better. The tape is easier to see than the string. Plus the tapes are easily retractable and reusable. We have these ones: http://www.eaieducation.com/Product/530045/Windup_Metric_Tape_100_30m.aspx

##BFPM