Tag Archive | gravity

Day 28: Do Unequal Masses Pull Equally On Each Other?

College-Prep Physics: Another round of voting, inspired by Preconceptions in Mechanics. Yesterday we determined that the Earth and tennis ball pull mutually on each other. But what about the strengths of the pulls?

VOTE #1 (Target scenario): How do the gravitational forces between the tennis ball & Earth compare? [Almost everyone said F(E on TB) > F(TB on E)]

VOTE #2: (Anchor scenario): How do the gravitational forces between 2 identical tiny masses compare? [Everyone agreed they were equal in size.]

Next we used the whiteboards to draw and label the gravitational forces between tiny mass A and tiny mass X. A surprise to me: most groups were not able to properly label the forces as F(A on X) and F(X on A) — they had the labels reversed. So it was important for me to go to each group and coach them for proper arrow labels and placement (arrows attached to objects, not hanging in mid-air).


How many pulls on A? How many pulls on X?

Then we added attached another tiny mass Y to tiny mass X. Does that change the interaction between A and X? [No.] Do we need to add more forces? [Yes.] Draw them.


How many pulls on A? How many pulls on XY?

Then we added tiny mass Z to mass XY. Do the existing arrows change? Do we have to add more arrows?

I also modeled the situation on the blackboard using magnetic hooks as the masses and rubber bands to represent the gravitational forces between the masses.


VOTE #3 (Bridge scenario): How many forces on A? How many forces on XYZ? So how do the strengths of the gravitational forces compare? [The same?!?]

VOTE #4 (Target scenario revisited): How do the gravitational forces between the tennis ball & Earth compare? [The same?!?]


VOTE #5: Jack weighs 800 N. How hard does Jack pull up on Earth? [800 N]

“But why doesn’t the Earth rise up to meet Jack if the pulls are equal?” Great (anticipated) question from the class. So we talked about how Earth’s 800 N pulling on Jack’s mass has much more of an effect than Jack’s 800 N pulling on the Earth’s entire mass. (And the Earth is like, “Jack, do you even lift?”)

Great lesson in all, though it took a lot of time because I wanted the kids to reach the conclusion on their own rather than me just telling them. I hope it sticks!


NGSS Science and Engineering Practice #6: Constructing Explanations

Day 27: Is Gravity a Mutual Force?

College-Prep Physics: On Friday, we discussed what causes gravity. Today, we discussed if it was a mutual force. We started by voting on the target scenario: “Does the tennis ball exert a gravitational force on the Earth?” Students shared their thoughts. In one class, everyone said yes, though I doubt everyone actually thought that. So as a way to encourage kids to think about alternate viewpoints, I asked, “Why might a thoughtful person claim that the tennis ball does not exert a gravitational force on the Earth?” In addition, the stigma/fear of sharing an incorrect answer is practically eliminated by framing it as “why might a thoughtful person claim …” rather than “put your hands up if you said … ”

Then we put the target scenario aside and voted on the anchor scenario: “Does Earth 2 exert a gravitational pull on the Earth?” The anchor scenario is one where most kids should have an intuition for the correct answer. We shared our thoughts again and came to consensus. (As an aside, I brought up the notion of a Counter-Earth, and blew a few minds.)

Then we moved to the bridging scenario, where we have the Moon (less massive than Earth 2, but more massive than a tennis ball): “Does the Moon exert a gravitational pull on the Earth?” We shared our thoughts again, several students mentioned tides as evidence, and we came to consensus.

What if we make the mass of the moon smaller and smaller, until it was the same as the mass of a tennis ball? We moved back to the target scenario and re-voted.

Lastly, I asked the students to discuss whether gravity was a one-way or two-way (mutual) force, based on our discussion. I should note that we have not discussed Newton’s Third Law yet. Tomorrow we’ll discuss if the mutual gravitational pulls between unequal masses are equal or unequal in size.

(The sequence of voting questions is based on those found in Preconception in Mechanics.)


NGSS Science and Engineering Practince #6: Constructing Explanations

Day 26: What Causes Gravity?

College-Prep Physics: Even though we now have a mathematical relationship between mass and weight, we still don’t know what causes Earth’s gravitational pull. So first, we took a short survey:
Download a copy here: GRAVITY Survey 2015

Then we went through each of the four claims in survey question 4 and did a testing experiment for each claim.


CLAIM #1: Earth’s Magnetism


CLAIM #2: Earth ‘s Rotation


CLAIM #3: Air Pressure


CLAIM #4: Earth’s Mass

We also compared characteristics of different planets using a table of planetary data.

This sequence of claims and questioning is based off one found in Preconceptions in Mechanics. On Tuesday, we’ll discuss the relative strengths of the gravitational pulls that 2 masses exert on each other.


NGSS Science and Engineering Practice #6. Constructing Explanations 

Day 26: Weightlessness ≠ No Gravity

College-Prep Physics: One of HW questions students struggled with today was:

Most people say the astronauts floating around on the International Space Station are weightless. Why is “weightless” a misnomer in this case? What’s really going on?

This wasn’t the first time we discussed this. We talked about how the astronauts are really falling around the earth because of their large tangential velocity and then we related that to making the bowling ball move in a circle by hitting it with a mallet. Students even asked if that meant astronauts feel that falling sensation in their stomachs all the time. So what happened? Why the struggle just a few days later?

Many students fell victim to what I call the “mashup” theory of learning: At first they thought there was zero-gravity on the ISS, but then in class we discussed that there was gravity. So the mashup of the 2 concepts is that there’s just much less gravity at the ISS, so the astronauts can still float.  And all that falling around the earth stuff?  Forgotten.

So I tried to demonstrate that “weightlessness” ≠ “no gravity” by dropping a mass hanging from a spring scale. I asked them to predict what would happen to the reading on the scale. Answers were all over the place. We filmed the drop using the slo-motion setting on my phone’s camera (a Motorola Moto X). We we played the video, we saw that the scale read zero (weightless, according to the scale) but clearly there was still gravity (that’s what made it fall in the first place). Same thing with the astronauts, but they are going forward fast enough that they never hit the ground.

These two Newton’s Canon applets came in handy (I forgot to use them during our first discussion about orbits):



(NOTE: For this post, I uploaded the video to YouTube and used YouTube video editor to do color correction, image stabilization, and have the movie playback even slower than recorded.)


Day 25: Gravitational Force Wrap-Up


College-Prep Physics: We wrapped up some traditional physics homework problems about mass and weight. The highlight of my day was when a student said, “I was absent Thursday when we went over the equations, so I just used the [weight vs. mass] graph I had in my lab notebook to solve the problems. Is that OK?”

YES! Of course!

Also, we discussed the WHY of gravity. I originally was going to show a clip from Carl Sagan’s Cosmos about the bending of space-time like a rubber sheet, but the clip had been removed due to copyright violations. Instead, I found this great clip from Brian Greene’s NOVA special The Fabric of the Cosmos:

Direct link to higher quality PBS website video

And here, physics teacher Dan Burns shows off his life-sized rubber sheet model:


Day 24: Gravitational Force Lab Debrief


College-Prep Physics: Today we debriefed the gravitational force lab where students found the relationship between mass and weight.

Why does everyone have similar slopes? Should they? What does the intercept mean? How would the graph change if we repeated the experiment on the Moon? On Jupiter?

(Tomorrow is a Staff Development Day and Monday is Columbus Day. So we’ll be back next Tuesday for Day 25.)


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

October 8, 2013 53835 PM EDT

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?


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?


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