Video: Using Newton’s Second Law to Explain the Deceleration of a Moving Object

Newton’s first law of motion states that an object in motion will not change its speed unless something pushes or pulls the object. A student gives a single, short push to a ball and the ball rolls along flat ground. The student observes that the ball slows down but does not see anything push or pull the ball. Which of the following conclusions that the student might arrive at is correct? [A] The ground is not as flat as the student originally thought, so the ball was actually rolling uphill. [B] Newton’s first law of motion is incorrect. [C] Earth’s motion around itself opposes the ball’s motion. [D] The ball is pulled or pushed by something that the student cannot see.

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Video Transcript

Newton’s first law of motion states that an object in motion will not change its speed unless something pushes or pulls the object. A student gives a single, short push to a ball and the ball rolls along flat ground. The student observes that the ball slows down but does not see anything push or pull the ball. Which of the following conclusions that the student might arrive at is correct?

Okay, before we show these conclusions, let’s see what’s happening here. What we’ve got is a section of flat ground and a ball that’s sitting on it. And we’re told that a student gives a single, short push to this ball. The ball rolls along the flat ground. But over time, the student notices that the ball slows down. Newton’s first law of motion says that this won’t happen unless something is pushing or pulling on the object, in this case the ball. The student doesn’t see anything that could be doing the pushing or the pulling. So the question is, what’s going on?

Here is a list of conclusions the student might make after seeing all this. A) The ground is not as flat as the student originally thought, so the ball was actually rolling uphill. B) Newton’s first law of motion is incorrect. C) Earth’s motion around itself opposes the ball’s motion. D) The ball is pulled or pushed by something that the student cannot see.

Now, these are all possible conclusions the student could draw. And we wanna pick which of these four is correct. Beginning with the first possibility, option A, this one says that the ground actually wasn’t flat, that the student thought it was. But this is contradicted by our problem statement, which tells us that, indeed, the ground the ball rolls on is flat. Now, the student might think it’s not because the ball is observed to slow down. But the ball doesn’t need to be rolling uphill for this to happen. This would occur for a ball rolling on a perfectly level surface as well. So we’ll cross out this answer choice.

Our second conclusion option is that Newton’s first law of motion is incorrect. Recall that this law says that the speed of an object won’t change unless it’s pulled or pushed by something else. Well, it is possible that Newton’s first law of motion is incorrect. But given how often this law has been tested, it’s very unlikely. For everyday-size objects such as this rolling ball, it’s much more likely that the first law of motion is correct and that something else explains what’s going on. We can tentatively cross option B off of our list and move on to options C and D.

Option C says that Earth’s motion around itself opposes the ball’s motion. Here’s what this might mean. Say that we’re looking at the Earth from such a perspective that the North Pole is in the very center. If we imagine a line going into and out of the screen, that line will go through the North and then the South Pole of the Earth. And it’s around this axis that the Earth rotates once every 24 hours. Now, our ball is on the surface of the Earth and therefore it’s on a rotating object.

Answer option C says that Earth’s motion around itself, say counterclockwise as we’ve drawn it, is opposing the motion of the ball. Say it would be in that direction. But here’s the thing about option C. The Earth was always rotating even before the ball started moving in the first place. Whether the ball rolls or not, quickly or slowly, the Earth’s rotation continues on. It’s a constant and therefore it won’t affect the motion of the ball to slow it down. Even before the ball started rolling, it was already moving with the Earth’s motion around itself. So option C is not a conclusion that our student should draw either.

This leaves us with option D, which is that the ball is pulled or pushed by something that the student cannot see. Now, this action the student takes of rolling this ball is common enough that we may have seen someone do it or done it ourselves. And in that case, we too observe that the ball eventually slows down, even if it’s rolling on flat ground. The reason for this is because of the force of friction between the ball and the surface it rolls on. This frictional force opposes the motion of the ball, causing it to slow down.

We could say that this is a push provided opposite the direction of the ball’s motion. We don’t see friction taking place. But we know because the ball is in physical contact with the floor it’s rolling on that friction will be there. Option D then will be our choice. The correct conclusion the student could draw is that the ball is pulled or pushed by something that the student cannot see.

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