A thoughtful student puts forth the following argument about when Newton’s 3rd l

Question

A thoughtful student puts forth the following argument about when Newton’s 3rd law applies.

"I understand why Newton’s 3rd law applies to contact forces, the forces objects exert on each other while touching, like when a truck hits a car. But I don’t think Newton’s 3rd law applies to gravity. When you drop a ball, the Earth exerts a gravitational force upon it, which makes the ball rush down to the Earth. But according to Newton’s 3rd law, the ball exerts just as big a force on the Earth as the Earth exerts on the ball. That can’t be right! The Earth doesn't “rush up” to meet the ball. This goes to show that the Earth exerts a bigger force on the ball than the ball exerts on the Earth. So, Newton’s 3rd law isn’t true for gravity."

1. First, what happens when the falling ball hits the Earth? Does Newton's 3rd Law apply? Explain how you know.





2. Now, consider the falling ball. Does Newton's 3rd Law hold or is the student correct in his argument? How would you respond to the student?

Explanation / Answer

Basically when the ball hits the ground equal impulse acts on the earth and the ball but the thing is that the mass of the earth is too large that the momentum of the earth is very very small... In the second question,yes equal force is applied on both the earth and the ball,but according to the second law of motion the acceleration of earth is very very low and more over earth is also under the action of other forces where as the ball is mostly under the only force of gravitation so what u observe is only the motion of ball and not the motion of earth...

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