# Recoil when ball is accelerated but not released

## Homework Statement

Will a skateboarder (initially at rest) experience recoil if he accelerates a ball as if about to throw but does not release the ball?

## Homework Equations

pinitial=pfinal when not external forces

## The Attempt at a Solution

I am confused with how to apply the momentum vectors for the throwing arm/ball and the body of the skateboarder.

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PeterO
Homework Helper

## Homework Statement

Will a skateboarder (initially at rest) experience recoil if he accelerates a ball as if about to throw but does not release the ball?

## Homework Equations

pinitial=pfinal when not external forces

## The Attempt at a Solution

I am confused with how to apply the momentum vectors for the throwing arm/ball and the body of the skateboarder.
When you throw a ball, you apply a forward force to it. newtons 3rd law say an equal sized force will act back on you - accelerating you.
If you don't release the ball, you must apply a backward force to stop that ball again. That means a forward force on you.

The ball first accelerates forwards, then accelerates backwards - but has a net movement forward.

You will first accelerate backwards, then accelerate Forwards - but have a net movement back.

The centre of mass of you and the ball will remain in the same place throughout.

Okay, that makes sense. Thanks so much. Just for clarification, I should take this to mean that for any case where a part of a system accelerates but is not released (ie if somehow rocket exhaust gases were released into a compartment of the rocket itself and not into space), there will be no recoil/equal and opposite momentum (ie the rocket will not move)? Basically, for some momentum to cause an equal and opposite reactant momentum, the 2 masses having momentum must separate?

PeterO
Homework Helper
Okay, that makes sense. Thanks so much. Just for clarification, I should take this to mean that for any case where a part of a system accelerates but is not released (ie if somehow rocket exhaust gases were released into a compartment of the rocket itself and not into space), there will be no recoil/equal and opposite momentum (ie the rocket will not move)? Basically, for some momentum to cause an equal and opposite reactant momentum, the 2 masses having momentum must separate?
That is correct, they must separate to get continuing motion.