# Recoil when ball is accelerated but not released

• katieoneill
In summary, when a skateboarder accelerates a ball as if about to throw but does not release the ball, there will be a recoil force on the skateboarder due to Newton's Third Law. However, because the ball is not released, the net movement of the system will remain the same and the skateboarder will return to their initial position. This applies to all cases where part of a system accelerates but is not released. The two masses must separate to continue motion and for the equal and opposite momentum to be observed.

## 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.

katieoneill said:

## 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?

katieoneill said:
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.