Recoil when ball is accelerated but not released

[katieoneill]

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.

 

[PeterO]

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.

 

[katieoneill]

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]

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.

 

[asteriatic]

I would approach this question by first defining the concept of recoil. Recoil is the backward movement of an object when it is released or fired with a force. In this case, the skateboarder is not releasing the ball, so technically there would not be a recoil. However, there would still be a change in momentum for both the skateboarder and the ball.

When the skateboarder accelerates the ball, they are applying a force to it, causing it to gain momentum in the forward direction. According to the law of conservation of momentum, the total momentum of the system (skateboarder and ball) should remain constant. Therefore, as the ball gains momentum in the forward direction, the skateboarder would experience an equal and opposite change in momentum in the backward direction.

However, since the skateboarder is not releasing the ball, the change in momentum for the skateboarder would be very small and may not be perceivable. This is because the mass of the skateboarder is much larger than the mass of the ball, so the change in velocity for the skateboarder would be very small compared to the change in velocity for the ball.

In conclusion, while there may be a small change in momentum for the skateboarder, it would not be considered a recoil in the traditional sense since the ball is not being released.