Two people in a boat and one person throws a ball

[Soniteflash]

1. Two people sit at opposite ends of a boat. Ther person at the front of the boat has a ball and throws it to the person sitting at the back of the boat who catches it.

What is the motion of the boat at the time immediaately after the ball is thrown and later after the ball is caught ?

Answer choices.

After Throw After the Catch
(A) Boat moves foward. Boat moves foward.
(B) Boat moves foward Boat moves backward.
(C) Boat moves foward. Boat does not move.
(D) Boat moves backwards Boat does not move.
(E) Boat moves backwards Boat moves foward.

2. Newton Third Law.

3. I honestly don't know. What i would say is that that the boat moves foward. After that I am just confused. What actually happens when the person at the back catches the ball? Doesnt he exert a force that us equal and opposite in direction?

I used my phone and the spacing is messed up. After the throw and Ablfter the Catch should be separated.

 

[AlephNumbers]

What actually happens when the person at the back catches the ball? Doesnt he exert a force that us equal and opposite in direction?

What does Newton's Third Law say?

 

[haruspex]

How about a conservation law instead?

 

[AlephNumbers]

How about a conservation law instead?

Yes, that is probably a better way to approach the problem.

 

[Soniteflash]

If object one exerts a force on object two, then object 2 exerts a force on object one that is equal in magnitude and opposite in direction.

 

[Soniteflash]

Conservation Law? What is being conserved here?

 

[haruspex]

Conservation Law? What is being conserved here?

What external forces act on the boat+occupants+ball system?

 

[Soniteflash]

Hmm, would the only external force be the force from throwing the ball?.

 

[haruspex]

Hmm, would the only external force be the force from throwing the ball?.

How is that external to the system?

 

[Soniteflash]

How is that external to the system?

Ok. Let me rethink. That would be an internal force then. I think that there is no external force acting on the boat+occupants+ ball system then.

 

[haruspex]

Ok. Let me rethink. That would be an internal force then. I think that there is no external force acting on the boat+occupants+ ball system then.

Well, no external horizontal force, which is all we need.
So what conservation laws apply?

 

[Soniteflash]

Conservation law. Are you speaking of conservation of kinetic energy?
Sorry I feel kinda dumb at the moment...

 

[haruspex]

Conservation law. Are you speaking of conservation of kinetic energy?
Sorry I feel kinda dumb at the moment...

Not that one. Two others to choose from...

 

[Soniteflash]

Not that one. Two others to choose from...

Uhm conservation of momentum maybe?

 

[J-Fly]

Well for every action there is an opposite reaction...

 

[haruspex]

Uhm conservation of momentum maybe?

Yes. Apply that to the before throw and after throw conditions.

 

[haruspex]

Well for every action there is an opposite reaction...

Yes, and conservation of momentum can be derived from that, but for the present problem it's more convenient to invoke the conservation law itself.