Practice Test Question- Momentum

[collegelife101]

A person on a sled is moving to the right along a frictionless surface. The person throws a ball straight upward (as seen in the person’s frame of reference). What will happen to the motion of the person and sled?

The answer is that it will stay the same. I kind of understand the explanation but don't understand one of the statements for the solution: Before the throw, the person, sled and ball all have horizontal velocity. When the person throws the ball upward, it will still have that same horizontal velocity even though the person thinks the ball is moving straight up. Since the horizontal momentum of the ball remains the same, the combined momentum of the person + sled must also remain the same.

If the person is throwing the ball upward, would there be velocity in the vertical direction? In this case, why would the horizontal velocity remain the same?

Thanks!

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[el_duderino]

The surface is frictionless. I suppose you could argue that the person, sled, and whatever mass/planet is supporting them all accelerate slightly in the "down" direction but that's clearly beyond the scope of the question. The horizontal velocity remains the same because there is no force applied in the horizontal plane.

 

[DrknoSDN]

They were breaking the velocity into components. So throwing it up does give it vertical velocity but because the force applied is perpendicular to the horizontal velocity, horizontal velocity does not change. Work is done elevating the ball to mgh instead of converting horizontal KE to vertical KE.

Like Jonnythan said, if there was no normal force it's different. If you're floating horizontally in space and throw a ball "up...?" your horizontal velocity must change because no work would be done against the lack of gravity. In that case only conservation of total momentum would apply.