Physics help! I appreciate the help! A best answer will be chosen.?

Question

1) Answer all that are correct-
You run and jump on a swing. During which of the following motions is the total momentum of you and the swing conserved?
running at the swing (and accelerating)
jumping on the swing (the "collision" between you and the swing)
swinging up on the swing
swinging down on the swing
2)Answer all that are correct-
You run and jump on a swing. During which of the following motions is the total mechanical energy of you, the swing, and the Earth conserved? (Assume that the swing is well made so friction can be ignored.)
running at the swing (and accelerating)
jumping on the swing (the "collision")
swinging up on the swing
swinging down on the swing
3) Momentum conservation occurs whenever
the net external force on the objects is zero
the work on the objects is zero
two objects are stuck together
two objects are pulled apart

Answer 1

Momentum is defined to be the mass of an object multiplied by the velocity of the object. The conservation of momentum states that, within some problem domain, the amount of momentum remains constant; momentum is neither created nor destroyed, but only changed through the action of forces as described by Newton's laws of motion. Dealing with momentum is more difficult than dealing with mass and energy because momentum is a vector quantity having both a magnitude and a direction. Momentum is conserved in all three physical directions at the same time.

Therefore, although the vectors may change, and momentum may be shared, the collision is a fully "elastic" collision and therefore momentum is conserved. The momentum increased during the run up to the swing, so it wasn't constant, and the feet that were pushing against the earth were pushed back upon by the earth, so technically this is not a good answer. The jump on the swing (the collision), however, is straight forward momentum conservation because it was an elastic collision; so "b" is a good answer. When the swing "swung up", momentum was converted into potential energy and at the point where the swing stopped, all momentum had been converted, thus not conserved (the outside force acting on the system was gravity); so this is not a good answer. When the swing "swung down", potential energy was converted into kinetic energy and, minus wind friction, momentum was restored (mass x velocity); so this, taken independently, is not a good answer either. However, if looked at as a closed system, one could argue that momentum was conserved from the point of impact to the point the swing returned to its starting point. However, technically speaking, if each part were treated as a separate event, momentum was not conserved during the upswing or the downswing. So "b", the collision (no change in momentum and no conversion taking place), is the only choice that is true without any qualifications.

2. Mechanical energy is not conserved on the run-up to the swing because you're converting internal energy into momentum. The collision with the swing conserves mechanical energy because the collision was elastic. The swing up and down did not conserve mechanical energy because it was expended to overcome gravity and was converted into potential energy. The swing down converted potential energy into mechanical energy, so it was not "conserved", it was converted. Again, the answer is the same as the conservation of momentum, and unless you look at the collision, swing up and swing down as a single event, mechanical energy was not conserved...except in answer "b".

3. Answer "a", when the net external force on the objects is zero

Answer 2

1. Momentum conservation occurs at the point of collision.

2. Mechanical energy is conserved when there are no external forces, I would say the point of collision qualify as conserved, swinging up and down on the swing are affected by gravity and wind resistance, accelerating is providing an external force also.

3. Momentum conservation occurs when two objects are stuck together.

Answer 3

thanks for two points=)