TPR practice test questions

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Apr 16, 2013
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These three questions are from the TPR MCAT Review Series Practice Test 1

1. A person on a sled is moving 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 the sled?
a. They will slow down
b.They will speed up
c.They will stay at their original speed
d. They will speed up at first and then slow to their original speed when the ball lands

The answer is c, because momentum is not conserved in the vertical direction
My thought process was as follows- (m1+m2)v= m1v2+m2v2
I thought that since the ball would separate from the sled +person..the velocity of each would decrease?

2. A medium with index of refraction is surrounded by air. The critical angle inside of the medium is then determined. The medium is then submerged in a fluid with index of refraction nfluid. If 1<nfluid<n, how is the critical angle affected?
a. It is measured inside of the medium and is smaller than its previous value
b. It is measured inside of the medium and is greater than its previous value
c.It is measured inside of the fluid and is smaller than its previous value
d. It is measured inside of the fluid and is greater than its previous value

The answer is b. For this question I know that sincriticalangle=n2/n1, but I'm just not sure how to wrap my head around this question to begin with.

3. Why is the electric potential on every point of the sphere constant? I thought that the electric potential is constant at every equidistant point from the center, but not the whole thing.

Thank you in advance!! Any help is appreciated! :)

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In question 1, acceleration of the sled will be cause by throwing the ball in any direction that is parallel to the surface the sled is sliding on. Because the ball was thrown straight up, perpendicular to the surface, the force of the ball being thrown pushes the sled down into that surface. Since the surface is immovable in the problem (it will almost always be if it is assumed to be the earth), the surface pushes back up with the same force equalling out the force and causing no vertical acceleration.

If you had a problem saying the boy through the ball either towards the direction he is traveling or away, then you would have a change in speed (increase in speed if thrown opposite the direction traveling or decrease if thrown towards the direction traveling).

You have to have some a force parallel to the direction of motion to have any acceleration occur. The force of the ball being thrown created a force completely perpendicular to the sled that was cancelled out by the normal force. Since there was no force parallel to the sled's motion, there is no change in velocity.

Question 2

Just assign numbers since they don't give you any like air=1, the fluid=2 and n=3. That should let you use the equation a little easier.

Question 3

You said it yourself, there is constant electrical potential at every point equidistant from the center. All points on a sphere's surface are uniformly equidistant from the center. Therefore, every point has a constant electrical potential.