Frictionless Puck in a Van Question?

Question

A puck is resting on the floor of a large moving van. Assume that the floor of the van is frictionless. The van and the puck are initially at rest, then the van undergoes a positive uniform acceleration.
A.) The puck will move forward with respect to the ground. (True or False)
B.) The acceleration of the puck with respect to the ground is (greater than, equal to, or less than) the acceleration of the van with respect to the ground.
C.) The magnitude of the acceleration of the puck with respect to the van is (equal to, less than, or greater than) the magnitude of the van's acceleration with respect to the ground.
D.) The puck will move toward the back of the van. (True or False)
E.) If the van turns to the right, the puck will hit the left wall. (True or False)
F.) The puck is stationary with respect to the van during this acceleration. (True or False)
Thank you!

Answer 1

"forjj" is right about one thing, Beauty - never trust answers online. In fact, none of the previous three responders got the problem entirely right.

A. False. The puck will remain stationary w.r.t. (with respect to) the ground.

B. Less than. The puck's acceleration w.r.t. the ground is zero.

(Black Angel got the right answer, but for the wrong reason. The question asks about the ground's perspective, not the van's.)

C. Equal to. The puck appears to the truck to accelerate backwards, but the magnitude is the same.

(At least the first three got this right. BlackAngel thinks the answer is "less than" because the acceleration w.r.t. the truck is negative, but problem asks about the magnitude, which means you ignore the sign and the direction, so the correct answer is "equal to.")

D. False. The puck holds still while the back of the van moves towards it. The puck is not moving towards the back of the van, rather the back of the van is moving towards the puck.

This question is worded badly, since it fails to ask whether you are watching the puck from the truck or from the ground. The correct answer would be true from the van's perspective, but false from the perspective of a person standing next to the truck. Since the puck is not moving (it is "at rest," according to the initial conditions stated in the problem), it makes sense to assume that the reference point is the ground, so the answer should be false. If your professor marks you wrong for answering false, you should point out that the (a) puck is not moving according to the problem statement (it is "at rest") and (b) the question cannot properly be answered, since it fails to provide a reference point.

E. True, assuming the back of the truck doesn't hit the puck first. The problem is formulated badly, since there is no way to know if it will hit the back or the side first. If the back of the truck hits the puck first, the puck could bounce to the right wall or even the front wall.

F. False. The puck is stationary w.r.t. the ground, but not w.r.t. the truck.

I grew up with a physics professor for a dad and took three years of college physics and engineering where we worked on problems like this, and I'm disappointed. The teacher did not give you enough information to answer (D) and (E). If you lose points on either one, let me know and I'll go T.P. his house.

Answer 2

A) false; no friction makes no forward movement.

B) Less than; if the van is moving with positive speed and the puck is standing still, then the puck has negative movement, in respect to the van.

C) Less than; see previous explination.

D) True; the van is moving FORWARD, so in respect to the van, the puck will move BACKWARD, thus to the back of the van.

E) True; equal and opposite reaction, according to Newton's first law of motion.

F) False; with respect to the van, the puck is moving backward (since in respect to the world, the van is moving forward).

You're welcome.

Answer 3

A. false
B. less than
C. equal to
D. false (the back of the van will move toward the puck)
E. true
F. false

Answer 4

a. true
b.equal to
c. equal to
d. true
e.true
f.false

Answer 5

a. F
b. equal
c. equal
d. f
e. t
f. t