Evaluate the following statements (T/F/N). As always, explain your reasoning. (A

Question

Evaluate the following statements (T/F/N). As always, explain your reasoning. (A) A normal force cannot do work. (B) If you hold an object in your hand and slowly lower your hand at constant speed, you are doing work on that object. (C) Kinetic energy is conserved when two identical objects, moving on a level, frictionless surface at equal speeds, collide head-on and stick together. (D) If a mass is swinging as a pendulum on a wire, the wire is doing no work on that mass. (E) A friction force always does work on an object. (F) If an object is moving in the negative x-direction, you would be doing negative work on it by pushing on it in the positive x-direction. (G) Units of power could be correctly expressed as N/(m

Explanation / Answer

(A) A normal force cannot do work.

False. A normal force can do work if the surface moves with the object. For example, when a person stands in an elevator and it moved upward, the normal force from the floor pushes the person upward and does work.

(B) If you hold an object in your hand and slowly lower your hand at constant speed, you are doing work on that object.

True. To do work you must simply apply a force and the object must move in the direction of the force (positive work) or opposite the direction of the force (negative work.)

(C) Kinetic energy is conserved when two identical objects, moving on a level, frictionless surface at equal speeds, collide head-on and stick together.

False. Kinetic energy is not conserved in this kind of collision, only momentum is conserved.

(D) If a mass is swinging as a pendulum on a wire, the wire is doing no work on that mass.

True. The force of the wire acts perpendicular to the motion of the mass, so this force does zero work.

(E) A friction force always does work on an object.

False. Static friction can hold an object in place. If the object does not move, the force of friction does zero work.

(F) If an object is moving in the negative x-direction, you would be doing negative work on it by pushing on it in the positive x-direction.

True. Negative work is done when the direction of the force is opposite the direction of motion.

(G) Units of power could be correctly expressed as N/(m·s)

False. Units of power are Watts, which are J/s which are N-m / s (they put the "m" in the wrong place in the question.)

(H) If you’re riding in car moving at constant speed along a circular curve in a level road, the car is doing work on you.

False. The car pushes you upward (i.e. supporting you) and perpendicular to the road (so that you turn in a circle.) But it does not push you forward or backward (i.e. in the direction of motion), so it does zero work on you.

(I) One “physics” definition of energy is: It’s the ability to do work.

True. This one's in the dictionary.

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