# Homework Help: A MCQ on Power

1. Dec 25, 2011

### tsw99

1. The problem statement, all variables and given/known data

I encountered this past exam paper question in Hong Kong when I was giving tutorial to a high school student. I am confused with the answer provided by the official authority. The question is (in exact wordings):

2. Relevant equations
3. The attempt at a solution
The friction f provides by the ground to propel the car is exactly cancelled by the component of weight down the inclined plane, hence the power is fv which is equal to mg(sin30°)v=1/2 mgv which is A. However the official answer is B. I have no idea how to arrive at the answer B and I believe the authority must have their own grounds. I even think that B is a "trap" choice when I first read this question.
Any help is appreciated. Thanks.

2. Dec 25, 2011

### Redbelly98

Staff Emeritus
It sounds like "the friction acting on the car" here refers to friction that is retarding the motion of the car, and not the friction you mention that is propelling the car forward.

Hope that helps.

3. Dec 25, 2011

### tsw99

Yours is a probable answer. Does that mean the car is skidding up the incline? So the friction does not provide the force to travel upward?
Frankly speaking, I never saw an electric toy car travels in this way. I believe it is a poorly set question.

4. Dec 25, 2011

### SammyS

Staff Emeritus
Generally speaking, contact friction, such as between the tire and the ground is not considered to do work.

The friction acting to impede the car as it goes up the ramp would likely be from the non-driven wheel-axle system and perhaps some piece of the car dragging on the ground.

5. Dec 26, 2011

### Redbelly98

Staff Emeritus
SammyS provided a good explanation, so I'll just add that no, I did not mean the car is skidding. Friction between the wheel and ground does provide the force to propel the car, however that is not the friction being referred to in the problem statement.