You are in your car at rest when the traffic light turns green. You place your coffee cup on the horizontal dash and hit the gas. The coffee cup does not slide as you accelerate forward. The work done by friction on your coffee cup is

a. equal to zero.

b. positive.

c. equal to the nonconservative work done by the car's engine.

d. negative.

e. equal in magnitude to the work done by the road on the car.

(d) Negative.

Explanation:

let's test each at a time.

(a) It can't be 0, because cup would slide back other wise.

(b) Positive, well if forward is positive, than the work done against the forward acceleration must be negative , so it can't be positive.

(c) Equal to non-conservative work done by the car's engine.

well no, because work done by car's engine dosen't go all of it into getting car to move, so it can't be that.

(d) negative, this look like it, because work that friction does must be nagative to counteract positive thrust of car which is positive and in forward direction.

(d) this can't be true.

So the answer is (d) negative.

an obstruction

the motion of a pendulum is a classic example of mechanical energy conservation. a pendulum consists of a mass (known as a bob ) attached by a string to a pivot point. as the pendulum moves it sweeps out a circular arc, moving back and forth in a periodic fashion.