Question

Let’s say you push the block from the bottom of the plane all the way up to the top of the plane, then let it slide back down.

11-1. While you are pushing the block up the plane, is the work done by gravity positive or negative? Why?

11-2. While you are pushing the block up the plane, is the work done by friction positive or negative? Why?

11-3. As the block slides back down the plane, is the work done by gravity positive or negative?

11-4. As the block slides back down the plane, is the work done by friction positive or negative?

11-5. What is the net work done by gravity over the entire cycle (being pushed all the way up and then sliding all the way back down)? Is it positive, negative, or zero?

11-6. What is the net work done by friction over the entire cycle (being pushed all the way up and then sliding all the way back down)? Is it positive, negative, or zero?

Answer 1

1)

When th block pushed up the plane, the gravitational force is directed downwards along the plane and its displacement vector is upwards along the plane ,

so work done

Work done by gravity is negative.

2) When the block is pushed up the plane, the frictional force is downwards along the plane, and diplacement vector is upwards along the plane.

Work done by friction is negative.

3)

When th block sliding down the plane, the gravitational force is directed downwards along the plane(inclined at angle ) . and its displacement vector is also downwards along the plane ,

so work done

Work done by gravity is positive.

4)

When the block is sliding down the plane, the frictional force is upwards along the plane, and diplacement vector is downwards along the plane.

Work done by friction is negative.

5)

Net work done by gravity over the entire cycle is zero.

6)

Net work done by friction over the entire cycle is negative.