A block is given an initial velocity down an inclined plane with friction. The b

Question

A block is given an initial velocity down an inclined plane with friction. The block comes to rest before the end of the inclined plane.

1)

What is the sign of the work done by gravity as the block comes to rest on the inclined plane?

positive

negative

zero

2)

What is the sign of the work done by friction as the block comes to rest on the inclined plane?

positive

negative

zero

3)

What is the sign of the work done by the normal force as the block comes to rest on the inclined plane?

positive

negative

zero

4)

What is the sign of the net work done (by all the forces) on the block as the block comes to rest on the inclined plane?

positive

negative

zero

5)

Now the block is given the same initial velocity down the same inclined plane (but with a smaller coefficient of friction). The block is moving faster than its initial speed as it approaches the end of the inclined plane.

What is the sign of the net work done (by all the forces) on the block as the block travels down the inclined plane?

positive

negative

zero

Explanation / Answer

1)

Work done by gravity = positive (as gravity has a component along the direction of displacement)

2)

Work done by friction = negative (as friction has a component opposite to the direction of displacement)

3)

Work done by normal force = 0 (as the normal force is perpendicular to the direction of displacement)

4)

Net work done = negative (as it comes to rest , so negative work must be done on it to bring it to rest)

5)

Net work done = positive (as its Kinetic energy at bottom is greater than at start)

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