Give an example where Work done by friction is positive and one where it is negative.
Sure! I can provide examples of both cases.
Example 1: Work Done by Friction is Positive
Let's say you push a box across a rough floor. In this case, you are applying a force in the direction of motion, but the frictional force opposes the motion. As a result, the work done by the frictional force is positive. This is because the force exerted by friction is acting in the opposite direction to your push, but the displacement of the box is happening in the same direction as your push. Therefore, the work done by friction is positive.
Example 2: Work Done by Friction is Negative
Now, let's consider a different scenario where a box is sliding down a rough incline. In this case, the force of gravity is acting down the incline, and the frictional force opposes the motion. Since the force and displacement are acting in opposite directions, the work done by the frictional force is negative. As the box moves down the incline due to gravity, friction acts in the opposite direction of the displacement, resulting in negative work.
To find out if the work done by friction is positive or negative, you need to understand the relationship between the direction of frictional force and the displacement of the object relative to that force. If they are in the same direction, the work done by friction is positive. If they are in the opposite direction, the work done by friction is negative.