Is work done against friction an extra work

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2. μ We can use p = FV, μ If it changes, we can only use the kinetic energy theorem to find work, but also power

3. Solution: by η= If W has / W total = GH / f * 3H = g / 3F, then f = g / 3 η= 450N / (3 * 75%) = 200N
work done to overcome friction 180J refers to the extra work done to overcome the friction between rope and pulley 180J, which is not contradictory to the extra work done to overcome the gravity of moving pulley
weve = GH = 450N * 2m = 900j
wtotal = FS = 200N * 6m = 1200J
to overcome the additional work done by the moving pulley is 1200j-900j-180j = 120J
120J = gdynamic h, we get gdynamic = 120J / 2m = 60N
that is, the gravity of the moving pulley is 60N
if the object falls, we say it is gravity work, if the pulling force pulls the object upward, it moves in the opposite direction to gravity, The work done by pulling force is sometimes referred to as work done by overcoming gravity.

4. I think there is a problem with the method. If the work done to overcome the friction, the friction is doing negative work on the object, and the negative work is extra work relative to the object. However, whether we can do extra work depends on the conditions. If there is an object on the conveyor belt, the conveyor belt works against the sliding friction of the object being transported (the conveyor belt is in the horizontal direction). The positive work that a conveyor belt does to an object is useful work. Because it does the work that people want. Delivery
it is better to ask when the work done by friction is not extra work. And it's easier to explain.

5. To do work against friction is to do negative work, and to do work against gravity is to transform gravitational potential energy into other forms of energy

6. If the direction of the friction force is consistent with the direction of motion, then the friction force does positive work, that is, useful work. If the direction of friction is opposite to that of motion, negative work is done.

7. In the inclined plane, the work done by friction is extra work. If there is any useful work, it is pushing things on the plane and generating heat by friction

8. Mechanical efficiency is useful work divided by total work (total work equals useful work plus reactive work), so useless work is also included in total work

9. The total work of combined external forces is the work of all external forces, including the work done to overcome friction
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