Hi,I have a question.

Question

Is the work of friction in water slides negative or positive and why?

Please I need a good explanation because I heard it is positive because of the water, but I am not convinced.

Answer 1

Ok. If I understood the question correctly, you are talking about force of friction in the water. In this case we are dealing with kinetic friction, because the body is sliding with respect to the water. This force depends on velocity of the body inside the water, its shape and other factors. Hence, it is not constant and varies with time. But it is still force of friction! it means that the mechanical energy will be dissipating - decreasing. Mainly it will be converted to heat (internal energy), small part of mechanical energy will convert to sound energy. Once I did estimation for myself and got the result about 5% of total mechanical energy. If the energy is decreasing, the only right conclusion could be made that the work in this case is negative. Hope you remember that work is change of energy of the system. If you subtract the bigger number from the smaller, the result will be negative.

By the way, Joule was the first, who used this fact practically. In thermally insulated container with water he placed a paddle that stirred when the masses pulled down the ropes and spun the device. Because it is being stirred, the temperature of the water raised due to friction between the water and the paddles. This is how he determined first time in the history of physics the numerical value of Mechanical Equivalent of Heat.

Sorry, but somebody misinformed you.

Good luck, hope now you are not so confused.

FYE. You remember that we also dealing with static friction force. Yes, this is the force that allows us to walk, cars - to move, hold something in our hands, etc. The work of this force is positive! :)))