Which of the following is true about high-energy mechanical devices? a. they usually need to be kept warm in order to preserve mechanical energy
b. they usually need to be cooled to dissipate thermal energy build- up and prevent damage. c. they usually convent radiant energy into electrical energy
d. during an energy conversion none of the energy is transferred as thermal energy because the devices are free of friction

B. They usually need to be cooled to dissipate thermal energy build-up and prevent damage.

Explanation:

High energy mechanical devices needs to be cooled down because they have moving parts which creates heat which may affect the other electrical and electronic component of the device. Because quality of material changes with the temperature. If high temperature occur might change the shape of that component and may affect the efficiency of the machine.

B.) They usually need to be cooled dissipate the thermal energy build-up and prevent damage

the answer is to check all of them! they all apply well at least i think so!

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.