Thermal/mechanical/potential/kinetic energy help

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I read the section on thermal/mechanical/potential/kinetic energy in my physics book and i seriously have NO idea how to apply what i learned in these problems. I am so confused, i never been this confused before in phsyics. I am getting very frustrated..man physics is so frustrating!!!

5) The temp. of a plastic cube is monitored while the cube is pushed 3.5 m across a floor at constant speed by a horizontal force of 15 N. The monitoring reveals that the thermal energy of the cube increases by 20 J. What is the increase in the thermal energy of the floor along which the cube slides?

6) A 41 g bullet with horizontal velocity of 496 m/s stops 13 cm within a solid wall. What is the change in mechanical energy

7) 5.8e6 kg of water fall 50 m over Niagara Falls each second. What is the decrease in gravitational potential energy of the water-Earth System.
 

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HallsofIvy
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5) The temp. of a plastic cube is monitored while the cube is pushed 3.5 m across a floor at constant speed by a horizontal force of 15 N. The monitoring reveals that the thermal energy of the cube increases by 20 J. What is the increase in the thermal energy of the floor along which the cube slides?

Since the cube moved 3.5 m with a force of 15 N, the total work done was 15(3.5)= 52.5 Joules. Where did all that work go? Since the speed remained the same, there was no increase in the kinetic energy. Since the floor was horizontal, there was no increase in potential energy. The thermal energy of the cube was increased by 20 Joules- that accounts for 20 Joules of the work. Where do you think the remaining 52.5- 20= 32.5 J went?

6) A 41 g bullet with horizontal velocity of 496 m/s stops 13 cm within a solid wall. What is the change in mechanical energy

This should be easy because it only asks about "mechanical energy". Here, that is kinetic energy. What was the kinetic energy before the bullet hit the wall? What was the kinetic energy after the bullet stopped (That's easy!)? The "13 cm" is irrelevant.

7) 5.8e6 kg of water fall 50 m over Niagara Falls each second. What is the decrease in gravitational potential energy of the water-Earth System.

In other words, what is the potential energy of a mass 5.8 x 106kg. raised 50 m (The "decrease" would be the loss as it falls back down that 50 m)?
 

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