Thermal/mechanical/potential/kinetic energy help

[nemzy]

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.

 

[HallsofIvy]

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)?

 

[M98Ranger]

I understand that learning about thermal, mechanical, potential, and kinetic energy can be overwhelming and confusing at first. Don't worry, you are not alone. Physics can be a challenging subject, but with practice and patience, you will be able to apply these concepts to various problems.

Let's break down each problem and see how we can apply what you have learned about thermal, mechanical, potential, and kinetic energy.

5) In this problem, we are given information about a plastic cube being pushed across a floor at a constant speed. The temperature of the cube is monitored and it is found that its thermal energy increases by 20 J. This means that the cube is gaining thermal energy as it is being pushed. The question asks for the increase in thermal energy of the floor along which the cube slides. To solve this, we need to understand that as the cube moves, it is causing friction with the floor, which in turn increases the thermal energy of the floor. So, the increase in thermal energy of the floor will also be 20 J.

6) In this problem, we are given the mass and velocity of a bullet and its stopping distance within a solid wall. To find the change in mechanical energy, we need to understand that mechanical energy is the sum of kinetic and potential energy. In this case, the bullet loses all its kinetic energy as it comes to a stop, and this energy is converted into heat and sound due to the impact with the wall. Therefore, the change in mechanical energy will be equal to the initial kinetic energy of the bullet, which can be calculated using the formula 1/2mv^2.

7) In this problem, we are given the mass of water and its height as it falls over Niagara Falls. The question asks for the decrease in gravitational potential energy of the water-Earth system. We know that gravitational potential energy is the energy an object has due to its position in a gravitational field. As the water falls, it loses height and therefore, its gravitational potential energy decreases. To calculate this, we can use the formula mgh, where m is the mass of water, g is the acceleration due to gravity, and h is the height of the fall.

I hope this helps you understand how to apply what you have learned about thermal, mechanical, potential, and kinetic energy. Remember, practice makes perfect, so keep working on problems and don't get discouraged. Physics may be frustrating at times, but it can also be very