# Homework Help: Mechanical energy?

1. Nov 7, 2008

### sweedeljoseph

1. The problem statement, all variables and given/known data
You use 556 N of constant force to push a crate 12 m horizontally across the floor, and then stop pushing, leaving the crate stopped in its new position:
a) How much work did you do on the crate?
b) What is the change in the total mechanical energy of the crate after being pushed (ingoring heat)?

2. Relevant equations
F(cos theta)d

3. The attempt at a solution
a) 6672 J
b) 0 J

i know how to find the first question. you use the equation i gave. F is 556 N, cos theta is 1, and d is 12 m.

i dont understand how the second part is 0. am i supposed to use a certain formula? i know PE and KE but i dont have a mass in the problem. so what am i supposed to use to figure out what mechanical energy is?

if you could help me it would be great
i have a test on next tuesday
tryna study now instead of later

thank you!:tongue2:
sweedeljoseph

2. Nov 7, 2008

### mgb_phys

Think about what happens rather than just the formulae.
Work is force * distance for the first part.

For the second part, what has changed about the energy of the box?
Is there anythign you can do to get more energy out of the system in it's second state than in it's original state?

3. Nov 7, 2008

### sweedeljoseph

i have no idea. i just wrote the question how it was on my paper. there are no resources to use as a reference for the problem. thats just it. im sure ill know how to solve it but im not really sure what im supposed to do. i know the answer is 0 thats it i dont know why.

4. Nov 7, 2008

### mgb_phys

Since work is force*distance, is there any force that is trying to return it to it's original position?

Imagine if you lifted the box up 1m, the force of gravity is trying to return it to it's original position so you can use this force to extract energy.
Similairly if there was a spring pulling it back you could use that.

5. Nov 7, 2008

### sweedeljoseph

would the mechanical energy be 0 because the crate isnt being lifted? its just being slid across the floor.

like when youre doing projectile motion? with the T chart of x and y values solving for v,vo,a,t,d the reason the y value of the vo will be 0 is because its staying in the air
something like that?

6. Nov 7, 2008

### mgb_phys

Yes pushing something along a level surface uses no energy except for friction.

7. Nov 7, 2008

### sweedeljoseph

thank you!
i understand it now
=]