# Energy Lost due to friction

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1. Jan 9, 2015

### GiantSheeps

1. The problem statement, all variables and given/known data
A team of dogs drags a 114 kg sled 1.92 km over a horizontal surface at a constant speed. The coefficient of friction between the sled and the snow is 0.124. The acceleration of gravity is 9.8 m/s 2 . Find the work done by the dogs.

Find the energy lost due to friction. Answer in units of kJ.

2. Relevant equations
I don't know, I think that might be part of my problem

3. The attempt at a solution

For the first one, I did 114 x 9.8 x 1.92 x 0.124 to get 265.982976, which was correct

but for the second part, I tried to do 114 x 9.8 x 1.92 to get 2145.024, which would be the energy with no friction, right? Then I subtracted the energy with friction from the energy without friction to get 1879.041, but that was wrong. What am I doing wrong? Wouldn't it make sense that you could find energy lost like this? Is there a formula?

2. Jan 9, 2015

### haruspex

You calculated the work done by the dogs. Where has that work gone? PE? KE? Anywhere else?

3. Jan 9, 2015

### GiantSheeps

Wouldn't it all have gone to KE?

4. Jan 9, 2015

5. Jan 9, 2015

### haruspex

How do you assess change in KE?

6. Jan 9, 2015

### GiantSheeps

I know that the kinetic energy formula is 1/2mv^2, so to find change in KE I would just use that formula to get the KE and subtract final KE from initial KE

7. Jan 9, 2015

### haruspex

Right. Is the speed changing?

8. Jan 9, 2015

### GiantSheeps

no, the speed is constant

9. Jan 9, 2015

### GiantSheeps

that wouldn't make the answer 0, would it?

10. Jan 9, 2015

### GiantSheeps

it isn't, can I have another clue, please?

11. Jan 9, 2015

### Nathanael

What is the change in energy of the sled?

12. Jan 9, 2015

### GiantSheeps

Wouldn't it also be 0? Becasue the sled is attached to the dogs

13. Jan 9, 2015

### Nathanael

So if the sled didn't gain any energy, what happened to the energy (from the work done by the dogs) from part one?

14. Jan 9, 2015

### GiantSheeps

It became friction energy?

15. Jan 9, 2015

### GiantSheeps

does that even make sense?

16. Jan 9, 2015

### Nathanael

Right.
Well... more correctly, you should say something like, "it was lost through friction." But you have the right idea.

It wouldn't be called "friction energy", it would be in the form of "thermal energy" (because the snow would be a bit warmer).

Haha, not exactly, but you have the right idea.

17. Jan 9, 2015

### GiantSheeps

Okay I think I'm starting to get it, all of the energy has to go somewhere, so in this case it all became thermal energy

18. Jan 9, 2015

### GiantSheeps

YES! Thank you so much!

19. Jan 3, 2016

### Sl0nimski

Wait, but that still doesn't answer the amount of energy lost! What's the formula?

20. Jan 3, 2016

### GiantSheeps

you don't need a formula for this one