# Work and Energy of snow

1. Feb 16, 2015

### kaspis245

1. The problem statement, all variables and given/known data
A sled is being pulled across a horizontal patch of snow. Friction is negligible. The pulling force points in the same direction as the sled's displacement, which is along the +x axis. As a result, the kinetic energy of the sled increases by 38% . By what percentage would the sled's kinetic energy have increased if this force had pointed 62o above the +x axis?

2. Relevant equations
The Work-Energy Theorem

3. The attempt at a solution

So I get, that the kinetic energy is reduced by 35% .

Correct answer: the kinetic energy increases by 18% .

2. Feb 16, 2015

### Bystander

If the problem statement is complete and correctly transcribed in your original post, without checking your math, you are correct. It would be best that you double-check the problem statement.

3. Feb 16, 2015

### kaspis245

The problem statement is correct 100%. Maybe I understood the problem incorrectly?

4. Feb 16, 2015

### Bystander

As written, what you've done looks correct --- the x-component of the force is 100 % for the first case, and is reduced for the second. Looks like a really lousy problem statement, and an even worse job of proof-reading/checking matches of answers with problems. Let us know whether anyone fesses up, or how they rationalize the answer that's claimed.

5. Feb 16, 2015

### BvU

I think it is understanding indeed. I fail to understand what W0 represents, for example.
You are given that Ekin + F $\cdot$ s = 1.38 Ekin

Now try to grasp what you are being asked. The 17.8% book answer is correct.

6. Feb 16, 2015

### kaspis245

Wo represents work with which other works (W1 and W2) are compared.

I understand your method, but I don't understand why my method didn't work.

7. Feb 16, 2015

### BvU

Still don't understand. If you write W1 = F $\cdot$ s , then I don't understand why you consider that to be 138 % of W0 instead of 38%

8. Feb 16, 2015

### kaspis245

As the problem says, the kinetic energy of Wo increases by 38% .

If I say, that the very first energy, with which we will compare other energies, is equal to 100% , then the second energy W1, which is along +x axis, must be equal to 138% (increases by 38%) .

9. Feb 16, 2015

### BvU

Fine with me. But then you must write W1 = W0 + F $\cdot$ s.
There is no friction, the original kinetic energy doesn't go away.

And energies are not along any axis. Energies are numbers. But I understand what you mean.

10. Feb 16, 2015

### Bystander

BvU has you pointed in the right direction --- stay with him, and forget everything I said --- I misread the problem horribly.