# Homework Help: Friction/ Newtons Laws

1. Jan 8, 2014

### KiNGGeexD

Hi I am stuck with a question which I have attached a photo of, it is one involving friction! I have done a few of these questions already and find it is when I am defining forces etc it's where I go wrong, not the easy computational part:(

I also have attached my attempt at the solution and have drawn my free body diagram as clear as possible:)

Any help would be great:) thanks

2. Jan 8, 2014

### voko

I see no mistake in your attempt so far.

3. Jan 8, 2014

### KiNGGeexD

Excellent I shall continue onwards and reply shortly:)

4. Jan 8, 2014

### KiNGGeexD

Ok I went on and done this to find the acceleration but the model answer is 3.71 m/s^2

5. Jan 8, 2014

### voko

In your computation you assumed that static friction will not be strong enough to prevent motion. Part (a) of the problem required you to determine whether this was the case.

6. Jan 8, 2014

### KiNGGeexD

It is not strong enough

7. Jan 8, 2014

### KiNGGeexD

It is the two fractions which are confusing me, kinetic friction is the friction between two moving bodies and static is between two non moving bodies would there be a case where both would have to come into a computation?

8. Jan 8, 2014

### voko

Can you demonstrate that?

9. Jan 8, 2014

### voko

I did not see #7 when I wrote #8. Please explain how you determined that static friction was not strong enough to prevent motion. That will help clarify what you confusion really is.

10. Jan 8, 2014

### KiNGGeexD

So for the block to move mg sinθ must be greater than static friction?

11. Jan 8, 2014

### KiNGGeexD

Sorry F cosθ

12. Jan 8, 2014

### voko

Yes, the horizontal component of force must be greater than the maximal static friction. The maximal static friction is given by $\mu_s N$.

13. Jan 8, 2014

### KiNGGeexD

14. Jan 8, 2014

### voko

I agree with your result, but it is not correct to write $F \cos \theta - \text{fr} = 0$. You should write $F \cos \theta - \text{fr} > 0$.

15. Jan 8, 2014

### CWatters

Check what value you used for μ.

I put 0.2 into your earlier equation and got 3.7 as the answer.

16. Jan 8, 2014

### KiNGGeexD

Oh! Haha I think I've got it now lol