# Calculating acceleration and brake force

1. Dec 30, 2008

### cyberhev

1. The problem statement, all variables and given/known data Hi all, the question for revision is; A car of mass 600kg travelling at 45m/s comes to rest in 12m when the brakes are applied. What is the average force exerted by the brakes?

2. Relevant equations Force = mass x acceleration V^2=u^2 +2as

3. The attempt at a solution I know I need to work out the acceleration but am having trouble correctly transposing that equation I think its this v^2-u^2 divided by s =2a, its what to do with the 2 to get a on its own, does it then become 1/2 s on the other side?

2. Dec 30, 2008

### Redbelly98

Staff Emeritus
You're using the correct equation, but it looks like you need some help in "solving for a" to get the acceleration.

Let's write out what you correctly have so far:

$$\frac{v^2-u^2}{s}=2a$$

Hopefully that helps you see better what to do ... if not, then post again.

p.s. I'm not quite sure if
means "(1/2)s" or 1/(2s).

3. Dec 30, 2008

### cyberhev

That is what i have so far, Im just not sure where the 2 goes so you just get a on its own, I was wondering the 2 then moves to the side of s to become1/2s.

4. Dec 30, 2008

### cyberhev

If I solve that equation with 2a. I did the following 45x45-0x0, divided by 12 = 168.75 x 2 =337.5. Is this the correct acceleration?

5. Dec 30, 2008

### Saladsamurai

$$\frac{v^2-u^2}{s}=2a$$

$$\Rightarrow \frac{v^2-u^2}{2s}=a$$

6. Dec 30, 2008

### cyberhev

Yes I have it, thank you, my transposition of equations knowledge is in there, but its 15 years or so since I used it! I often write the suvat list out, i find it very helpful.

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