# Is there anything missing in this motion equation?

1. Apr 20, 2013

### stephenn

hello
have found this interesting question (interesting to me).... along with an equation that is supposed to allow the answer..... but I plug in the data and never get the same result as is correct.

could someone set it out with the data so it flows to give -690N (if correct).... thanks
(not an exam question or anything.... I found it and would like it as a model to find average brake force.... cheers)

A 1100-kg car traveling at 27 m/s starts to decelerate and comes to a complete stop in 578.0 m. What is the average braking force acting on the car?
answer: (a) -690 N (b) -340 N (c) -410 N (d) -550 N

the answer is said to be (a) -690N
and the suggested equations are these:

Use Newton's second law in the form:
F = ma

And this equation of motion:
v2 = u2 + 2as

Where:
m = mass
F = braking force
v = final velocity
u = initial velocity
s = displacement

You end up with an equation like this:
F = (mv2)/2s

and the answer is -690 N.

...................................
appreciate thoughts, thanks

2. Apr 20, 2013

### rock.freak667

F = (mv^2)/2s gives 693 N.

You could have gotten the same answer by using conservation of energy:

Loss of KE by car = work done by friction force

1/2(mv^2)=Fs

3. Apr 20, 2013

### stephenn

Thanks.... gotit in end....
didn't bother with the end equation...... simply used first 2 equations in own right:

* started with v2=u2+2as
* then plugged that into F=ma
* this got me to the answer

All done
Thanks for input, much appreciated