# Forces, find net with velocity and distance

yogoes123

## Homework Statement

Driver brakes 1420kg car moving at 64.8km/h [W]. Car slows down and comes to stop after 729m [w]

find net force

a=v/t f=ma

## The Attempt at a Solution

t=40.5 a= 0.44

f=1420(0.44) f= 624.8

what am i doing wrong?

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Staff Emeritus
Hi yogoes123. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

Your first mistake is probably in how you worked out that 40.5 secs, as you haven't shown the equation you used.

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yogoes123
I used v=d/t swapped it around and solved for t=d/v and that gave me 40.5s, is that wrong?

Staff Emeritus
I used v=d/t swapped it around and solved for t=d/v and that gave me 40.5s, is that wrong?
That equation relates time to the distance travelled at a constant speed.

yogoes123
So it would be correct, no? I cant figure out what i did wrong.

Staff Emeritus
f=1420(0.44) f= 624.8

ans is: 316N

what am i doing wrong?
You [somehow] worked out a figure of 624.8N

How to explain that halving?

yogoes123
The TB answer is 316, my answer is 624.8, sorry I didnt make that clear.

Staff Emeritus
So it would be correct, no? I cant figure out what i did wrong.
As I emphasised "at a constant speed". At no time here is the vehicle travelling at a fixed speed, the vehicle is constantly slowing.

To begin over....

you need equations relating distance, initial velocity, final velocity (= 0 here) etc.

So, write down some suitable candidates.

yogoes123
I thought it starts to slow down after the 729m. Are you saying that is it slowing down within 729m?

Staff Emeritus
I thought it starts to slow down after the 729m. Are you saying that is it slowing down within 729m?
At the 729m mark, it is motionless. Where speed is constant it has acceleration = 0 (for straight line travel).

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yogoes123
Ok I think I got this, I used Vf^2=Vi^2+2ad

Then I did a= 18^2/2/729 and i got 0.2222222 then I used F=ma and got 316, thanks so much for clearing that up.

Staff Emeritus
Good. 