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

Homework Equations

a=v/t f=ma

The Attempt at a Solution

t=40.5 a= 0.44

f=1420(0.44) f= 624.8

My answer is 624.8N

TB Answer: 316N what am i doing wrong?

 

[NascentOxygen]

Hi yogoes123. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

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

 

[yogoes123]

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

 

[NascentOxygen]

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 traveled at a constant speed.

 

[yogoes123]

So it would be correct, no? I can't figure out what i did wrong.

 

[NascentOxygen]

f=1420(0.44) f= 624.8

ans is: 316N


what am i doing wrong?

You [somehow] worked out a figure of 624.8N
then you decided to halve it, and call that your answer?

How to explain that halving?

 

[yogoes123]

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

 

[NascentOxygen]

So it would be correct, no? I can't figure out what i did wrong.

As I emphasised "at a constant speed". At no time here is the vehicle traveling 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?

 

[NascentOxygen]

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).

 

[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.

 

[NascentOxygen]

Good.