# Homework Help: Forces help

1. Sep 9, 2008

### pat666

1. The problem statement, all variables and given/known data
A car of mass 2 (metric) tonnes is traveling at 20m/s. caculate its kinetic energy (can and have done) and the force required to stop it in 10 s.

2. Relevant equations

3. The attempt at a solution
I have found the KE = 4 *10^5 but i cant figure out how to calculate the force required to stop it in 10s. I think the equation for this mite be E/t=FV

The answer is 4*10^3N

2. Sep 9, 2008

### danago

To change it speed from 20m/s to 0m/s requires a deceleration of how much? What net force is required to produce this deceleration (what law relates net acceleration of a body to the net force)?

3. Sep 9, 2008

### pat666

so is it F=ma
that would give F=2000*-20
which = 4*10^4 not 10^3
also the force is positive but i suppose you just swap it because it is in the opposite direction to the original?? Also what then puts the 10s into the equation??

4. Sep 9, 2008

### danago

Yes use F=ma, but think about what value of 'a' you have used. a, acceleration, measures the rate at which speed is changing. If the speed changes from 20 to 0 over a period of 10 seconds, what is the acceleration?

5. Sep 9, 2008

### pat666

ok i think i get it now the acceleration should = 20-0/10
which = 2
so F= 2000*2
=4*10^3
thankyou so much
This website is ausome...

6. Sep 9, 2008

### danago

No problems And yes this website is the best haha.