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posted by Maggie .
I have NO idea how to do this! any help is great!
A car with a mass of 1330kg is traveling at 28m/s. The driver applies the brakes to bring the car to a rest over a distance of 79m. Calculate the retarding force acting on the car.
From this i can see that it gives me the
Mass:1330kg
the inicial velocity: 28m/s
and the distance: 79m
i'm trying to find the force, but i can't think of a formula using the known variables!!

you can find the force doen through W=Fd, where W is work done,F is force applied, and D is the displacement(79m)
Work done also equlals change in kinetic energy, which is 521360J, sp the force applied is 6599.5 N. 
... we havn't even learned about Kinetic energy yet... my teacher must be losing it.... haha

You have to use three formulas for this one, I'm afraid.
First you need to find out how long it takes for the car to decelerate during those 79m.
Then you have to figure the acceleration, knowing the speed and the time.
And finally you have to translate that acceleration into a force.
For the first part you'll need this formula (to find out the time taken to stop):
Distance = (Final Speed + Initial Speed ) x Time/2
79 = 28 x T/2
T = 5.64s
For the second step, you have the following formula (to find the acceleration):
Final Speed = Initial Speed  Acceleration x Time
0 = 28  A x 5.64
A = 4.96m/s^2
Finally, you have the force formula:
Force = Mass x Acceleration
F = 1330 x 4.96
Force = 6596.8N
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