# Could someone help w/ a problem

Hey I thought i had this figured out but when i submitted it it was wrong.

A car with a mass of 1150 kg moves at 16 m/s. What braking force is needed to bring the car to a halt in 10s?

I set it up as
KE=1/2mv^2
=1/2(1150)(16)^2
=147200J

then v/2= 16/2=8

force= 147200/8
=18400 N

Could someone tell me what I did wrong?
Thanks

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Doc Al
Mentor
I set it up as
KE=1/2mv^2
=1/2(1150)(16)^2
=147200J
You found the energy.

then v/2= 16/2=8
And then the average speed.

force= 147200/8
=18400 N
Huh? Why would you divide KE by average speed?

Maybe you are thinking of:
Power = Force * Speed?

If so, what is the average power required?

nrqed
Homework Helper
Gold Member
Hey I thought i had this figured out but when i submitted it it was wrong.

A car with a mass of 1150 kg moves at 16 m/s. What braking force is needed to bring the car to a halt in 10s?

I set it up as
KE=1/2mv^2
=1/2(1150)(16)^2
=147200J

then v/2= 16/2=8

force= 147200/8
=18400 N

Could someone tell me what I did wrong?
Thanks  What equations are you using?? I don't follow your reasoning of taking half the speed and so on...

Umm I'm not sure. Maybe I have it set up all wrong. I just need the breaking force in Newtons.

Doc Al
Mentor
We know what you need, we're trying to understand what you did. If you weren't thinking of power, you might wish to consider how much work is required to stop the car.

Or you can use kinematics: What's the car's acceleration?

It doesn't give the car's acceleration

malawi_glenn
Homework Helper
F = m*a

find acceleration

Doc Al
Mentor
It doesn't give the car's acceleration
But you have speed and time--everything you need to figure out the acceleration using kinematics.

if the car moves at 16 m/s the the acceleration would be constant right?

malawi_glenn
Homework Helper
if the car moves at 16 m/s the the acceleration would be constant right?
but the car shall stop in 10s.. hence an acceleration is done.

Doc Al
Mentor
You may assume that the acceleration is constant.

Ok I wrote out everything the problem has in it so how do i set it up to solve it?

Doc Al
Mentor
Did you find the car's acceleration?

No could u tell me what it is or how to get it.

malawi_glenn
Homework Helper
average a = delta(V)/delta(t) , i.e. how much the velocity changes during a certain interval of time.

so the acceleration would be -1.6

malawi_glenn
Homework Helper
m/s^2 , always keep the units.

so the magnitude of the force is then?

so the acceleration is -1.6

wait what?

malawi_glenn
Homework Helper
wait what?

you can delete your double post by pressing the "edit" button then choose "delete post".

The question was what force is needed to stop the car, right?..

yeah thats right, so what do i do to set it up

malawi_glenn
Homework Helper
I / we have alreade told you that...

Ok so I'm not that great in Physics so do ya mind drawing me a picture?

malawi_glenn