Could someone help w/ a problem

  • Thread starter btkale
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  • #1
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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
 

Answers and Replies

  • #2
Doc Al
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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?
 
  • #3
nrqed
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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
:confused::confused: What equations are you using?? I don't follow your reasoning of taking half the speed and so on...
 
  • #4
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Umm I'm not sure. Maybe I have it set up all wrong. I just need the breaking force in Newtons.
 
  • #5
Doc Al
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We know what you need, we're trying to understand what you did. :wink:

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?
 
  • #6
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It doesn't give the car's acceleration
 
  • #7
malawi_glenn
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F = m*a

find acceleration
 
  • #8
Doc Al
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It doesn't give the car's acceleration
But you have speed and time--everything you need to figure out the acceleration using kinematics.
 
  • #9
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if the car moves at 16 m/s the the acceleration would be constant right?
 
  • #10
malawi_glenn
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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.
 
  • #11
Doc Al
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You may assume that the acceleration is constant.
 
  • #12
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Ok I wrote out everything the problem has in it so how do i set it up to solve it?
 
  • #13
Doc Al
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Did you find the car's acceleration?
 
  • #14
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No could u tell me what it is or how to get it.
 
  • #15
malawi_glenn
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average a = delta(V)/delta(t) , i.e. how much the velocity changes during a certain interval of time.
 
  • #16
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so the acceleration would be -1.6
 
  • #17
malawi_glenn
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m/s^2 , always keep the units.

so the magnitude of the force is then?
 
  • #18
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so the acceleration is -1.6
 
  • #19
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wait what?
 
  • #20
malawi_glenn
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wait what?

what are you asking about?...

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?..
 
  • #21
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yeah thats right, so what do i do to set it up
 
  • #22
malawi_glenn
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I / we have alreade told you that...
 
  • #23
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Ok so I'm not that great in Physics so do ya mind drawing me a picture?
 
  • #24
malawi_glenn
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------- see post #7 in this thread. -----

it is the second law:

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
 

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