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I need help with my Physics Homework

  1. Jun 29, 2009 #1
    I need help with my Physics Homework!!!

    A Car of mass 1000kg decelerates from 20m/s2 to rest in a time of 5 seconds. Calculate the braking force of the car, assuming that there is no friction.


    Ans:
    20 m/s2 / 5 seconds = 4m/s2
    Force= ma= 1000 * 4 = 4000N

    Can anyone explain this to me?
     
  2. jcsd
  3. Jun 29, 2009 #2
    Re: I need help with my Physics Homework!!!

    Welcome to PF. :)
    We're here to help you do your homework, not to do it for you. Please show your attempt at the solution first, or we won't help.

    First make sure you got all the relevant data written down right.
    Was the initial velocity of the car 20 m/s? (NOT m/s², those are the units of acceleration!)

    Data:
    Mcar = 1000 kg
    Vi = 20 m/s
    thalt = 5 s
    Vf = 0 (The car breaks to a halt)

    What formula do you know linking initial velocity, final velocity, and acceleration?
    What formula do you know linking acceleration, mass and force?
     
  4. Jun 29, 2009 #3
    Re: I need help with my Physics Homework!!!

    4ms is acceleration .. the formula is vt=vo-at (if decelerates) after that you can multyplie with car mass 1000 kg (f=m.a)
     
  5. Jun 30, 2009 #4
    Re: I need help with my Physics Homework!!!

    Yes, I know the answer is 4000N, and the solution is there, I got the answer wrong in my test paper, I nned to know why the correct answer is 4000N.

    The given solution was:
    20 m/s divided by 5 seconds = 4m/s2
    Force= ma= 1000 X 4 = 4000N

    I need someone to explain step-by-step...
     
  6. Jun 30, 2009 #5

    Pengwuino

    User Avatar
    Gold Member

    Re: I need help with my Physics Homework!!!

    Starting with Newton's 2nd Law: F = ma = mass * acceleration. We know the mass obviously. The acceleration, however, for a constant force, is the change in velocity divided by the change in time, that is:

    [tex]
    \frac{{\Delta v}}{{\Delta t}} = \frac{{20m/s}}{{5s}} = 4m/s^2
    [/tex]

    Now to find the force, it's simply:

    [tex]
    F = ma = 1000kg*4m/s^2 = 4000N
    [/tex]
     
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