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Homework Help: Forces acting on a moving car

  1. Mar 22, 2014 #1
    1. The problem statement, all variables and given/known data
    When the car is at rest, normal reaction 1 and 2 act on the car's front wheel and rear wheel respectively . The problem is when the car starts to accelerates by rotating the wheels ,how would the normal reaction force change?

    2. Relevant equations

    3. The attempt at a solution
    I dont understand why the normal reaction force action on the rear wheel will increase after accelerating. I am much obliged if one can help me :)
  2. jcsd
  3. Mar 22, 2014 #2


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    Hello wai, and welcome to PF. Please help us helping you with working out number 2 and 3 a little more. That way you get more appropriate help, and it avoids you just getting an answer without really being helped or learning anything.

    In the case of the car you want to draw a free body diagram of an accelerating car. It doesn't leave the ground and it stays level, so there are several balances to be drawn up.

    Perhaps one hint: "normal reaction force action on the rear wheel will increase after accelerating" is wrong. That should be "during".

    Perhaps another one: ever heard of a "wheelie" ? Might have something to do with accelerating !

    Do you really need more hints ?
  4. Mar 22, 2014 #3
    sorry for the poor description haha.i know that the change in normal reaction has something to do with friction, and the moment produced by friction about the center of gravity, but i really cant get why the R2 will increase.
  5. Mar 22, 2014 #4


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    There are at least 2 ways to look at it...


    1) At what height does friction with the road act?
    At what height is the centre of mass?
    Draw a diagram.

    2) Which way do the wheels turn? Think about torque.
  6. Mar 22, 2014 #5
    i have one question to ask : if the car moves to the right,the wheels rotate clockwisely, so the friction action the wheels is on the right?
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