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Homework Help: Help with simple force problem

  1. Oct 17, 2004 #1
    A 1000 kg car pushes a 2000 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 4500 N.

    What is the magnitude of the force of the car on the truck?

    Tried this:
    1000kg*9.81m/s^2 = 9810 N

    What is the magnitude of the force of the truck on the car?

    Tried this:
    2000kg * 9.81m/s^2 = 19620N

    Is wasn't right
    I need help
  2. jcsd
  3. Oct 17, 2004 #2
    Why have you taken 9.81 (g) as your acceleration. You already know the force the car exerts when the driver accelearates and the mass of the car this should allow you to get the correct acceleration and this should give you the right answer for the force required for the truck
  4. Oct 17, 2004 #3

    Doc Al

    User Avatar

    Staff: Mentor

    For some reason, you have calculated the weight of each vehicle. But that is not relevant for this problem.

    Start by using Newton's 2nd law to find the acceleration of the car + truck. Once you have the acceleration, apply Newton's 2nd law to the truck alone to find out the force that the car must be pushing on the truck.
  5. Oct 17, 2004 #4
    the acceleration of the car will be 4500N/1000kg =4.5m/s/s

    will the acceleration of the truck be 4500N/2000kg =2.25m/s/s
    if it is, will the force of the car on the truck be 2000kg*(2.25+4.5)=13500N
    and will it be the same as the force of the truck on the car.
  6. Oct 17, 2004 #5
    You're not accounting for the fact that the car has to push not only the car but also the truck too. And since the two objects are connected to each other, for the purposes of this problem, you can treat the car and truck, as one point object. The car has to use the force that the wheels generate to push both objects together.

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