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Please check over

  1. Nov 19, 2006 #1
    When you step on the accelerator to increase the speed of your car, the force that accelerates the car is, the force friction of the road on the tires, the force of your foot on the accelerator, the force of the engine on the drive shaft, or the normal force of the road on the tires.

    When the car begins to accelerate, new forces come into play. The rear wheels exert a force against the ground in a horizontal direction; this makes the car start to accelerate. When the car is moving slowly, almost all of the force goes into accelerating the car. This is why I think the answer is the force friction of the road on the tires because the car is exerting a force in the horizontal direction against the ground.


    Sphere X, of mass 2kg is moving to the right at 10m/s. Sphere Y, of mass 4 kg, is moving to the left at 10m/s. The two spheres collide head on. The magnitude of the impulse of X on Y is: twice the magnitude of impulse of Y on X, 1/2 the magnitude of Y on X, 1/4 the magnitude of impulse, four times the magnitude of impulse of Y on X, or the same magnitude of impulse of Y on X.


    I take it that it would be the same magnitude of impulse of X on Y as Y on X--> due to P(before) =P(after)


    Thank you :biggrin:
     
  2. jcsd
  3. Nov 19, 2006 #2

    PhanthomJay

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    I agree with your answers, but your reasoning is not quite correct. In part 1, consider that the car might have front wheel drive. Or 4WD. And is it not the force of the ground on the car that accelerates it ,as a consequence of Newton III? Same for part 2, think Newton III.
     
  4. Nov 19, 2006 #3
    Assume the car wheels rest on a horizontal surface and rotate without slipping for simplifcation.

    When a tire rotates on a surface (without slipping), the point of the tire that is in contact with the surface remains stationary (otherwise it would slip). The point wants to move one direction, but something prevents it from doing so? Some force must be present. Static friction enables the point to be stationary (otherwise the car would slip and we would have kinetic friction). The direction of the static friction points in the opposite direction of the tangential velocity of the point in contact with the road.

    PhantomJay,

    While it true there is a normal force acting on the car, the normal force itself will not cause the car to accelerate on a horizontal surface as it points perpendicular to the direction of motion; however, the frictional force (static) points along a line parallel to motion and contributes to a non-zero net force and thus a net acceleration. The frictional force is proportional to the magnitude of the normal force, it not not proportional to the normal force vector.

    If the frictional force were proportional to the normal force vector, you could mathematically show that friction can't do work!! A mistake I once assumed...

    You could also argue that in an isolated system of the car and the earth, the car will not accelerate without any of the given options.
     
    Last edited: Nov 19, 2006
  5. Nov 19, 2006 #4

    PhanthomJay

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    I didn't mean to imply that the normal force of the ground on the car accelerates the car forward. I meant to say that the horizontal force of the ground on the car (that is, the friction force), accelerates it. Newton III in its finest hour.
     
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