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Homework Help: Not intuitive-friction causes motion?

  1. Oct 29, 2012 #1
    Not intuitive--friction causes motion?

    Hi. I'm having difficulty conceptualizing this situation:

    Consider two objects, mass 1 and mass 2. Mass 2 is on mass 1 and mass 1 is being pulled by a rope. There is a frictional force between mass 1 and mass 2, but none between mass 2 and the surface. Now my confusions/questions are as follows:

    How is the acceleration of mass 2 caused by the frictional force between the two objects? It is known that friction retards motion, so how does this force supposedly help object 2 accelerate? It would be appreciated if you can help me visualize/provide intuition for this situation (how friction causes acceleration, that is).
  2. jcsd
  3. Oct 29, 2012 #2


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    Re: Not intuitive--friction causes motion?

    Imagine standing on the bed of a truck with an unsecured load, just as it starts to accelerate. What will happen to the load? Now think about what the looks like from the ground.

    Mass 1 is the truck; mass 2 is the load. Does that help?
  4. Oct 29, 2012 #3
    Re: Not intuitive--friction causes motion?

    Well, the unsecured load will fall off the truck. So, to keep the the object on the accelerating truck, we would need friction. So, the friction keeps the object "accelerating" with the truck. I think what's confusing me is how it is accelerating; specifically, it is at rest on the truck but with respect to the ground, it is accelerating. So, is the free body diagram and acceleration with respect to the ground or the truck itself? Thanks in advance.
  5. Oct 29, 2012 #4


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    Re: Not intuitive--friction causes motion?

    Always with respect to the ground when using newton 2 and free body diagrams with real forces.
  6. Oct 30, 2012 #5
    Re: Not intuitive--friction causes motion?

    This is more of a relative velocity question. As you said, with respect to the bottom mass, the top mass is not moving. The idea is the same as you driving inside a car. The car is moving, and with respect to the car, you are not moving. However, with respect to the ground the car is moving, so you must be moving, since you are moving at the same speed as the car. Similarly, because the bottom mass moves, the top mass must also move with respect to the ground. Of course, there is no net force technically being applied on the top mass, so the actual net force on it is solely provided by the static friction, not the kinetic friction. There is no external force on the top mass, so the net force is solely given by the static friction. That should answer your question.
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