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Netwons Third Law

  1. Mar 16, 2009 #1
    How can two objects of different mass accelerating at the same speed impart an equal an opposite force on each other when they collide?

    I understand Force to be equal to Mass times Acceleration. How can the Force and Acceleration be equal while the Masses are different?

    I'm probably confusing myself so I eagerly await your help. Thanks.
  2. jcsd
  3. Mar 16, 2009 #2
    I'm going to assume that by "accelerating at the same speed" you mean they are traveling the same constant speed and not actually accelerating.

    As a practical example, let's say an SUV is going 50 mi/hr and and a mini cooper is also going 50 mi/hr before they collide.

    You are right that the forces experienced by both would be the same, but their accelerations would be different. Since F=ma, they can't experience the same F and a when they have different m. Rearrange to a=F/m, and you see that the SUV experiences a smaller acceleration (deacceration in this case because the force is in the opposite direction of velocity) because the mass is larger.

    It might be easier to visualize if you consider momentum. Force is also defined as the change in momentum over time. The SUV will not change it's velocity so much because it has a higher momentum, but it's change in momentum will be the same because it has a high mass. The mini cooper doesnt have a high mass, but its change of momentum is just as high because it GREATLY changes its velocity (in fact it will start to go backwards).
  4. Mar 16, 2009 #3
    Got it. Thanks a ton.
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