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Trouble understanding newtons first law and f=ma

  1. Aug 22, 2010 #1
    In reading newtons first law,
    An object at rest or in motion will remain the same unless acted on by an outside force.

    also known and understood in newtons 2nd is
    Force = mass x acceleration.

    An object "A" moving at a constant velocity has no acceleration therefor no force.
    Object "B" is stationary. When object A hits object B, B will cease to be stationary.

    Where did the force to move object B come from if A has no force?
  2. jcsd
  3. Aug 22, 2010 #2
    Actually Newton's 2nd law is F=dp/t (change in momentum over time), so object A has mommentum, and loses some which is given to B.
  4. Aug 22, 2010 #3


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    Force isn't an inherent property of an object. That's your error - your statement that "An object 'A' moving at a constant velocity has no acceleration therefore no force" sounds like you are assuming that an object can have an inherent "force" that it can transmit to other objects.

    In your example, while object A is moving at a constant velocity, there are no forces acting on it, but when it hits object B, there is definitely a force that occurs at impact, which serves to both slow down object A and accelerate object B. The exact details of the collision are somewhat complicated, depending on the properties of each object, but there is definitely a force that occurs between them at impact.
  5. Aug 22, 2010 #4
    Your what I could not quite manifest in my mind. Thanks.

    Got it. Newtons laws are best explained in terms of momentum, some reason F = ma sticks out more in high school text. Force is not an inherent property.
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