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Really simple gravitational question

  1. Sep 30, 2010 #1
    I seem to be brain dead and I just want to make sure of something (no homework problem). If I am at a cliff and I let an object fall via free fall of course the gravitation force approx. 9.8 N will act on the object. If I give it an initial acceleration or exert a net force, the object will fall to the ground quicker than if I just let the object fall off my hand without that force (for example throwing it) correct? I just seem to be brain dead with the logic of this and it has been stuck in my head for the last few minutes...sorry for such a simple question
  2. jcsd
  3. Sep 30, 2010 #2


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    First, the force will only be 9.8N if the object has a mass of 1kg. Otherwise, the force will be different.

    Second, the acceleration will be the same no matter what you do. However, if you give it some initial downward velocity (such as throwing it), it will reach the ground faster.
  4. Sep 30, 2010 #3
    Since the object is weightless (if thrown down off a cliff) the force would be 9.8N m/s squared regardless wouldn't it? It should depend on the mass at that point...am I getting something confused here?

    Of course the acceleration would be the same; I guess the only different would be the initial downward velocity (such as throwing it) but all the other forces such as the force of gravity would be equal since the initial downward velocity (the throw) is an external nonzero net force....so away from that everything equals out but I am just confused as to your initial point as not taking into account air friction....a feather and a rock should fall at the same rate regardless of its mass...
  5. Sep 30, 2010 #4
    A Weightless object and a massless object are two different things. When you are standing in an elevator, your weight is the force exerted by the floor of the elevator upwards so that you don't sink into it. ie the force balancing the force of gravity acting downwards. your mass doesn't change whether you are in free fall or if the elevator is moving up or down, but your weight changes.

    The acceleration due to gravity is 9.8 m\s that means that the rock and feather will both experience the same change in velocity over the same period of time neglecting air resistance, but the force on the two are completely different.

    To make the rock change it's velocity at the same rate as the feather a much higher force is needed ( F=ma )

    The reason the acceleration is the same for both objects is that acceleration is inversely proportional to mass ( a=F\m) but the gravitational force of gravity is directly proportional to the mass!! ( F= -Gm1m2\r^2)

    so if the mass of one object is double that of the other, it gets accelerated only half as fast by an equal force on the two, but the gravitational force on it is double that of the lighter object.
  6. Sep 30, 2010 #5
    Thanks so much for the explanation. I was just a little confused as I have been studying so many equations in my Conceptuals Physics class lately. Thank you again for the explanation to clarify things for me!
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