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Which is larger? the weight or the force?

  1. Jun 17, 2005 #1
    Hello, I just started physics about a month ago and I’m having a bit of trouble. It may seem silly but I just have a hard time wrapping my mind around the concept of comparing amount of weight to amount of force.
    So anyway my question is while you throw a ball upward, which is larger: the weight of the ball or your upward force on the ball?
    Thank you very much for any assistance you can offer.
     
  2. jcsd
  3. Jun 17, 2005 #2

    StatusX

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    The total force on the ball is its weight (mg) plus any other force on it. When the ball is in your hand as you throw it up, you apply an upward force greater than mg so that there is a net acceleration, giving it an upward velocity. As soon as you let go, the only force acting on it is gravity (ignoring air resistance, which just acts to slow it down whatever direction its going). The net force is down, and the ball accelerates downward until it loses its upward velocity and begins to fall back down to earth.
     
  4. Jun 17, 2005 #3
    Also, to clarify the weight of an object is a measure of the force of gravity acting on it.

    A 1lb object has mass 0.45kg

    The force of gravity on a 0.45 kg is 0.45*9.8 = 4.45N

    So you get the conversion factor for the force of gravity,

    1lb = 4.45N (only on Earth).

    An object always WEIGHS the same (again, on Earth) but the net FORCE acting on it is independant of that. For example, if I'm holding a 1lb object in my hand the net force on it is zero, however it doesnt weigh 0 lbs, it weighs 1lb.

    Cool?
     
  5. Jun 17, 2005 #4
    I think perhaps whozum did not mean independant in the sense that the weight has no bearing on the net force. For if this were the case, his arm would require 0 force to hold the mass in the air.

    I preume he meant that the net force of an object is not solely determined by the weight of an object. In your example, the ball is moving upwards, therefore the upwards force is greater, as it is OVERCOMING the gravitational force pulling the object down

    -NewScientist
     
  6. Jun 17, 2005 #5

    HallsofIvy

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    If you are simply holding the ball, at rest, the force you are applying to the ball is equal to the weight of the ball. Throwing it upward, you are giving the ball an acceleration so the force must be greater than the weight of the ball.
     
  7. Jun 17, 2005 #6
    Thank you HallsOfIvy for expressing my idea far better than I could!

    -NewScientist
     
  8. Jun 18, 2005 #7

    HallsofIvy

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    No, I'm just "simpler" than you are!
     
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