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Some Newton's Law Questions.

  1. Oct 15, 2006 #1
    I am having trouble with some of these questions:

    1. A ball is held in a person's hand. a) Identify all the external forces acting on the ball and the reaction to each. b)If the ball is dropped, what force is exerted on it while it is failling? Identify the reaction force in this case. (Neglect air resistance)

    My Work:
    a) actions:
    downward gravity force exerted onto the ball
    upward normal force exterted by the hand on the ball

    upward gravity force exerted by the ball??
    I think this is because the ball is on the hand while the gravitational force is acting upon it thereforce have an opposite upward gravitational force to make the net force zero?
    downward force exerted by the ball onto the hand

    b) If the ball is dropped, the only force exerted on it while falling would be the downward gravitational force. The reaction would be the upward gravitational force exerted by the ball on Earth. Why is this? If the force acting upon the ball while failling is the gravitational force, and there is no air friction, why would there be an upward gravitational force? Wouldn't the reaction be the acceleration of gravity(-9.8m/s^2)??
  2. jcsd
  3. Oct 15, 2006 #2
    Also this one:

    2. If a car is traveling westward with a constant speed of 20m/s, what is the resultant force acting on it??

    Would the resultant forces be air resistance and road friction??
  4. Oct 15, 2006 #3


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    There is no such thing as an 'upward gravitational force'. The ball is falling, the gravitational force acting on the ball is constant, and it causes constant gravitational acceleration. It's as simple as that.

    Regarding the first case, the action is the gravitational force, and the reaction is the force exerted on the ball by the person's hand.
  5. Oct 15, 2006 #4


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    Homework Helper

    Read Newton's second law more carefully. Constant velocity implies that the resultant force equals zero, if you ignore air resistance, friction, etc.
  6. Oct 15, 2006 #5


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    Staff: Mentor

    Friction between road and tires allows the propulsion system (motor, transmission, wheels/tires) to propel the car viz a viz traction.

    Air resistance is a factor, by virtue of the car velocity. A mass traveling through a fluid, either gas or liquid, will loose momentum via collisions with the fluid molecules.

    Gravity always pulls/acts downward, and friction opposes motion.

    See -



  7. Oct 15, 2006 #6
    for problem number 1, the reactions answers are:

    a) an upward gravitational force exerted by the ball on Earth.

    b) The reactions is an upward gravitatinal force exerted by the ball on Earth.

    The upward gravitational force for a) is because the hand is supporting the downward gravitational force(or weight) of the ball right??

    What about the reactions for b)?? How does it have an upward gravitational force? I don't get it.
  8. Oct 15, 2006 #7
    So for the resultant force acting on the car traveling at a constant speed would be air resistance and the ground friction?
  9. Oct 15, 2006 #8
    Another Problem that I'm not sure of:

    A rubber call is dropped onto the floor. What forces causes the ball to bounce?

    Are the forces that cause it to bounce the downward gravitational pull and the reaction of the drop which is when the ground hits the ball??
    What is the correct way to say "the force that the ground exerts on the ball, or when the ground "hits" the ball, causing it to bounce??"
  10. Oct 15, 2006 #9


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  11. Oct 15, 2006 #10
    thanks for the link, it really helped! :smile:
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