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Homework Help: True or False:Gravitation & Circular Motion Concepts

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    Give reasons why you regard each of the following statements as true or false. Include appropriate equations and explanations using underlying basic physics concepts and laws.
    a) The condition for uniform circular motion of any object is that its velocity and speed must be constant, its acceleration must be directed towards the center of the circle and the force on the object must be tangent to the circle.
    b) you throw a ball straight up. Neglecting air resistance, the resultant force acting on the ball is zero during the throwing process and also at the maximum height.
    c)a spaceship is traveling from the earth to the moon. At the midpoint of the distance from the center of the earth to the center of the moon, the gavitational force exerted by the earth on the spaceship will be equal in magnitude to the gravitational force exerted by the moon on the spaceship.

    2. Relevant equations
    for a): F_c = m(v^2/r) where a_c = v^2/r
    for b) i assume the resultant force means the net force, which i dont think is zero during the throwing process, because there is acceleration. at the top, there is also a resultant force, because even though the veloc. is zero at the top, the ball is still accelerating. so, i think it would be: Fnet= Fg + Fa does not equal zero.
    for c) the gravitational force will not be equal in magnitude at the midpoint of the distance--the spaceship would have to be closer to the moon to have the same gravitational force exerted on it by the moon as the grav. force exerted by the earth (because the moon is less massive, it has less gravitational pull). im not quite sure how to make an equation for this situation but i think it should be a ratio.

    3. The attempt at a solution
    see above.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 6, 2009 #2
    oh, a) false because velocity is never constant for circular motion, and also the force is towards the center of the circle, like acceleration.
    b) false
    c) false
  4. Apr 6, 2009 #3


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    a) is correct except so long as you are understanding ... |V| is constant in a circular motion. The direction changes of course.

    b) seems that it would be sufficient to say that g is constant throughout and the ball is subject to m*g at all times.

    c) seems that saying the mass of the earth and moon are unequal, means that the distance of net 0 pull must be unequal from each.
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