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    moonjutt92's Avatar
    moonjutt92 Posts: 1, Reputation: 1
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    #1

    Jun 12, 2011, 04:02 AM
    Free fall motion
    Answers for A body freely falling from top of building and a body rolling down an inclined plane,both are released at same time.which will reach ground first?explain please
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Jun 12, 2011, 07:13 AM

    Use F = ma

    Both bodies are released at the same time at the same height. The only thing that will tell whether one reaches the ground before the other is the acceleration towards the ground, and that is dependent on the force they experience.

    What is the force acting vertically on the first body? And what is the force acing vertically on the second body?

    Post your what you think :)
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #3

    Jun 13, 2011, 06:53 AM

    Moonjutt92 - not knowing what level of physics you are studying, note that the question says that the body is "rolling" down the inclined plane. This implies that there is friction between the inclined plane and the body, and that the body has a moment of inertia that must be overcome for it to roll. If in you haven't yet studied moment of inertia for rotating bodies you can probably ignore this, and instead simply concentrate on the component of force due to gravity in the direction tangential to the inclined plane.
    elscarta's Avatar
    elscarta Posts: 118, Reputation: 20
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    #4

    Jun 13, 2011, 08:53 AM
    Given that the question states nothing about the shape of the body it is not possible to consider moments of inertia for rotating bodies. Also note that that the angle of the inclined plane is not given either so rather than trying to solve the problem with algebra all you need to do is consider what Conservation of Energy tells us about the situation.

    The body that falls freely to the ground converts all of its Potential Energy into translational Kinetic Energy while the body that rolls down the inclined plane converts some of its Potential Energy into rotational Kinetic Energy and the remainder into translational Kinetic Energy. This means that the rolling body will have a smaller velocity when it reaches the ground compared to the free falling body.

    A smaller velocity together with the fact that the inclined plane is longer that the vertical drop of the free falling body quite clearly tells us that the rolling body will take longer to reach the ground.

    NOTE: If the inclined plane was frictionless then the body would slide down the slope instead of rolling down it and so reach the bottom with all of its Potential Energy converted into translational Kinetic Energy. This means that it would have the same velocity at ground level as the free falling body, but it would still take longer to reach the ground as it needed to travel further than the free falling body.
    Ldog69's Avatar
    Ldog69 Posts: 9, Reputation: 1
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    #5

    Jul 1, 2011, 09:45 PM
    because one body is falling directly down.and da other down an inclined plane... da oder one down da incline plane is bound to have lesser accleration...
    a=g sin ( )... () contain da angle of incidence... and as sin is always less den 1 for inclination... it is bound to be less den g.
    hence da object falling from da building shall hit da ground first.

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