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Math Help - confused with this physics question

  1. #1
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    confused with this physics question

    A tennis ball is dropped from a height of 2.0 m and it rebounds to a height of 1.5 m . If the ball is in contact with the floor for 0.110 s , what is the acceleration of the ball when it is in contact with the floor ?

    I am a bit confused with the directions .

    For the downward motion , g=-9.81m/s^2 , s=-2.0m and u =0

    And when it rebounds which will be the upward motion , g=-9.81m/s^2 . s=1.5

    That is what the book says .

    For the downward motion , isnt the acceleration due to gravity supposed to be negative since it is in the direction of gravity and the displacement , s , why is it negative ??

    And for the upward motion , why is the displacement positive ??
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  2. #2
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    Quote Originally Posted by thereddevils View Post
    A tennis ball is dropped from a height of 2.0 m and it rebounds to a height of 1.5 m . If the ball is in contact with the floor for 0.110 s , what is the acceleration of the ball when it is in contact with the floor ?

    I am a bit confused with the directions .

    For the downward motion , g=-9.81m/s^2 , s=-2.0m and u =0

    And when it rebounds which will be the upward motion , g=-9.81m/s^2 . s=1.5

    That is what the book says .

    For the downward motion , isnt the acceleration due to gravity supposed to be negative since it is in the direction of gravity and the displacement , s , why is it negative ??

    And for the upward motion , why is the displacement positive ??
    You're dealing with vector quantities. Vectors have magnitude and direction. In straightline motion, the direction is specified by +ve or -ve. So you have to make a choice which direction is the +ve direction and which direction is the -ve direction. Your book has made the choice that down is negative and up is positive.
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  3. #3
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    Quote Originally Posted by mr fantastic View Post
    You're dealing with vector quantities. Vectors have magnitude and direction. In straightline motion, the direction is specified by +ve or -ve. So you have to make a choice which direction is the +ve direction and which direction is the -ve direction. Your book has made the choice that down is negative and up is positive.

    Ok .. so lets say i take downwards as the negative direction , do i say that the displacement , velocity and acceleration are all negative ?
    But i thought the acceleration should be positive as it is in the direction of the gravity ?

    But consider projectile motion , when an object is being projected at an angle from a building , lets say i take the upwards direction as positive , do i also say that the acceleration is -9.81m/s^2 throughout its motion ?
    Last edited by thereddevils; July 20th 2009 at 05:51 AM.
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  4. #4
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    Quote Originally Posted by thereddevils View Post
    A tennis ball is dropped from a height of 2.0 m and it rebounds to a height of 1.5 m . If the ball is in contact with the floor for 0.110 s , what is the acceleration of the ball when it is in contact with the floor ?

    I am a bit confused with the directions .

    For the downward motion , g=-9.81m/s^2 , s=-2.0m and u =0

    And when it rebounds which will be the upward motion , g=-9.81m/s^2 . s=1.5

    That is what the book says .

    For the downward motion , isnt the acceleration due to gravity supposed to be negative since it is in the direction of gravity and the displacement , s , why is it negative ??

    And for the upward motion , why is the displacement positive ??
    Choose the origin and direction you will call positive before you start, it does not matter what the choice is, but a nice choice will make everything easier.

    With this in mind and 350 years experience, take the point that the ball bounces as 0, and the upward direction as positive.

    Then the ball starts from s=2, with acceleration -g (that is downwards, as we have defined + as upwards)

    Now calculate the velocity when the ball hits the ground, and its velocity on rebound. The (avaerage) accelleration is the change in velocity divided by the time it takes.

    CB
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