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addiaz2714087
Joined: 09 Mar 2010
Posts: 9
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 Posted: Sun May 02, 2010 4:35 pm Post subject: |
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| mgsin(30)=50N is the force downwards, and 20N is the force upwards....so how is the block moving upwards? Shouldnt there be a 30N force moving the block downwards? |
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gmosud2257
Joined: 29 Apr 2010
Posts: 1
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| Posted: Fri May 07, 2010 9:24 pm Post subject: |
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| yup I was wondering the same thing |
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mcat_premed3832
Joined: 19 Oct 2006
Posts: 413
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| Posted: Sat May 08, 2010 12:00 am Post subject: |
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| Quote: | | mgsin(30)=50N is the force downwards, |
Yes, so that means that if there is no force to counteract the force downwards, then the block would certainly accelerate downwards. If you wanted the block not to accelerate downwards, then you would have to push upwards (i.e. parallel to the plane) with exactly a force of 50 N. IF you push upwards with any force greater than 50 N (even 50.000000001 N) then the block MUST accelerate upwards according to F = ma (Newton's Second Law which states that if there is a net force then there must be acceleration).
So the total force must be the 50 N (to counteract the downward force) PLUS the force calculated to accelerate the block by 2 m/s^2 (which is 20 N) = a total of 70 N.
Please let me know if the above is not clear.
Going deeper: a counteractive force of 50 N does not necessarily mean that the "block is not moving" but rather that the block is in an inertial frame of reference. This means that either the block is not moving or it is moving with constant velocity. Either way, in accordance with Newton's Second Law, if there is no net force, the block cannot accelerate. |
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