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    hannahphysics's Avatar
    hannahphysics Posts: 2, Reputation: 1
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    #1

    Oct 13, 2011, 12:46 AM
    forces and velocity
    I'm having trouble with an assignment question which is due this coming Monday 17th October '11.
    I've worked out my free body diagram and cofficient of friction easily enough, but now the questions asks for the final velocity after 8 seconds if a force of 441.4 Newtons was acting on it pulling it along a flat surface.

    initial velocity obviously = 0
    time = 8 sec
    force acting on object = 441.4 N

    final velocity = ?

    I've been through my equations of motion, and they all require either a displacement or an accelaration and we are given neither.

    ay help would be great :)
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Oct 13, 2011, 01:39 AM
    I don't have the complete question, so I can't tell for sure, but if I understood it, it must be a question like that:

    "Two bodies with known mass are tied together with a light inextensible string, passing over a smooth pulley."

    sort of question. From that, you can use F = ma to get an acceleration and thus, a velocity.

    But as said before, I can't say for sure. Please, do come back and confirm, or explain more about the question :)
    hannahphysics's Avatar
    hannahphysics Posts: 2, Reputation: 1
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    #3

    Oct 13, 2011, 05:22 PM
    Not quite. I'll write the question as it appears on the assignment.

    A horizontal force of 900N is applied to a 260kg box initially at rest on a rough horizontal surface where te coefficient of friction with the box is 0.18.
    a/ draw a free body diagram showing all the forces acting on this box, including its weight and the normal reaction force.
    b/ if this force acts for a total of 8 seconds, what will be the boxes final velocity?
    c/ the same surface mentioned above is now inclined at an angle of 16 degrees. What will be the acelration of the box dow the slope if it slides with no external force being applied?

    OK so that's the whole question. At the moment I'm kind of stuck on part b, but looking at it, part c looks like a bit of a challenge as well.


    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #4

    Oct 13, 2011, 10:42 PM
    Ahh, it's even easier! :p

    Okay, can you find the net force acting on the box, parallel to the plane?

    If you can get that, you can get the acceleration from F = ma

    Remember that here, the net force is given by:



    If you're not sure about something, feel free to ask :)
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