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Two basic M1 dynamics questions i'm stuck on (+ve rep) watch

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    First one:

    A body of mass 5kg rests horizontally on a rough horizontal ground. It is being pushed with a force of 3N. The coefficient of friction between the body and the ground is 1/7.
    i) Find the magnitude of F.
    Finding Reaction Force:
    R - 5g = 5 x 0
    R = 5g

    Finding Friction:
    F = uR
    F = 1/7 x 49 = 7N


    ii) State whether the body will remain at rest or accelerate from rest along the ground.
    The body will remain at rest (because the friction force > pushing force) <--is that correct?




    Question 2:
    A small stone of mass 0.4kg is projected vertically upwards from the bottom of a pond full of water with speed 10m/s. As the stone moves through the water it experiences a constant resistance of magnitude 3N. Assuming that the stone does not reach the surface of the pond, find
    a) the greatest height above the bottom of the pond that the stone reaches
    u = 10
    v = 0
    t = ?
    a = ?

    I'm not sure which suvat formula to use here, any pointers?


    THANKS! +ve rep up for grabs
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    Your first ones are correct.

    For Question 2, try using f=ma to get the value of the acceleration. You are given f and m! Good luck.
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    First one is correct.
    Second one, is there a diagram or something showing a distance? If there is, v^2 = u^2 + 2as should work. If not, v = u + at having used F=ma to get acceleration should do.

    And your rep power is zero, btw.
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    for the second one do F = ma with F = -0.4g-3 and m = 0.4
    a = F/m = -9.8

    then v^2 = u^2 + 2as with v taken as 0 (when the particle reaches its max height it stops) gives
    0 = 100 + 2x-9.8xs
    100 = 19.6s
    s = 100/19.6 = 5.1m
 
 
 
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Updated: January 20, 2010
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