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Resolving forces - OCR M1 Questions

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    Hi guys,

    I'm struggling to see where I'm going wrong with these questions, if any one could help I'd be thankful:

    1) A shopper pushes a supermarket trolley in a straight line towards her car with a force of magnitude of 20N, directed downwards at an angle of 15 degrees to the horizontal. Given that the acceleration of the trolley is 2.4ms^2, calculate its mass. Also find the magnitude of the normal contact force exerted on the trolley by the ground.

    - I found the mass to be 8.05kg (3 s.f) which is correct however when it came to finding the normal contact force I was confused. I did R+20sin15 = 8.05(9.8), which led me to the wrong answer - can anybody explain why?


    2) A child's toy of mass 5kg is pulled along level ground by a string inclined at 30 degrees to the horizontal. Denoting tension in the string by TN find, in terms of T, an expression for the normal contact force and deduce that T cannot exceed 98.

    I got this right, but think it was more down to guess work than anything else -if somebody could post their method for it, it would really help.

    Thanks in advance.
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    (Original post by Choppyy)
    Hi guys,

    I'm struggling to see where I'm going wrong with these questions, if any one could help I'd be thankful:

    1) A shopper pushes a supermarket trolley in a straight line towards her car with a force of magnitude of 20N, directed downwards at an angle of 15 degrees to the horizontal. Given that the acceleration of the trolley is 2.4ms^2, calculate its mass. Also find the magnitude of the normal contact force exerted on the trolley by the ground.

    - I found the mass to be 8.05kg (3 s.f) which is correct however when it came to finding the normal contact force I was confused. I did R+20sin15 = 8.05(9.8), which led me to the wrong answer - can anybody explain why?
    I don't know what your reasoning is for arriving at that equation, however:

    Sum of vertical forces is zero, since the trolley isn't going into the ground or flying up into the air.

    So magnitude of downwards forces = magnitude of upward forces.

    Viz. mg + 20 sin15 = R
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    (Original post by ghostwalker)
    I don't know what your reasoning is for arriving at that equation, however:

    Sum of vertical forces is zero, since the trolley isn't going into the ground or flying up into the air.

    So magnitude of downwards forces = magnitude of upward forces.

    Viz. mg + 20 sin15 = R
    I did it as I didn't realise the force was acting downwards - cheers, don't need any more help with those now!

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