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# Resolving forces - OCR M1 Questions

1. 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.

2. (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
3. (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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