[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?


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.

[ghostwalker]

(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

[Choppyy]

(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!