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physics p2 help!

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01 october QS 3(c) n 8(b)
02 may QS 3(b)(iii) and 4(d)why in downward direction???
 
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W01:
Q3 [c]
(i) The weight of the lid acts at the center of gravity that is C, and it acts directly downwards.
(ii) Tension always acts OUTWARDS from the point, hence it'll be towards the left on the cord.
(iii) The force of the hinge on the lid is the REACTION to the weight, it'll act directly upwards at the point E.

I'm not sure about Q8 :)

J02:

Q3
(iii) All you need to understand is that the weight of the rod acts at the center of gravity. Now, you can take moments about ANY point, e.g A (like in the mark scheme) or even about X directly.

Taking moments about X (left corner of the rod):

950*(x) = 570*(0.4) + 380*(2.1)
x = 108cm = 1.08m

Q4 [d]
In [c], you calculated the change in momentum and it came to be positive while taking the upwards direction of velocity as positive. Hence, momentum is acting upwards in the ball. However, momentum /must/ be conserved in the system. Hence, to counter the upwards momentum of the ball, the plate exerts a downwards momentum.
 
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