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# Moments help watch

1. STRUGGLING WITH MOMENTS IN PHYSICS

A uniform metre rule of mass 80g is balanced on 2 knife edges place at the 10cm and 65cm marks
A block of metal of mass 100g is placed on the metre rule

Calculate upward force on the rule by each knife edge when the block is placed at the left hand end of the rule
(ii) at the 50cm mark
2. Let's call the point at 10cm A and 65cm B.

The centre of mass of the ruler is at 50cm, and the mass of the block is at i) the left and ii) the middle.

i) taking moment around point A, we have 100x0.1 to the left and 80x0.4 to the right, for a total of (32-10) to the right (=22)

65-10 = 55, 22/0.55 = 40 so B is supporting 40g. 180-40 = 140 so A is supporting 140g

ii) again taking moment around point A, we have 180x0.4 = 72 to the right.

72/0.55 = 131 (roughly) so B is supporting 131g and A is supporting 49g

If I've skipped over any steps you need to know feel free to ask.
Let's call the point at 10cm A and 65cm B.

The centre of mass of the ruler is at 50cm, and the mass of the block is at i) the left and ii) the middle.

i) taking moment around point A, we have 100x0.1 to the left and 80x0.4 to the right, for a total of (32-10) to the right (=22)

65-10 = 55, 22/0.55 = 40 so B is supporting 40g. 180-40 = 140 so A is supporting 140g

ii) again taking moment around point A, we have 180x0.4 = 72 to the right.

72/0.55 = 131 (roughly) so B is supporting 131g and A is supporting 49g

If I've skipped over any steps you need to know feel free to ask.
I got something similar to you but the answers say FA = 1.68N AND FB = 0.09N

ii. FA = 0.59N FB = 1.18N
4. Oh, my answers are in grams. If the solutions are in newtons then divide by 1000 and multiply by 9.8.
Oh, my answers are in grams. If the solutions are in newtons then divide by 1000 and multiply by 9.8.
ohhh thats what i did as well
i got so confused lol
do you know good physics a level resources online?
6. (Original post by AsianG99)
ohhh thats what i did as well
i got so confused lol
do you know good physics a level resources online?

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