# a level physics-moments

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Thread starter 1 year ago
#1
could someone help me with this question, the answers are 0.51N at 100mm mark,0.69 at the 800mm mark
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1 year ago
#2
(Original post by student144)
could someone help me with this question, the answers are 0.51N at 100mm mark,0.69 at the 800mm mark
uniform ruler means the centre of mass is at 500mm

draw a diagram to help you and then take moments about the supports.
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Thread starter 1 year ago
#3
(Original post by Joinedup)
uniform ruler means the centre of mass is at 500mm

draw a diagram to help you and then take moments about the supports.
and how do you work that out? because i don have the mass of the two weights
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1 year ago
#4
(Original post by student144)
and how do you work that out? because i don have the mass of the two weights
I don't know why you think there are 2 weights... Did you post the right question?
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Thread starter 1 year ago
#5
(Original post by Joinedup)
I don't know why you think there are 2 weights... Did you post the right question?
sorry what I meant is don't you need the weight to work out the moments of the support idk
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1 year ago
#6
(Original post by student144)
sorry what I meant is don't you need the weight to work out the moments of the support idk
so the rod is uniform, meaning the weight of the ruler is acting from the centre. Draw a diagram with the two supports, and their forces (which go upwards). Then take moments and solve for the upward force from each support.

N.B. the total moments cw = 0 and you will need to take moments about one of the supports to eliminate one variable from the equation and make it solvable
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1 year ago
#7
because it states the ruler is uniform you know its weight is going to act through the centre (500mm). use this to set up a diagram of the moments representing the knives as reaction forces at 100mm and 200mm. you can then use the principle of moments to generate two equations where the two reaction forces (i call them R1 and R2) are unknown. rearrange the equations into the form R1= .... then equate them to eliminate R1. you can then solve for R2.
once you’ve found R2 you know the system is in equilibrium to therefore the sum of the forces acting down (weight of ruler) is equal to the sum of forces acting up (R1 and R2). this means R1 R2 = 1.2 therefore R1 =1.2 - R2.

sorry this was a mess of an explanation but i’ll attach a picture of my working to hopefully make it easier to understandAttachment 873804
Last edited by jprimr0se; 1 year ago
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