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# Physics AS Level Mechanics - Help with an exam question watch

1. "A uniform metre of weight 1.2N rests horizontally on two knife-edges at the 100mm mark and 800mm mark. Sketch the arrangement and calculate the force due to each knife edge."

I cannot figure this out for the life of me, and have been to my teacher for more or less every question so far in the booklet - so im too embarrassed to admit i still cant do them to her. Can someone help?
2. (Original post by Lucie J)
"A uniform metre of weight 1.2N rests horizontally on two knife-edges at the 100mm mark and 800mm mark. Sketch the arrangement and calculate the force due to each knife edge."

I cannot figure this out for the life of me, and have been to my teacher for more or less every question so far in the booklet - so im too embarrassed to admit i still cant do them to her. Can someone help?
I guess it's a uniform meter ruler...
you can say a few things already, the CoM will be at 500mm
The vertical components of the contact force at both contact points will add up to 1.2N (because it is at rest)
Have you tried taking moments about one of the knife/ruler contact points yet?
3. Yeah, i tried taking the moments, but i don't know how
4. (Original post by Lucie J)
"A uniform metre of weight 1.2N rests horizontally on two knife-edges at the 100mm mark and 800mm mark. Sketch the arrangement and calculate the force due to each knife edge."

I cannot figure this out for the life of me, and have been to my teacher for more or less every question so far in the booklet - so im too embarrassed to admit i still cant do them to her. Can someone help?
Remember that if you have two unknowns in a moments question, always take the moment about one of them because at that point the moment is 0, eliminating one of the unknowns, enabling you to work out the other unknown.
5. (Original post by Lucie J)
Yeah, i tried taking the moments, but i don't know how
Moment = force x perpendicular distance from the line of action to the pivot

the sum of the anticlockwise moments = the sum of the clockwise moments

Set up an equation with one unknown in order to work it out
6. draw a diagram, list the magnitude and direction of the forces acting as an arrow

list your formulas, find one that you think will work (if you don't know use all), rearrange equation, find out how many variables- if more than one then more than one equation might be needed, plug in what you have and then get back to me if you find the answer

asking here on TSR is a short term solution, if you have to spend 3 hours on this question- then do- take a break, take a walk and comeback then try it with a fresh mind

read over whole chapter carefully again then try
7. (Original post by Lucie J)
"A uniform metre of weight 1.2N rests horizontally on two knife-edges at the 100mm mark and 800mm mark. Sketch the arrangement and calculate the force due to each knife edge."

I cannot figure this out for the life of me, and have been to my teacher for more or less every question so far in the booklet - so im too embarrassed to admit i still cant do them to her. Can someone help?
Firstly, don't be embarrassed about not understanding it. That's fine - A Level physics is hard - that goes for A Levels in general! Teachers are there to help you out and I'm sure yours has no problem with it.

Okay so you want to consider moments about the centre of mass of the ruler. This will be at 500 mm because it is a uniform ruler so the mass is evenly distributed and it is half way across. The first thing to do is draw a diagram.

Anticlockwise moments =
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Perpendicular distance x force: (0.500-0.100)*f1 =0.4f1 (f1=force from knife 1)

Clockwise moments =
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Perpendicular distance x force: (0.800-0.500)*f2 =0.3f2 (f2=force from knife 2)

Clockwise moments = anticlockwise moments as this system is at equilibrium.

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Therefore 0.4f1=0.3f2.

Also, you can resolve forces in general. Consider the vertical forces acting on the ruler due to its weight and the knives.
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R(up) f1+f2=1.2N

You can solve with simultaneous equations.

Note: the directions I have given are entirely subjective.

Hope this helps.

EDIT: I completely agree with others on this thread - TSR isn't the best solution. Hopefully this explains it a bit and helps you on your way to understanding, but if not, don't be afraid to ask your teacher. There is absolutely nothing to be embarrassed about. There are also loads of good YouTube videos for AS Physics. Also don't be afraid to leave something if it is not working and come back to it. Going round in circles for hours won't do you any good.

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Updated: November 11, 2017
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