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Help? AS Physics vectors

I'm studying for my physics AS exams (yes, at midnight...) and I'm using the 'AS Physics Essential' OCR-approved book. There's an exercise in it (page 11 question 3 for those who have it) whose answer (at the back of the book) i'm not sure is right. I can't really show or explain the question without the picture, since its probably pretty important, so for those of you who have the book, could you please tell me why the book says that the vertical component is 6.1kN and the horizontal component is 2.2kN, rather than the other way round?

I'm hoping it's just an error in the book, rather than a misunderstanding on my part. Either way, I can't stop thinking about it and it's driving me crazy, so I need to know. :eek:

Reply 1

teachercol

9

I dont have a copy of the book with me - but there are several errors in the book that I know.

Reply 2

~*starlight*conquest*~

OP

11

Well, that's good enough for me! Thank you! ^^ My life can go on! =D

Reply 3

~*starlight*conquest*~

OP

11

Argh, here's another problem I can't figure out. This time i can write it out:

A uniform plank of length 5.0m rests horizontally on two bricks, which are 0.5m from either end. A child of weight 200N stands on one end of the plank and causes the other end to lift, so it is no longer supported at that end.

Calculate:

a) the weight of the plank

Answer at the back of the book says it's 50N, but i don't know how to work it out.

Reply 4

teachercol

9

Draw a diagram and mark all the forces acting.

Note - the plank will lift when the normal force on the brick at that end becomes zero.

Take moments about the other brick.

Reply 5

~*starlight*conquest*~

OP

11

It doesnt seem to work. could you go through it step by step?

Reply 6

teachercol

9

Take moments about the brick nearest the child. There is no force from the other brick when the plank lifts.

The weight acts in the middle which is 2.0 m from the brick .
The child is 0.5 m from the brick

W x 2.0 = 200 x 0.5

W = 50N

Reply 7

~*starlight*conquest*~

OP

11

Oh, I see! Thank you so much!

Reply 8

GMZOHAIB

3

Original post by teachercol

Take moments about the brick nearest the child. There is no force from the other brick when the plank lifts.

The weight acts in the middle which is 2.0 m from the brick .
The child is 0.5 m from the brick

W x 2.0 = 200 x 0.5

W = 50N

Why you put them equal to eachother??

Reply 9

Eimmanuel

Study Forum Helper

15

Original post by GMZOHAIB

Why you put them equal to eachother??

They are using the principle of the moment to solve the problem.
Sum of anticlockwise moments about a pivot = Sum of clockwise moments about a pivot
W x 2.0 = 200 x 0.5
You can review the concept using the following links.
https://www.youtube.com/watch?v=HOXGiaEThdg
http://www.excelatphysics.com/principle-of-moment.html

Please do not post on an old thread in future. If you need help, you can start a new thread and make reference to the old thread to ask your question.

Help? AS Physics vectors

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