A level math help.

mrwonderful

Pls help

OP

im supposed to express w in term of W. myoo is a sixth root 3. AB is in equilibrium. i took moments about A but I keep getting W cancelling out please help.
image-bb2b328e-c3b3-47e6-86b5-d9b81c59f98a7406129060654171906-compressed.jpg.jpeg

image-bb2b328e-c3b3-47e6-86b5-d9b81c59f98a7406129060654171906-compressed.jpg.jpeg

Reply 2

Notnek

21

Original post by mrwonderful

im supposed to express w in term of W. myoo is a sixth root 3. AB is in equilibrium. i took moments about A but I keep getting W cancelling out please help.
image-bb2b328e-c3b3-47e6-86b5-d9b81c59f98a7406129060654171906-compressed.jpg.jpeg

Please post the full question along with all your working which shows the problem you’re having.

Reply 3

mrwonderful

OP

10

image-af3e4377-43d2-4528-ba82-2fdb29e1fc714328541808941613134-compressed.jpg

Original post by Notnek

Please post the full question along with all your working which shows the problem you’re having.

Attachment not found

its question 9

(edited 4 years ago)

20190505_164451.jpg164.8KB

15570712017525868020604154958401.jpg175.3KB

Reply 4

Notnek

21

Original post by mrwonderful

Attachment not found

its question 9

I can have a look in around 15 mins

Reply 5

Notnek

21

Original post by mrwonderful

Attachment not found

its question 9

Can you please post your diagram again? I can't see it in your first post anymore.

Reply 6

mrwonderful

OP

10

Original post by Notnek

Can you please post your diagram again? I can't see it in your first post anymore.

Original post by Notnek

Can you please post your diagram again? I can't see it in your first post anymore.

image-64cd2270-5f1c-4f63-b09c-574d02bcee1e6991449709828093479-compressed.jpg.jpeg

Reply 7

Notnek

21

Original post by mrwonderful

x

There appear to be two w's in your working. One is the weight of the ladder but what's the other one?

Reply 8

mrwonderful

OP

10

Original post by Notnek

There appear to be two w's in your working. One is the weight of the ladder but what's the other one?

part c of the question says a load of weight w ia applied at B. The weight of the ladder is W. and i am to express w in terms of Big W

Reply 9

Quirky Object

20

Original post by mrwonderful

part c of the question says a load of weight w ia applied at B. The weight of the ladder is W. and i am to express w in terms of Big W

If you're answering part (a) then don't use any information that isn't given until part (b)/(c).

The wall is smooth and in part (a) there is no weight hanging at B, so there is only one force acting at B. The force you're looking for is the normal reaction of the wall on the ladder, so you don't need to know what

\mu

is (just as well because you're never given a value for it).

Reply 10

mrwonderful

OP

10

Original post by Quirky Object

If you're answering part (a) then don't use any information that isn't given until part (b)/(c).

The wall is smooth and in part (a) there is no weight hanging at B, so there is only one force acting at B. The force you're looking for is the normal reaction of the wall on the ladder, so you don't need to know what

\mu

is (just as well because you're never given a value for it).

i got part A and B correct, i am just struggling a bit with part c.

Reply 11

Notnek

21

Original post by mrwonderful

i got part A and B correct, i am just struggling a bit with part c.

Try taking moments about B instead. Post your working if you're still stuck.

Reply 12

Quirky Object

20

Original post by mrwonderful

i got part A and B correct, i am just struggling a bit with part c.

Oh sorry, part c wasn't in the picture you posted so I assumed you were just overcomplicating part a :lol:

can you post part c?

Reply 13

mrwonderful

OP

10

Original post by Quirky Object

Oh sorry, part c wasn't in the picture you posted so I assumed you were just overcomplicating part a :lol:

can you post part c?

Reply 14

mrwonderful

OP

10

Original post by Notnek

Try taking moments about B instead. Post your working if you're still stuck.

ok ill try that but why doesnt it work with moments about A?

Reply 15

Notnek

21

Original post by mrwonderful

ok ill try that but why doesnt it work with moments about A?

EDIT: After actually trying the question I've realised that it's impossible wherever you take moments :smile:

You have to think carefully about what variables you have and what kind of equation you will arrive at if you take moments. You have

R_B = \frac{\sqrt{3}}{6}(W+w)

So if you want an equation just involving

W

and

w

then you're going to need to eliminate

R_B

.

If you take moments amount A then that's going to give you an equation involving

R_B

,

W

and

w

but you already have an equation with those variables so this isn't going to help.

If instead you take moments about B then you'll get an equation involving

W

and

R_B

(since

R_A

can be written in terms of

R_B

). So then you'll be able to write

R_B

in terms of

W

and eliminate it from the above equation.

(edited 4 years ago)

Reply 16

Notnek

21

@mrwonderful I recommend that you don't use the app when posting in the maths forum mainly because maths formatting doesn't show up properly (you may notice that in my post above) and attachments are very buggy. Use the mobile site instead if you're using your phone.

Reply 17

mrwonderful

OP

10

Original post by Notnek

Try taking moments about B instead. Post your working if you're still stuck.

image-0d76a963-9bf6-4ac2-8017-b2ac315abaec4955006305236479466-compressed.jpg.jpeg

ok so i took moments about B and Big W cancelled out!

Reply 18

Notnek

21

Original post by mrwonderful

X

After trying the question myself I've realised that it's impossible! Then I checked my (updated) version of the textbook and realised that they have corrected a typo. Here's the new version of the question:

\mu

should be

\frac{\sqrt{3}}{5}

.

Thanks Pearson for wasting both of our times :grumble:

By the way for future questions please try to post everything in a single post. It was tricky looking through all your posts to find all the information as I looked through your working.

Reply 19

mrwonderful

OP

10

Original post by Notnek

After trying the question myself I've realised that it's impossible! Then I checked my (updated) version of the textbook and realised that they have corrected a typo. Here's the new version of the question:

\mu

should be

\frac{\sqrt{3}}{5}

.

Thanks Pearson for wasting both of our times :grumble:

By the way for future questions please try to post everything in a single post. It was tricky looking through all your posts to find all the information as I looked through your working.

i knew it! it just wasnt working.with the myoo value given. I now got w is equal to a quarter big W

A level math help.

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