Help with a maths problem

peige

peige

peige

Soldato
Joined
18 Oct 2002
Posts
3,717
Location
Sussex
Trying to help with a maths question on an assigment my Son is doing, he's had several stabs at it but is struggling, his collage seem not interested in helping dispite being asked.
As a side issue I'm thinking about getting one of those skype type tutors to help as "lecturing" maths just doesn't seem to work.

Anyway, can you help with below? I'm good at SOHCAHTOA but this is beyond me :o

 
My maths is a bit rusty but from the looks of this you can solve this through a combination of extending the diagram and the cosine rule. Will attach an image with a solution in a few minutes

https://postimage.org/

Someone correct me if im wrong but that should work, obviously i've used 180 degrees in my example and i should have used radians so it should be Pi instead.

Edit: Just realised my trickery isn't the solution theyre after as they want to use both rules. Will look again
 
Last edited:
Nevermind, you can use the sine rule to work out the phase angle afterwards, didn't see this was required. Rule one of any question is to actually read the damn thing.

sinA/a = sinB/b

Where a = V2, A= phase angle, b = V3, B = Pi - phi(?)

Someone confirm this :p
 
He's studying Engineering level 3 which has a lump of A leveled maths stuck in for good measure. Just like me, enjoys the pratical and the relavent theory but struggles with some of the more abstract theory :) Years on I still use Sin/Cos/Tan to calculate CNC cordinates or missing features on drawings but some of this other stuff is hard to find relavent!

Reeve, top work! Thats sparked with the little he understood to get him going, he's now working the calculations out and drawing out the diagram :cool:

The question says he needs the use the Sin rule too, so now we are puzzelling over that!

/edit Ah, I see you've noticed that!
 
Used to do stuff like that in A level maths, but I've forgotten most of it. Was only four years ago. :D
 
Reeve said:
sinA/a = sinB/b

Where a = V2, A= phase angle, b = V3, B = Pi - phi(?)

Someone confirm this :p
Click to expand...

He has no idea what phi is so we don't think that can be part of the answer given that he's supposed to have been taught how to do it and doesn't recall phi at all :o
 
peige said:
He has no idea what phi is so we don't think that can be part of the answer given that he's supposed to have been taught how to do it and doesn't recall phi at all :o
Click to expand...

I thought that was the name of the greek symbol comprised of | and o. I wasn't sure of its name which is why i put the question afterwards.
 
Reeve got the cosine law bit and his drawing correct:

V3 = sqrt(V1^2 + v2^2 - 2*V1*V2*cos(180 - phi))

Then, call the angle between V2 and V3 theta and calculate it like this:

V1 / sin(theta) = V3 / sin(180 - phi), or after rearranging:

theta = sin^-1((V1 / V3) * sin(180 - phi))

Note that sin^-1 is the inverse sine function.

Plug in your numbers and off you go.
 
I don't have any notes to hand but this seems largely like a resolution of forces type question I did in foundation year for my ongoing mech eng degree...perhaps search for something similarly named and draw out what you need.

I think you calculate the sine/cosine parts of each 'force' and then do a sum of forces and so on.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom