x Turn on thread page Beta
 You are Here: Home >< Physics

# Don't understand sin/cos concept in free-body diagrams!! watch

1. I understand trig and sohcahtoa (eg: sin(alpha) = opposite length/hypotenuse length) but not which one to use in which situation!!

Right, there is a slope with a trolley going up it, and that is attached to a pulley (using a light/massless string) with a bucket on the other end of the string that goes up and down depending on the trolley going down or up the slope respectively.

Anyway, I don't understand why for the trolley I should use these equations:

Sum of Fx = 0 (I get that bit) = T - W1sin15o

Sum of Fy = 0 = n - W1cos15o

Why not use cos in the first equation and sin in the 2nd? I don't get it! And why would sumFx = T - ... and sumFy = n - .... ????

I do not get it and cos and sin crop up so much in this mechanics unit and I love Physics but I'm missing some random bit of knowledge that will make this all make sense and make me not end up hating life! Okay, maybe exaggerating, but not really...if I fail an exam cos of cos or sin..ANYWAY.

Thank you!
2. (Original post by PhysicsGal)
I understand trig and sohcahtoa (eg: sin(alpha) = opposite length/hypotenuse length) but not which one to use in which situation!!

Right, there is a slope with a trolley going up it, and that is attached to a pulley (using a light/massless string) with a bucket on the other end of the string that goes up and down depending on the trolley going down or up the slope respectively.

Anyway, I don't understand why for the trolley I should use these equations:

Sum of Fx = 0 [SIZE=1](I get that bit) [SIZE=3]= T - W1sin15o

Sum of Fy = 0 = n - W1cos15o

[SUP]Why not use cos in the first equation and sin in the 2nd? I don't get it! And why would sumFx = T - ... and sumFy = n - .... ????

I do not get it and cos and sin crop up so much in this mechanics unit and I love Physics but I'm missing some random bit of knowledge that will make this all make sense and make me not end up hating life! Okay, maybe exaggerating, but not really...if I fail an exam cos of cos or sin..ANYWAY.

I don't think Fx necessarily has to be 0 - it won't be if the mass is accelerating down or up the slope.
Just to confirm: is n the normal force?

You use cos and sin because they're trigonometric identities, and what you effectively have is a triangle of forces. Hopefully this helps.

Basically, if you draw your incline with an angle alpha, the angle between the force of gravity (mg - the hypoteneuse) and the force perpendicular to the plane due to gravity will also be alpha (you can work this out just by filling in angles using basic geometry, explained here). So the opposite side from the angle (the force parallel to the incline) is given by , and the force perpendicular to the plane (adjacent to the angle) is given by

That's why you use sin and cos.
3. (Original post by Hype en Ecosse)
I don't think Fx necessarily has to be 0 - it won't be if the mass is accelerating down or up the slope.
Just to confirm: is n the normal force?

You use cos and sin because they're trigonometric identities, and what you effectively have is a triangle of forces. Hopefully this helps.

Basically, if you draw your incline with an angle alpha, the angle between the force of gravity (mg - the hypoteneuse) and the force perpendicular to the plane due to gravity will also be alpha (you can work this out just by filling in angles using basic geometry, explained here). So the opposite side from the angle (the force parallel to the incline) is given by , and the force perpendicular to the plane (adjacent to the angle) is given by

That's why you use sin and cos.
Ahhh thank you!! Khan Academy helped tonnes! Thanks

I have another qu if you don't mind!

If a = g(m1 - m2)/(m1 + m2)
why does T = g(2m1m2)/(m1 + m2) ??
4. (Original post by PhysicsGal)
Ahhh thank you!! Khan Academy helped tonnes! Thanks

I have another qu if you don't mind!

If a = g(m1 - m2)/(m1 + m2)
why does T = g(2m1m2)/(m1 + m2) ??
I'm afraid I'm not a mystic! You'll have to be specific and tell me what we're working with here. What's the question say, and what are all the objects given in the diagram (if there's one)?
5. (Original post by Hype en Ecosse)
I'm afraid I'm not a mystic! You'll have to be specific and tell me what we're working with here. What's the question say, and what are all the objects given in the diagram (if there's one)?
Sorry!

It's just turned midnight here, and after trial and error I seem to be figuring out answers and understanding concepts better and I figured out the qu I just posted re a and T, but I'm going to leave it for the night and sleep (as I have Uni in but 9 hours)!

Thanks for the help earlier though

Nightio

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 17, 2013
Today on TSR

### Loughborough better than Cambridge

Loughborough at number one

### Can I date a girl with no boobs?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE