math help!

[Omegachi]

so i am doing this CE problem on vectors... and i ended up with these two equations...

Fx= 600cos60i + Tsin(theta)i = 1200
Fy= 600sin60j + Tcos(theta)j = 0

... i guess my algebra is rusty.. how do i solve for T and theta?

to visualize the problem, its pretty much a upside down triangle with the two forces pulling upwards in two positive x angle, the resulting force is 1200lbs upwards...

please help

 

[dighn]

well you can solve for T using Fx, which will give you somenumber/sin(theta)
sub that inti Fy so you'll get 600sin60 + somenumber/tan(theta) = 0 which should be easy to solve, with theta solved you can get T

 

[Omegachi]

hmm... for some reason my theta comes out to be really small...

 

[dighn]

Originally posted by: Omegachi
hmm... for some reason my theta comes out to be really small...

make sure it's on deg not radian (since you seem to be using degrees). should be 120/-60 deg (probably the former)

 

[Omegachi]

its on degrees...

 

[Omegachi]

hold up, i'll show you the problem... maybe i made a mistake

 

[dighn]

i dunno what you are doing wrong but using the equatiions you gave me, i get 120 degrees for theta and 1000 something for T

 

[Omegachi]

The post is to be pulled out of the ground using two ropes A and B. Rope A is subjected to a force of 600lb and is directed at 60(degrees) from the horizontal. If the resultant force acting on the post is to be 1200lb, vertically upward, determine the force T in rope B and the corresponding angle (theta).

diagram

 

[dighn]

pic does'nt work man

 

[Omegachi]

try now

 

[Omegachi]

are my force vectors correct?

 

[dighn]

sorry i got confused earlier (mixed up 60 and theta... need sleep i do)

i think Fx= 600cos60i + Tsin(theta)i = 1200 should be Fx= 600cos60i - Tsin(theta)i = 1200

 

[Omegachi]

yea, i fixed that. that still doesn't work

 

[dighn]

ok i found out how i got confused... i somehow forgot about it wtf?!

anyway

Fx should be 0
Fy should be 1200

so the euqations should be

Fx= 600cos60i - Tsin(theta)i = 0
Fy= 600sin60j + Tcos(theta)j = 1200

 

[Omegachi]

oh... hahaha... i am an idiot.

 

[dighn]

haha don't sweat it. just choose a coordiante system and stick with it

np ( dont wanna make a new post hahaha)
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[Omegachi]

thanks dighn!

 

[DaFinn]

Wheres the poll... I vote "idiot"