Question

A uniform 120-lb beam is held in a vertical position by a pin at its lower end and a cable at its upper end. A horizontal force (magnitude P) acts as shown in the figure. If P=75 lb, what is the tension in the cable?

a. 31 lb
b. 46 lb
c. 41 lb
d. 21 lb
e. 51 lb

Answer 1

Clockwise torque:
P * 4 ft
75*4 ft

Counter-clockwise torque:
T * 5 ft * sin(35)

Clockwise torque = Counter-clockwise torque:
75 * 4 ft = T * 5 ft * sin(35)

Is this going almost right?

Answer 2

You're almost there, but you need to reconsider where the cable tension is acting, which is to say its distance from the pin about which there should be zero moment.

Answer 3

So the 4ft should be switched with the 5 ft?

Answer 4

The beam pivots at the bottom, so what's the distance from the bottom to where the tension acts on the beam?

Answer 5

It's 9 feet. So would I have to replace the 5 with a 9?

Answer 6

Right. The force P is acting 4 ft from the pivot, while the tension force is acting 9ft from the pivot.

Answer 7

I'm getting 58.1149
So I guess E is the answer since it's the closest to it

Answer 8

You should have cos(35) instead of sin(35).

Answer 9

Oh Ok, thanks. I'm getting now C if I round up

Answer 10

That's correct.