HELP!!!

[Jordan Premo]

I've got a problem here that I can't solve, and I have an exam in the
morning.. it's the only one I can't get, or even start.

This is from the sample final they had us try out:

A farmyard gate 4m long and 2m high weighs 600N. It's centre of gravity
is at its centre and is supported by hinges at A and B. To relieve the
strain on the top hinge a cable CD is connected at an angle of 30 degrees
as shown. The tension in the cable is such that the horizontal force at
hinge A is equal to zero.

a) Determine the tension in the cable.

b) Determine the magnitude of the horizontal component of the force on
hinge B.

c) Determine the combined vertical force exerted on two hinges.

d) If the cable were not in place, what would be the magnitude of the
horizontal component of the force on hinge A?

And here's a mockup of the diagram:

|D
|\
| \
| \
| \
| 30 \ C
-------
|A |
| | 2m
| |
-------
B 4m

I really have no idea what to do... please help!

[Mu-Pi]

Post by Jordan Premo
I've got a problem here that I can't solve, and I have an exam in the
morning.. it's the only one I can't get, or even start.

What time is your exam?

[[e-mail] (formerly)]

Post by Jordan Premo
I've got a problem here that I can't solve, and I have an exam in the
morning.. it's the only one I can't get, or even start.
A farmyard gate 4m long and 2m high weighs 600N. It's centre of gravity
is at its centre and is supported by hinges at A and B. To relieve the
strain on the top hinge a cable CD is connected at an angle of 30 degrees
as shown. The tension in the cable is such that the horizontal force at
hinge A is equal to zero.

Determine the torques, using hinge A as a center. The gate is not
accelerating rotationally. There should be three terms, one from the
cable, one from the weight of the gate, and one from hinge B. Torque is
force cross distance (or the product of force time the shortest distance to
the line of action). The only tricky one is the cable.

From this three component equation, only two terms have components in the
vertical direction, the cable, and the weight of the gate.

Post by Jordan Premo
a) Determine the tension in the cable.

You have enough hints for this.

Post by Jordan Premo
b) Determine the magnitude of the horizontal component of the force on
hinge B.

Once you get a), then the horizontal force at B has only one component in
the formulation above to oppose it, and that is the horizontal component of
the cable force.

Post by Jordan Premo
c) Determine the combined vertical force exerted on two hinges.

Vertical component of the cable force less the weight of the gate.

Post by Jordan Premo
d) If the cable were not in place, what would be the magnitude of the
horizontal component of the force on hinge A?

Sum the moments about hinge B. The horizontal component of A comes right
out.

Post by Jordan Premo
I really have no idea what to do... please help!

I sure hope you can remember this during an exam...

David A. Smith