# Simple physics question

1. Dec 15, 2003

### jazzchan

hi everyone...

I am new comer and new for phsyics..

I have a problem for the following question.

/|
/ |
/ mass
/
/__________
A B

From point A to B is a simple bridge mad from uniform 6 m long platorm. A rope of negligible mass is attached to other end point A. this rope passess over a smaill frictionless pulley and supports a 110 kg mass on its other end. The angle between the rope and the bridge is 30 deg.

would u let me know the mass of the bridge is necessary to consdier to find the tension of the rope ??

I know it is a very simple question....but it is hard for me ..a poor old man....

thanks a lot..

Last edited: Dec 15, 2003
2. Dec 15, 2003

### Staff: Mentor

I don't fully understand the problem, but if the system is in equilibrium the tension in the rope must equal the weight of the hanging mass.

3. Dec 15, 2003

### jazzchan

the question said that the bridge at point B is frictionless hinge and mass is 110 kg.

is it mean there is a frictionless support at point B ?? if yes.......i think i shall consider the mg of the bridge and the reaction force at point B

is right ??

thanks..

4. Dec 15, 2003

### HallsofIvy

Yes, a "frictionless hinge at B" means the B-end of the bridge is fixed in place but that the angle can change (that's the hinge part). Find the torque around B and set it equal to 0.
(Since you are taking the torque "around B" you don't need to worry about the force at that end. The two forces involved are the vertical component of force due to the 110 kg mass and the weight of the bridge which, since it is uniform, you take to be concentrated at the middle.

(and don't "poor old man" me. I'll bet I could beat you in both the "poor" and "old" departments!)

5. Jan 2, 2004

### jazzchan

od u mean that the tension of the rope

the toque = +T*L*sin 30 - mass of the bridge *g*L/2 - mass of 110kg*g = 0 ??
is right ??

thanks