Where Should the Fulcrum Be Placed to Balance the Meter Stick?

[pat666]

Homework Statement

A metre stick of negligible mass has 1 .2kg mass at its 35cm mark and a .4kg mass at its 75cm mark. where will the fulcrum be if it is balanced?

Homework Equations

net torque = 0

The Attempt at a Solution

ive done more complicated questions then this but for some reason i can't figure this one out (logically its more complicated..). I don't know what to do since the radius of revolution will be different for both masses.

 

[ehild]

Let be the fulcrum at mark "x" and write the torque of both weights with respect to the x mark.

ehild

 

[pat666]

i tried that but that keeps making the torque radius the same for both masses and it will never be in equilibrium
.2x+.4x=0 x=0 left gravity out because it just cancels

 

[pat666]

? I am sure I am missing something simple but i really can't think of it

 

[ehild]

What is torque at all? Is not it the product of force and the distance of the force line from the rotation axis?

ehild

 

[pat666]

yes that's what it is by definition but to be in equilibrium there has to be two different rotation radii since the masses are not equal. I cannot figure out what to do because of this?

 

[ehild]

make drawing.

ehild

 

[pat666]

i have it didnt help me

 

[ehild]

Show your drawing. Without it, I do not understand what is in your mind.

ehild

 

[pat666]

the fulcrum goes at some unknown place between them. didnt bother labeling it because its such a simple drawing.

 

[ehild]

Show all the forces and their arms.

ehild

 

[arielthw]

you can draw it!


http://www.convertmpgtowmv.com" .

 

[pat666]

who can draw it-- wtf are you talking about??

 

[ehild]

I gave up. I wanted a drawing something like this. I made it with Paint in 2 minutes.
d1=35 cm, d2= 75 cm, F1 = 1.2 g, F2=0.4 g, the pivot is at mark x.
What are the conditions of equilibrium?
How do you calculate the torque of a force F with respect to a point, say with respect to the left edge of the metre stick?


ehild

 

[xxChrisxx]

this is a statics problem. What should you do with any statically determinate statics problem?

Answer:

Free Body Diagram

Sized for effect. Seriously, get used to drawing them now OP, they are so useful.

 

[pat666]

net torque and force are 0, still i end up with two variable to solve for though

 

[xxChrisxx]

net torque and force are 0, still i end up with two variable to solve for though

You also have 2 equations.

 

[pat666]

what are my two equations, i have T=rFsin(90) and T=Ia but i don't think i can use that

 

[xxChrisxx]

Have you drawn a free body diagram?

There are two equations:

Sum of vertical forces.
Sum of moments.

You should have 2 unknowns, so those 2 equations are enough to solve the problem.