To be balanced, or not to be
Classical Mechanics
Level
3
Taking into account that the rods are massles, are rods A and B balanced?
Rod A is balanced, but rod B is not.
Both rods are balanced.
Rod A is not balanced, but rod B is.
Neither rods are balanced.
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1 solution
The moment of force of an object is the product of it's force and the arm of it's force, and we write it like this: F x bF = MF
All the forces in this problem are just weight.
We can calculate if rod B is balanced by calculating the moment of force of the 2 baloons forces.
The red baloon has 5 units of weight and is 1 unit away from the wire so it's moment of force is: 5 x 1=5
The magenta baloon has a weight of 3 units and is one unit away from the wire so it's moment of force is: 3 x 1=3
Since 5>3, rod B isn't balanced.
Now to see if rod A is balanced. The purple baloon has a weight of 6 units and is 3 units away from the wire, so that means it's moment of force is: 6 x 3=18
The yellow baloon has a weight of 2 units and is 1 unit away from the wire, so it's moment of force is: 2 x 1=2
Rod B has a weight of 8 units and is 2 units away from the wire, so it's moment of force is: 8 x 2=16
Adding those up we get: 18=18
So the corect answear is : Rod A is balanced, but rod B is not!