Question

A balance and torque experiment is conducted with a balanced ruler (that is, the pivot point is in the middle).

Length of the ruler: 1 m

Mass of the ruler: 650 g

Weight 1: 4 N. This weight is placed at the 15-cm mark on the ruler.

Weight 2: 3 N. This weight is placed at the 58-cm mark on the ruler.

We want to balance the ruler by supporting it with a spring scale at the 20-cm mark.

What reading should we expect on the spring scale?

Answer 1

Note:

If you will try to balance force in upward and downward direction you will get (4+3+6.5)N ...which is not incorrect. It is total force in upward direction which includes hinge force also. So to reduce calculations (due to hinge force) , we are using Torque balancing equation about Hinge so that we can ignore hinge forces.