Question 2
A wooden strip fixed at its centre needed a force F to turn it when pulled with one spring balance. If balances are now attached at both ends and pulled in opposite directions, what force must each one apply?
The right answer is the one marked above.
F/2. Both balances turn the strip the same way round, so their moments add. Each one therefore needs only half the original force.
It is wrong to think that because the forces are opposite they cancel. They act on opposite sides of the axis, so as far as turning goes they add.