Question

a. give an example of an object that has zero net force acting on it, but experienced a net torque

b. give an example of an object that has zero net torque acting on it, but experiences a net force.

Answer 1

a)

• Consider a rod AB. Apply an upward force of magnitude (say F) at one end of the rod. Also apply a downward force of same magnitude F at another end of the rod.
• Now, the net force on the rod is zero. Because forces applied on the rad are equal in magnitude but opposite in direction.
• But the forces will accelerate the rod. Because there is a distance between the two forces applied and they will accelerate the rod. Therefore the net torque is not zero and the rod will rotate.

b)

• Rotational velocity of the object is changed the net torque acting on it. While the linear velocity of the object is changed by the net force which is acting on it.
• For example while pushing an object on the table. Assuming low friction between the object and the table, the pushing will move the object horizontally and there is no ration. Therefore the net torque is zero and the net force in non-zero.