Question

Your younger brother is supposed to mow the lawn using a push mower. He reasons from Newton's third Law that the mower will push back with the same force he exerts on the mower; therefore nothing will move and attempting to mow the lawn is pointless. What is wrong with his reasoning?

Answer 1

Solution:

The action -reaction pairs are existing on different objects in this situation.

The person pushing the lawn mower with a force F1 is pushed back by the lawn mower with an Equal reaction force -F1 in the opposite direction.

Another action reaction pair of forces is the push of the person on the ground F2 . This force experiences an equal but opposite reaction force -F2 , which is the push of the ground on the person.

These are different action reaction pairs on differnt objects . That gives a resultant net force which is Not zero. So the person is able to push the lawn mower and move.

The pairs of forces here are :

Force on mower by your brother F1 = - Force of Mower on your brother ( -F1)

Force of your brother on the ground (F2) = -Force of ground on your brother.(-F2)

Since the forces F1 and F2 are not equal to each other , these forces produce a net force =mass x acceleration , thus enabling the lawn mower to move.