Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.                                                     This is an example of newtons second law.The bigger man is clearly going to have the most inertia because he is bigger.the little boy will only have a little inertia compared to the man because he is so little. The most amount of inertia will win over the least amount of inertia because inertia is basically a force.Newtons second law has to deal with applying the biggest amount of inertia  to the littlest amount of inertia.

These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is: For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects.                                                          This is a example of the third law of motion.The man is exerting a force against the lawn mower in response the lawn mower exerts a equal and opposite force against the man.Newtons third law states "for every action there is an equal and opposite reaction".This is the same as when you are walking a dog and you are pulling the dog towards you and then the dog is pulling away from you.In this case you are pushing the lawn mower forwards and the lawn mower is pushing against you.