Question

Mitchael thinks that if a wagons pull is equal and opposite my pull then the net force will always be zero, so the wagon can never move. Since it is at rest it remains at rest according to newtons 1st law. How can i convince that Mitchael that if he pulls on the wagon, it will move? and how does it describe newtons 3rd law?

Answer 1

Let's observe that reaction forces (like those defined by Newton's Third Law) are INTERNAL.

Let's recall that Newton's Second Law accounts for external forces.

Let me paint this picture for you. You are standing on a boat. You tie a rope to the boat in some fashion. Pulling on the rope will not cause the boat to move because the tension force exerted on the rope acts on the boat, but our feet are also attached to the boat. Therefore, this force is INTERNAL.

Now back to the wagon example. We pull on a rope attached to the wagon. There is an equal and opposite tension force in the rope that acts on us and the wagon. This tension is internal. However, we are planted on the ground. Opposing our pulling force is a friction force between our feet and the ground. Assuming we don't slip and slide on the dirt and sand (we are wearing cleats) this frictional force will be GREATER than that the rolling resistance of the wagon wheel. This frictional force and rolling resistance are EXTERNAL. Therefore, the net EXTERNAL force is greater than zero. As a result, the wagon will move.

Answer 2

yes, basically the action and reaction forces are acting on different bodies. Hence the net force is not zero.