Solving Dynamics Questions: Newton's Third Law Explained

[darksoulzero]

Homework Statement

A father is pulling his child on a sled in the snow. How is it possible that the father can pull the child and sled forward according to Newton's Third Law.

Homework Equations

The Attempt at a Solution

I understand the basic concept of this law,but when it is applied I don't really understand how to explain.
What I think is occurring is that the sled and the father is an action-reaction pair and the father is able to move the sled because of another action-reaction pair between the father's foot and the ground. The sled moves because when the father walks his feet exert a force on the ground and the ground exerts a force back onto the father that allows him to move himself as well as the sled with his child on it.

 

[Doc Al]

That's not bad.

The key is to understand that just because the father and the sled exert equal and opposite forces on each other that doesn't mean that those forces produce equilibrium: The forces act on different objects.

Look at each 'system' in turn:
The sled: There's a force on it due to the father pulling it. Since there's a net force on it, it accelerates.
The father: The sled pulls back on the father, but the ground pushes him forward. Since there's a net force on him, he can accelerate.
The 'sled + father' as a whole: There's an external force of the ground acting on the system, so it can accelerate.