Often people have the following difficulty with Newton's Third Law:
"If A pushes B, then B pushes A with an equal and opposite force. If these forces are equal and opposite, they cancel, producing a net force of zero. This means that neither object can accelerate, which means that Newton's Laws predict that nothing can ever move."
(See The Horse & Cart Problem.) What's going on?
Newton's Third Law really does say that if A pushes B, then B pushes A with an equal and opposite force. However, these forces DO NOT CANCEL because they influence the motion of different objects. The force that A exerts on B influences B's motion, and the force that B exerts on A influences A's motion. The force on B can cancel with other forces on B - but NOT with forces on A(and vice versa).