1.Isolate the two objects and consider one at a time. Analyze what
forces are acting on each of the objects independent of each other.
Only these forces are involved in the acceleration of an object.
2. The walker must push off from the ground with both a downward
and backward push to the ground from their feet. The opposite force
(forward and upward) is returned back to the person from the ground,
so the person walks forward.
3. The equal and opposite forces referred to in Newton's third law
of motion are acting on different objects. The horse will pull on the
cart and the cart will pull on the horse. The cart will have an
unbalanced net force acting on it (if neglecting friction) and would
accelerate.
Return to third law
of motion lesson.