What external interactions are talking place when a person walks on the ground? And when a bicycle turns?

Question

If the Law of Conservation of Momentum is to be obeyed, both these situations have external interactions taking place. But what are the, and what causes them?

Answer 1

When you walk, your foot is pushing against the ground (actually the entire planet Earth), so the "external" interaction you're refering you is the Newton's 3rd law occuring between you and the Earth. As you step on the ground, the Earth pushes back with equal and opposite force. This helps you to get off the ground so that you can take another step. Of course, the Earth is pushed away from you with the same force you used to step down, but since the earth is so big and so much more massive than you, the effect is completely neglegible.

BTW, friction is certainly another "external" interaction, but it is only responsible for keeping you from sliding around on the ground so that you can walk. It is not responsible for you walking.

But in the case of the bicycle, friction plays a major part in turning the wheels as you step on the pedals.

Answer 2

The external interactions are caused by friction. Without friction, a bicycle wheel would just slide across the ground instead of turning. But friction resists the wheel's attempt to slide forward, causing the wheel as a whole to roll forwards over the bottom point. Similarly, friction is what gives your feet traction to push your body forwards. Friction is the only external force in both situations, apart from air resistance.

Answer 3

FRICTION!!!! no more no less...c;