# What force enables us to walk?

When we walk, we apply a force $F$ on the earth at an angle $\theta$ . Thus, by Newton's third law, $$F_{\text{me, earth}}=-F_{\text{earth,me}}$$ Therefore, the earth exerts a reaction force on us, the horizontal component of which enables us to walk. The force I apply also makes the earth move an extremely tiny bit but not too much because of its gigantic mass.

Is this explanation correct? What role does friction play in all of this?

• You forgot to mention atmospheric force!! Mar 13 '17 at 5:49
• Possible duplicate of With Newton's third law, why are things capable of moving? May 18 '18 at 11:35
• I don't see how that could be a duplicate, the OP seems to understand Newton's 3rd law. May 18 '18 at 13:23
• The reaction force is the friction force, isn't it? May 18 '18 at 14:10

Your answer is correct. If you don't have friction you cannot apply a horizontal component to the force.

Let's focus on the foot.

If you are standing still of course there is no friction, every force is just acting on the vertical. But let's think of what happens when you start to walk. If you think about it you're just, in a way, "pushing the floor" in the opposite direction of your walking. Thus the friction force acts in the same direction of your walking.

In the picture above you can see the forces that the Earth exerts on your foot:

Think about walking on a very slippery plate of ice. You can't walk because your foot won't stay put. Your foot would go backwards, as your foot is exerting a force in that direction. When the friction comes to play it opposes that force and your foot can stay still.

The moving-forward movement can finally take place thanks to BOTH friction force and the work of your knee and hip articulation.

Its the frictional force. When you walk your feet push the ground backwards and according to Newton's third law, the ground pushes you forward.

The ability to overcome the gravitational force and atmospheric force which requires energy.

Can you walk without a reaction force in space??

So reaction force is necessary which is provided by one leg and the other leg alternately during walking.

That's how human evolved !!!