# How can you push the ground backwards?

Whenever we come across " The physics behind walking" we often get answers stating that we push the ground backwards then it pushes feet forward and we walk ( very brief answer ) but the thing that's perplexing me is How can somebody on the top of ground push the ground backwards ?

It's very cross - intuitive as in real life we always see that whenever we push or pull something backward or forward we do it by applying force to the sides of the object. We never say that we pull or push a flat object backwards or forward by applying the force on the top of a flat object.

This push to the ground that is stated to be applied by us cannot be the normal force as it is always perpendicular to the surface. So what kind of force is being stated in those explainations ?

What I personally think is that it is normal reaction as no surface is perfectly flat and there are many curves on that so it is pretty much possible but in the end I don't want to keep any misconceptions.

• "We never say that we pull or push a flat object backwards or forward by applying the force on the top of a flat object". It's the component of the downward force in the horizontal direction (tangent to the surface) that pushes or pulls a flat object by means of static friction. Jun 30 at 12:27
• You can convince yourself that normal force isn't the one responsible for being able to walk, although it's responsible for you to not fall into the ground (so you can walk), by imagining walking on a frictionless surface - its impossible (normal force is still present). Basically friction is what lets you walk. Jun 30 at 12:40
• Jun 30 at 12:45
• “Whenever we push an object we do it by pushing the side”... False. Put a book on the table. Put your hand on top of the book. Move the book around. Jun 30 at 17:31
• “whenever we push or pull something backward or forward we do it by applying force to the sides of the object. We never say that we pull or push a flat object backwards or forward by applying the force on the top of a flat object.” You might want to deal some cards. I bet you do it by applying the force on the top of a flat object.
– Dale
Jun 30 at 19:33

the answer is depicted in the first image you posted. There is a force $$F_{push}$$ applied on the floor, by the runner. This force is neither perfectly horizontal nor vertical. It is oblique to the floor, but you can treat it as if it was a composition of two forces: a horizontal component $$F_h = F_{push} \, \cos{\theta}$$ and a vertical component $$F_v = F_{push} \, \sin{\theta}$$. Now, both of this components have a corresponding "reaction force" (according to Newton's 3rd Law). In particular, the reaction to the horizontal component is $$F^*_h = - F_h$$ (it has the same magnitude but the opposite direction) which is a force applied on the runner by the floor. This force is simply friction, and is the one responsible for accelerating the runner forward.

Friction allows you to do it! What you say about having to push the side of an object to make it move is quite wrong. Simply place a book on a table top, and you will find you can make it slide around by pressing on the top of it. When you walk, your leg imparts a force on the ground at an angle. The vertical component of the force supports your weight, while the horizontal component propels you forward. If it were not for friction, instead of propelling you forward, your foot would slide backwards along the surface of the ground, as it might do on very slippery ice.

• Well the friction itself acts only when there is a force along a surface and you say friction allows me to apply a force along the surface isn't that a type or circular reasoning. Jun 30 at 15:25
• The friction IS the force acting along the surface. Without friction there is no tangential force of this kind. The tangential force acting on both objects in contact is what we call friction force.
– nasu
Jun 30 at 15:31
• As another example, friction is also what lets you hold a cylindrical glass upright in your hand. You're not supporting the glass from the bottom at all, the supporting force comes entirely from friction from your hand on the vertical walls of the glass. Jun 30 at 18:49

Two others have already answered but sometimes you need to hear the same thing in other words. It seems like you understand newtons 3rd law that for every force there will be an equal and opposite force. If I understood your question correctly then what is perplexing for you is that a "flat" surface is able to give a reaction force to the side (in other words not normal to the flat surface). This can only be done if there exist friction. Imagine what would happen if a person tried to run on an ideal non-frictional surface. He would just kick himself away in the normal direction. Now since there exist friction then your last picture is really good here, it shows that surfaces actually have microscopic obstacles which give rise to the friction. This means that one could kick on the flat surface and push oneself a bit more to the side instead of in the normal direction.

• You also say that we are able to apply a tangential force on the surface as there is friction but friction itself comes only when there is a tangential force isn't it a circular reasoning. Jun 30 at 23:05
• No I don't think so. Try to imagine friction as what would happen in the macroscopic case for example a box that is stuck to horizontal ground. To achieve a force that is tangential to the ground is now easy it's just to push against one of the vertical sides of the box. The reaction from the box is friction if we scale down the box to be microscopic instead.
– ludz
Jul 2 at 10:22