\(\def \u#1{\,\mathrm{#1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \ten#1{\times 10^{#1}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\)

Static Friction

Suppose there is a very heavy box that you need to move. You push and push and push on it, but it just will not budge. Why won't it move? If you say "because it is too heavy" you are only indirectly right: after all, if the same box were sitting on ice it would be a lot easier to move, even though it has the same weight. And if we look at the force diagram of the box (that is, a diagram showing all the forces acting on the block), we see that the weight of the box is vertical and the normal force you are exerting on the box is horizontal, so the weight can't be the force that is balancing your push.

What is balancing your push is a new type of force, called static friction. Static friction is the force that prevents one surface from sliding past another: unlike the normal force, static friction is always parallel to the surface of contact. In this case, there is a static frictional force between the box and the floor, and so the static frictional force is horizontal, and points to the left to balance your pushing.

Note that the floor can exert a normal force and a static friction force on the same object, and this is commonly the case. Even your hands might be exerting a static frictional force on the box here, if your hands are pushing a little downward on the box as you try to move it.