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

Static Equilibrium

When an object is not moving, it must be true that the forces acting on that object are balanced with each other. We say that such an object is in static equilibrium.

For example, if a block sits on a table, it feels the force of gravity pulling it downward. Because the block isn't moving, there must be another force that balances it: the normal force of the table pushing the block upwards. (We know it's a normal force because it is a push due to the table touching the block.) These two forces must be equal: if the block has a weight of 50N, then the normal force is also equal to 50N.

Suppose we attach a rope onto the same 50N block and pull it gently upward with a tension force of 10N. Now there are three forces acting on the block: tension and the normal force pointing up, and weight pointing down. If the forces balance, what must the normal force from the table be? (Think about it before turning the page!)