When you lay in bed on your phone, you’re at rest. You aren’t moving. To get nerdy for a moment, you’re in equilibrium. Well, one kind at least.
Equilibrium essentially means no change.
As we learned about Newton’s First Law, something at rest will stay that way until something moves it. In other words, it will stay in equilibrium. But remember, something will also keep moving until something stops it.
As long as something is moving at a constant speed in a straight line, it is also in equilibrium.
Standing on a scale is an excellent example of equilibrium. What’s weird though is that what’s happening on the scale is actually happening anytime you are simply standing on the ground beneath you. The same can be said about a book sitting on a desk, or you holding something steady out in front of you!
What about equilibrium in movement? That’s where friction comes into play. When we talked about net force, we used two people as an example. But often, moving one object requires moving over another. Like sliding a fridge across the floor, or sliding a box on concrete. To move the object, you have to overcome not only the object’s inertia but the resistance provided by friction.
Say you’re sliding a desk across the room. If you manage to do so at a constant speed in a straight line, the desk is in equilibrium. Why? The force you are pushing into the desk is equal to the force of the friction!
An object at rest is in what is called static equilibrium
An object in motion at a constant speed in a single direction is in dynamic equilibrium.