Equilibrium
Net force and net torque are both zero in equilibrium.
About this topic
An object is in equilibrium when the net force is zero and the net torque is zero, so it does not accelerate or begin rotating. It can be at rest (static equilibrium) or moving at a constant velocity.
For problems, sum forces in the x and y directions and sum torques about a convenient pivot to zero. Choosing the pivot at an unknown force removes that force from the torque equation, which simplifies the algebra. Bridges, ladders, shelves and cranes must be in equilibrium to be safe.
Example: a 4 m uniform plank of 20 kg rests on two supports at its ends with a 10 kg child standing in the middle. Each support carries half the total weight, 147 N. If the child moves toward one end, that support carries more and the other less. Architects and structural engineers calculate such forces for every building.
Misconceptions: equilibrium does not mean the object is at rest, since constant velocity also counts. Also, balanced forces alone do not guarantee equilibrium: two equal and opposite forces at different points can still twist the object, which is why the torque condition is needed.
Key ideas
- Static equilibrium: nothing moves
- Components sum to zero
- Used in bridges and cranes
Quick check
Use cases: where Equilibrium helps
- Homework: review the key idea behind Equilibrium before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Equilibrium without notes.
- Visual learning: play with the interactive lab to see how Equilibrium changes when you adjust its inputs.
- Teaching a friend: explain Equilibrium in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Equilibrium?
Net force and net torque are both zero in equilibrium.
What will I learn in Equilibrium?
Static equilibrium: nothing moves. Components sum to zero. Used in bridges and cranes.
Is Equilibrium part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Equilibrium lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Equilibrium?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.