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Net force and equilibrium
Beginner
Zero net force means zero acceleration.
ΣF = 0If the net force is zero the acceleration is zero: the body is at rest or moving at constant velocity (translational equilibrium). All forces form a closed polygon.
A 10 kg lamp hung by two cables is in equilibrium when the vertical components of the tensions sum to 98 N.
Equilibrium doesn't mean no forces, only balanced forces.
In more detail
An object is in equilibrium when the net force on it is zero, so its acceleration is zero. It can still be moving, if its velocity is constant.
Real-world uses
- Bridges and buildings are designed so that forces balance and they do not accelerate.
- A hanging lamp stays at rest because the tension balances its weight.
- A hanging picture is in equilibrium when the tension in its cord balances its weight, so it neither falls nor rises.
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Frequently asked questions
Does equilibrium mean at rest?
Not necessarily. Constant velocity also means zero net force.
How do I check equilibrium?
Resolve forces into components and show that the sum in each direction is zero.
Does zero net force mean the object is stationary?
No. It means constant velocity, which can be zero or any steady value.
Related topics in Newton's Laws & Forces
Newton's third law · Weight and normal force · Free-body diagrams · Tension · Atwood machine · Connected bodies