Equilibrium & Net Force
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 4 practice questions.
The net force is all the forces on an object added together, with directions. If the net force is zero, the object doesn’t accelerate: it stays at rest or keeps moving at a constant velocity. If the net force isn’t zero, the object accelerates in the direction of the net force.
1. Adding forces to get the net force
Forces along one line add like signed numbers. Pick a positive direction (say right = +), give each force a sign, and add.
A force is a push or a pull, measured in newtons (N) with a spring scale or force sensor.
Equilibrium means the net force is zero. That’s true for a book sitting still and for a hockey puck gliding at a steady speed on smooth ice.
2. Net force decides the acceleration, not the motion
The acceleration always points the same way as the net force. The velocity doesn’t have to.
A car moving right while braking has a net force to the left. It keeps moving right, but it slows down.
An object moving in a circle at constant speed is not in equilibrium: its direction keeps changing, so it is accelerating.
3. Example, then try it
A box is pushed right with 40 N while friction pulls left with 25 N. What is the net force, and what does the box do?
Right = +: Fnet = 40 − 25 = 15 N to the right.
The net force isn’t zero, so the box accelerates to the right.
A crate is already sliding right at 2 m/s. A person pushes it right with 50 N, friction acts left with 20 N, and a rope pulls it left with 30 N. What happens next?
Choose + : right.
Add the forces: Fnet = 50 − 20 − 30 = 0 N.
Zero net force means zero acceleration.
So its velocity doesn’t change: it keeps sliding right at 2 m/s.
Answer: it keeps moving right at a constant 2 m/s. It does not stop, and it doesn’t need a net force to keep going.
4. Common mistakes
A spacecraft far from any planet coasts at 300 m/s with its engines off. What is the net force on it?
Nothing pushes or pulls on it, so the net force is zero, and it keeps moving at 300 m/s in a straight line.
Ready to practice?
4 questions on Equilibrium & Net Force, each with a hint and an explanation for every wrong answer.