Beginner · 10 min
Forces and motion
Combine forces along a line and connect net force to acceleration.
Find the net force
A force is a push or pull caused by an interaction. Forces have direction and are measured in newtons (N). The net force is the combined effect of all forces on an object. Forces in the same direction add; opposing forces subtract. Balanced forces give a net force of zero. This means no acceleration: an object can remain at rest or continue at constant velocity.
Worked example
Two horizontal forces act on a box: 9 N to the right and 5 N to the left. Net force = 9 − 5 = 4 N to the right.
Force, mass, and acceleration
For an object of constant mass, net force equals mass multiplied by acceleration: F = m × a. Use the net force, after including opposing forces such as friction. A larger net force causes a larger acceleration for the same mass. With the same net force, an object with more mass accelerates less. Acceleration is in the direction of the net force.
Worked example
A 2 kg trolley has a net horizontal force of 6 N. Acceleration = 6 ÷ 2 = 3 m/s²
Draw the forces
- Draw a box with a 10 N force to the right and a 6 N force to the left. Find the net force and mark its direction.
- Change the leftward force so the forces balance. Describe what happens if the box was already moving at a constant velocity.