Big idea
When forces are unbalanced, there is a resultant (net) force, and the object's motion changes — it accelerates.
01Resultant force
When forces do not cancel out, there is a leftover force called the resultant force (or net force). It has both a size and a direction.
To find it, add forces acting in the same direction, and subtract forces acting in opposite directions.
Example. A box is pushed right with 30 N, and friction pushes left with 10 N. Resultant force = 30 − 10 = 20 N to the right.
02Newton's Second Law
Newton's Second Law links force, mass, and acceleration. A bigger resultant force causes bigger acceleration. A bigger mass is harder to accelerate.
Force (N) = mass (kg) × acceleration (m/s²) — F = ma
Example. A 2 kg trolley is pushed with a resultant force of 6 N. Acceleration = F ÷ m = 6 ÷ 2 = 3 m/s².
03Effects of unbalanced forces
An unbalanced force can make an object:
- Speed up (accelerate)
- Slow down (decelerate)
- Change direction
- Change shape (e.g. a spring stretching)
04Friction and air resistance
Friction is a force that opposes motion between two surfaces in contact. Air resistance (drag) is a similar force from air pushing against a moving object.
Both usually act to slow moving objects down, and both turn some kinetic energy into heat.
| Term | 한국어 | Meaning |
|---|---|---|
| resultant force | 합력(순수 힘) | The overall leftover force after combining all forces |
| net force | 순력 | Another name for resultant force |
| mass | 질량 | The amount of matter in an object (kg) |
| friction | 마찰력 | A force that opposes motion between surfaces |
| air resistance | 공기 저항 | Drag force from air acting on a moving object |
| decelerate | 감속하다 | To slow down |
Quick check — tap a question to reveal the answer
A 5 kg object accelerates at 2 m/s². What resultant force is acting on it?
F = ma = 5 × 2 = 10 N
Two forces of 15 N and 5 N act on a box in opposite directions. What is the resultant force?
15 − 5 = 10 N, in the direction of the larger force.
Name two effects an unbalanced force can have on an object's motion.
Any two of: speeding up, slowing down, changing direction, changing shape.