What a force can do
A force is a push or a pull. A resultant force (net force) acting on an object can change its speed, change its direction, or change its shape. When forces are balanced (resultant force = 0), the object stays at rest or moves at constant velocity.
Key formula
Newton's second law: F = ma
where F = resultant force (N), m = mass (kg), a = acceleration (m s⁻²). One newton is the force that gives a 1 kg mass an acceleration of 1 m s⁻².
Force and acceleration
For a fixed mass, acceleration is directly proportional to the resultant force (a ∝ F). For a fixed force, acceleration is inversely proportional to mass (a ∝ 1/m). Acceleration always acts in the direction of the resultant force.
Worked example
A resultant force of 30 N acts on a 6 kg box. Find its acceleration.
F = ma, so a = F ÷ m = 30 ÷ 6 = 5 m s⁻².
If friction of 6 N now opposes a 30 N push, resultant force = 30 − 6 = 24 N, so a = 24 ÷ 6 = 4 m s⁻².
Balanced and unbalanced forces
Balanced forces produce no acceleration. An unbalanced (resultant) force produces acceleration in its direction.
Remember
- Resultant force = mass × acceleration.
- Find the resultant force first (subtract opposing forces).
- Zero resultant force means constant velocity or rest.