What forces do
A force is a push or a pull, measured in newtons (N). A force can change an object's shape, its speed, or its direction of motion. Forces are vectors, so their direction matters. When several forces act, they combine into a single resultant force.
Key idea
Resultant force F = m × a (Newton's second law). A resultant of zero means the forces are balanced, so a body stays at rest or keeps a steady velocity.
Balanced and unbalanced forces
When forces are balanced the resultant is zero and there is no acceleration. When they are unbalanced there is a resultant force, and the object accelerates in the direction of that resultant. Friction and air resistance are forces that oppose motion.
Worked example
A 2.0 kg trolley is pushed with a resultant force of 6.0 N. a = F ÷ m = 6.0 ÷ 2.0 = 3.0 m/s² in the direction of the force.
Springs and Hooke's law
A force can stretch a spring. Hooke's law states that the extension of a spring is proportional to the load, provided the limit of proportionality is not exceeded. A graph of load against extension is then a straight line through the origin.
Remember
- Force is a vector, measured in newtons.
- Resultant force F = ma.
- Balanced forces → no acceleration.