Summary
A force is anything that can change an object's motion or deform it: pushing, pulling, gravity, friction. Forces are measured in newtons (N) and have direction, which is why they are drawn as arrows. When several forces act on a body, what matters is their sum: the net or resultant force.
Isaac Newton summed up how they work in three laws. The first, inertia: a body stays still or moves at a constant speed as long as no net force acts. The second: the more force, the more acceleration, and the more mass, the less (F = m · a). The third, action and reaction: if one body pushes another, the other pushes back with an equal and opposite force. Two everyday forces are gravity, which pulls us towards the Earth (weight), and friction, which opposes motion.
Key points
- A force changes an object's motion or deforms it.
- It is measured in newtons (N) and has direction (drawn as arrows).
- The net force is the sum of all the forces acting on a body.
- 1st law (inertia): with no net force, motion does not change.
- 2nd law: force = mass × acceleration (F = m · a).
- 3rd law: every action has an equal and opposite reaction.
- Weight is the force of gravity; friction opposes motion.