Push a shopping trolley and it rolls; slam the brakes on a bike and it slows; squeeze a stress ball and it squashes. Every one of those is a force at work — a push or a pull that can change an object’s speed, direction or shape. We measure forces in newtons (N), named after Isaac Newton. A newton is roughly the weight of a small apple resting in your hand.
Two families of force
Every force falls into one of two families, depending on whether the objects have to be touching:
- Contact forces — the objects must touch for the force to act. These include friction, air resistance, tension (a pull in a rope or spring) and normal contact (the push a table gives a book resting on it).
- Non-contact forces — these reach across empty space, no touching required. The three to know are gravity, magnetism and the electrostatic force (static electricity). Think of them as invisible hands acting at a distance.
| Force | Family |
|---|---|
| Friction | Contact |
| Air resistance | Contact |
| Tension | Contact |
| Gravity | Non-contact |
| Magnetism | Non-contact |
| Electrostatic | Non-contact |
Resultant force: adding forces up
Usually several forces act on an object at once. The resultant force is the single force left over once you combine them all — the overall push or pull. Picture a tug-of-war: if both teams pull equally hard, the rope doesn’t move. That is a resultant of zero.
- Forces in the same direction — add them.
- Forces in opposite directions — subtract the smaller from the larger; the resultant points the way of the bigger force.
- Balanced forces cancel out, giving a resultant of 0 N. A balanced object stays still, or keeps moving at constant velocity — it does not speed up or slow down.
Force, mass and acceleration
An unbalanced (non-zero) resultant force makes an object accelerate — change its velocity. How much it accelerates depends on its mass. This is captured by one of the most important equations in physics:
Worked example: a 2 kg trolley accelerates at 3 m/s². What force is needed? Don’t reach for the calculator blindly — first write the equation, then slot the numbers in: F = m × a = 2 × 3 = 6 N. To find acceleration instead, rearrange to a = F ÷ m: a 5 kg mass feeling 20 N accelerates at 20 ÷ 5 = 4 m/s².
Memory hook: “Fma” — Force makes things accelerate. And when forces balance, the resultant is a big fat zero.