In short: A force is a push or a pull, measured in newtons (N). The net force is all the forces on an object added together with their directions: if it is zero the object stays still or keeps a steady velocity, and if it is not zero the object accelerates.
What is a force?
A force is a push or a pull. Forces can:
- make a still object start moving
- make a moving object speed up, slow down or stop
- change the direction of motion
- change the shape of an object (squash, stretch, bend)
Force is measured in newtons (N). An apple weighs about 1 N. A force is a vector, so it has a size and a direction. We measure force with a spring balance (newton meter).
Contact and non-contact forces
| Contact forces (objects must touch) | Non-contact forces (act at a distance) |
|---|---|
| Friction: resists sliding between surfaces | Gravity: pulls masses together |
| Air resistance (drag): friction from air or water | Magnetic force: between magnets and some metals |
| Normal force: a surface pushing back on an object | Electrostatic force: between charged objects |
| Tension: pull through a rope or string | |
| Applied force: a push or pull from a person or machine | |
| Upthrust (buoyancy): push up from a liquid or gas |
Free body diagrams
A free body diagram shows one object as a dot or box, with an arrow for every force on it. Longer arrows mean bigger forces.
Net force (resultant force)
The net force is what you get when you add up all the forces on an object, taking direction into account.
- Forces in the same direction: add.
- Forces in opposite directions: subtract. The net force points the way of the bigger one.
Balanced and unbalanced forces
| Net force | What happens |
|---|---|
| Zero (balanced) | Object stays still, or keeps moving at constant speed in a straight line |
| Not zero (unbalanced) | Object accelerates: speeds up, slows down or changes direction |
- Friction
- Air resistance
- Normal force
- Tension
- Upthrust
- Gravity
- Magnetic
- Electrostatic
- Add same direction
- Subtract opposite
- Zero = balanced
- Start or stop
- Speed change
- Direction change
- Shape change
Quick summary
- A force is a push or pull, measured in newtons (N). Force is a vector.
- Contact forces: friction, air resistance, normal, tension, upthrust, applied.
- Non-contact forces: gravity, magnetic, electrostatic.
- Net force = all forces added with direction.
- Balanced (net force zero): still, or constant velocity. Unbalanced: acceleration.
- Free body diagrams show every force on one object as an arrow.
Quiz
Sign in to take the quiz
13 questions with instant marking. It is free, and your scores are saved to your account so you can carry on from any device.
Turn on JavaScript to sign in and take the quiz.
Try 3 questions now
Which is a non-contact force?
- A Friction
- B Tension
- C Magnetic force
- D Normal force
Show answer
C: Magnetic force
Magnets can attract or repel without touching.
The unit of force is the:
- A Joule
- B Watt
- C Newton
- D Kilogram
Show answer
C: Newton
Force is measured in newtons (N).
Forces of 40 N right and 15 N left act on a trolley. The net force is:
- A 55 N right
- B 25 N right
- C 25 N left
- D 55 N left
Show answer
B: 25 N right
Opposite directions subtract: 40 − 15 = 25 N, in the direction of the bigger force (right).