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 |
Forces
Contact
- Friction
- Air resistance
- Normal force
- Tension
- Upthrust
Non-contact
- Gravity
- Magnetic
- Electrostatic
Net force
- Add same direction
- Subtract opposite
- Zero = balanced
Effects
- 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.
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