Scalars, vectors and interacting forces
Scalars and vectors
A quantity has a size (magnitude) only. A quantity has a size and a direction.
| Scalars | Vectors |
|---|---|
| Distance | Displacement |
| Speed | Velocity |
| Mass | Weight |
| Time, temperature, energy | Force, acceleration, momentum |
A vector can be drawn as an arrow. The length of the arrow shows the size, and the way it points shows the direction.
Speed and velocity show the difference: "20 m/s" is a speed, while "20 m/s north" is a velocity.
Key terms
Contact and non-contact forces
A is a push or a pull that acts on an object because it interacts with another object. Force is a , measured in .
act only when the objects are touching:
- opposes sliding between surfaces.
- opposes motion through air.
- Tension: in a stretched rope or string.
- Normal contact force: a surface pushing back on an object resting on it.
act when the objects are apart:
- between any two masses, such as you and the Earth.
- Electrostatic force: between charged objects.
- Magnetic force: between magnets, or a magnet and a magnetic material.
Forces always come in pairs: when the Earth pulls you down, you pull the Earth up.