05 Speed and velocity
- In a car, the speedometer in front of the driver shows, for example, "80 km/h".
- It tells us how fast the car moves but nothing about its direction.
- This quantity is called speed.
- To describe the motion completely we need the direction too, for example "80 km/h toward the east" — this is velocity.
1Definition
Speed
- distance covered per unit time
Velocity
- displacement per unit time
2Type of quantity
Speed
- scalar (magnitude only)
Velocity
- vector (magnitude and direction)
3Sign
Speed
- always positive
Velocity
- positive in one direction, negative in the opposite direction
So the speedometer measures speed, not velocity, because it does not give the direction of travel.
Note: in the problems of this book, when we say "velocity" we use it as the quantity that gives the complete description of the motion; for straight-line motion the direction is just a plus or minus sign.