Speed and Velocity
Speed and Velocity
Speed is how fast something moves.
Velocity is speed with a direction.
Saying Spot the Dog runs at 9 km/h (kilometers per hour) is a speed. But
saying he runs 9 km/h westwards is a velocity.
Speed
Velocity
magnitude
magnitude and direction
Example:
60 km/h
60 km/h North
Example:
5 m/s
5 m/s upwards
Has:
Imagine something moving back and forth very fast: it has a high speed, but a low (or zero) velocity.
Speed
Speed = Distance / Time
Example: A car travels 50 km in one hour.
Its average speed is 50 km per hour (50 km/h)
Average vs. Instantaneous Speed
The examples so far calculate average speed: how far something travels over a period of time.
But speed can change as time goes by. A car can go faster and slower, maybe if there are stop lights or if there
is a lot of traffic.
So there is also instantaneous speed: the speed at an instant in time. We can try to measure it by using a very
short span of time (the shorter the better).
Example: Sam uses a stopwatch and measures 1.6 seconds as the car travels between two posts 20 m
apart. What is the instantaneous speed?
Well, we don't know exactly, as the car may have been speeding up or slowing down during that time, but we
can estimate:
20 m / 1.6 s = 12.5 m/s = 45 km/h
It is really still an average, but is close to an instantaneous speed.
Constant Speed
When the speed does not change it is constant.
For constant speed, the average and instantaneous speeds are the same.
Velocity
Velocity is speed with a direction.
It is actually a vector ...
... as it has magnitude and direction
Because the direction is important velocity uses displacement instead of distance:
Speed = Distance / Time
Velocity = Displacement / Time in a direction.
Example: You walk from home to the shop in 100 seconds, what is your speed and what is your velocity?
Speed = 220 m / 100 s = 2.2 m/s
Velocity = 130 m / 100 s East = 1.3 m/s East
You forgot your money so you turn around and go back home in 120 more seconds: what is your
round-trip speed and velocity?
The total time is 100 s + 120 s = 220 s:
Speed = 440 m / 220 s = 2.0 m/s
Velocity = 0 m / 220 s = 0 m/s
Yes, the velocity is zero as you ended up where you started (the displacement = 0)