The velocity of an object is its speed in a particular direction. Velocity is a vector quantity because it has both a magnitude and an associated direction. To calculate velocity, displacement is used in calculations, rather than distance.
Unlike distance, which is a scalar quantity, displacement is a vector quantity. It includes:
When people run, fall, cycle or travel in a car or a plane their speed will change. They may speed up - acceleration, or slow down - deceleration. Acceleration is the rate of change of velocity. It is the amount that velocity changes per unit time. The change in velocity can be calculated using the equation:
The average acceleration of an object can be calculated using the equation:
This is when:
If an object is slowing down, it is decelerating (and its acceleration has a negative value). Some typical values for acceleration in metres per second (m/s²) include:
|Method of travel||Typical acceleration in m/s²|
|Running from a standing start||6|
|Family car overtaking another car on a motorway||4|
|Skydiver falling under the action of gravity||10|
|Rollercoaster going into a loop||30 - 60|
|Race car leaving the starting grid in a race||14|
A car takes 8.0 s to accelerate from rest to 28 m/s. Calculate the average acceleration of the car.
final velocity, (v) = 28 m/s
initial velocity, (u) = 0 m/s (because it was at rest - not moving)
change in velocity, (∆v) = (28 – 0) = 28 m/s
A car takes 25 s to accelerate from 20 m/s to 30 m/s. Calculate the acceleration of the car.
final velocity, (v) = 30 m/s
initial velocity, (u) = 20 m/s
change in velocity, (∆v) = (30 – 20) = 10 m/s
A bicycle is moving at 10 m/s when the rider applies the brakes and stops in 5 seconds. What is the acceleration?
final velocity, (v) = 0 m/s
initial velocity, (u) = 10 m/s
change in velocity, (∆v) = (0 − 10) = -10 m/s
The answer is a negative acceleration so the bicycle is decelerating.