Speed and velocity refer to the motion of an object. Distance-time and velocity-time graphs can be a useful way of analysing motion.
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:
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:
change in velocity = final velocity - intial velocity
This is when:
The average acceleration of an object can be calculated using the equation:
This is when:
If an object is slowing down, it is decelerating - in this case, its acceleration has a negative value.
A car takes 8.0 s to accelerate from rest to 28 m/s. Calculate the average acceleration of the car.
First calculate change in velocity:
final velocity = 28 m/s
initial velocity = 0 m/s (because it was at rest – not moving)
change in velocity = (28 - 0) = 28 m/s
Then use the values to calculate average acceleration:
acceleration = 28 ÷ 8
acceleration = 3.5 m/s2
A car takes 25 s to decelerate from 30 m/s to 20 m/s. Calculate the acceleration of the car.
First calculate change in velocity:
final velocity = 20 m/s
initial velocity = 30 m/s
change in velocity = (20 - 30) = -10 m/s
Then use the values to calculate average acceleration:
acceleration = -10 ÷ 25
acceleration = -0.4 m/s2
The acceleration value is negative here because the car is slowing down or decelerating.