# Changing speed

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## Accerleration

A change of speed per unit time is called accerleration, if the speed is increasing the object is accerlerating, if the speed is decreasing the object is decelerating.

The formula for measuring acceleration is:

acceleration = change in speed (or velocity)

time taken

• Acceleration is measured in metre per second squared (m/s2)
• A negative acceleration shows the object is decelerting, which means slowing down, it is the opposite of acceleration which is speeding up
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## Relative velocitites

Velocity is a vector - this means it has both size (speed) and direction.

When two cars are moving past each other, their relative velocity is:

• the sum of their individual velocities if they are going in opposite directions
• the different of their individual velocties if they are going in the same direction.
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## Speed-time graphs

The area under a speed-time graph is equal to the distance travelled.

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## Circular motion

A vehicle may go around a roundabout at a constant speed but it is accelerating. This is because its direction of travel of travel is changing; it's not going in a straight line. The driver needs to apply a force towards the centre of the roundabout to change direction. The gives the vehicle an acceleration directed towards the centre of the roundabout.

• ANY OBJECT MOVING ALONG A CIRCULAR PATH MOVES AT A TANGENT TO THE CIRCLE OR ARC OF A CIRCLE
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## Key words

SPEED - how fast an object travels, speed = distance/time.

ACCELERATION - a measurement of how quickly the speed of a moving object changes.

DISTANCE - a measure of how far something is, measured in units such a metres.

VELOCITY - how fast an object is travelling in a certain direction, velocity = displacement/time.

RELATIVE VELOCITY - vector difference between the velocities of two objects.

FORCE - a push or pull which is able to change the velocity or shape of a body.

STRAIGHT LINE - a line of constant graident.

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