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On the move

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Speed and acceleration are words that we often use to describe motion. We talk about high-speed trains. We talk about the acceleration of a sprinter or a Formula 1 racing car. In Physics these words have a precise meaning.

We use words like speed, constant speed, average speed and uniform acceleration to describe different types of motion. We can also use graphs to represent these motions. These graphs are called speed-time graphs.

In your Standard Grade Physics course you are expected to be able to draw and interpret speed-time graphs that represent situations involving constant speed and uniform acceleration.

You may wish to print a hard copy for your reference.

Speed and acceleration - before you start

In your Standard Grade Physics course you are expected to know and be able to use each of the following terms.

  • average speed
  • instantaneous speed (normally referred to as "speed")
  • acceleration.

You will be expected to use these terms in doing calculations on motion and in describing different types of motion. Make sure that you can remember what is meant by each of these terms. Study the wise up definitions for each of the terms.

  • Average speed is defined by this mathematical relationship:

average\; speed\; = \; {distance\; travelled \over time \;taken} \cr \cr

Average speed is measured in metres per second (m/s).

 

Video: Calculating average speed

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  • Instantaneous speed is the speed of an object at a particular moment in time. Instantaneous speed is measured in metres per second (m/s).
  • Acceleration is defined as follows:

\eqalign{ acceleration\; & = & \; {final\; speed\; - initial\;speed \over time \;taken} \cr}

Acceleration is measured in metres per second (m/s2)

Sometimes when we are describing motion we use the terms "steady speed" or "constant speed".

An object is travelling at a steady or constant speed when its instantaneous speed has the same value throughout its journey. For example, if a car is travelling at a constant speed the reading on the car's speedometer does not change. The speedometer reads the speed of the car at each moment in time throughout the car's journey. In cases like this, where the motion involves constant speed, the instantaneous speed of the object can be worked out using the relationship.

speed\; = \; {distance\; travelled \over time \;taken} \cr \cr

We also sometimes refer to a moving object as having a "constant acceleration" or a "uniform acceleration". A constant or uniform acceleration means that the speed of the object changes by the same amount every second.

 

Video: Acceleration and force

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When an object's speed is decreasing with time (ie slowing down). The object's speed is changing and so, by definition, the object is accelerating. However we often refer to this type of motion as a "deceleration".

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