P2aL1 See How it Moves

Key Words

Average Speed - total distance divided by the total time taken.

Distance - length between two points

Distance time graph - a graph in which distance (vertical axis) is plotted against time (horizontal axis).

Slope - how steep the graph is

Speed - distance divided by the time.

Stationary - something that is standing still.  (NB: Stationery - paper, pens, etc.)

Ticker Timer - a machine that prints 50 dots a second onto a paper strip.  You can work out the speed from this.

Time - period in which something happens (or doesn't).

Ultrasonic motion sensor - an instrument connected to a computer that detects movement.

Test Yourself

Homework
Physics GCSE
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Grade E

We can plot the movement of an object using a distance time graph.

  • A shallow slope means that the object is moving slowly.  The slope is the gradient, which gives us the speed.  A straight line tells us that the speed is constant.

  • A steep slope means that the object has a high speed.

  • A horizontal line tells us that the object is stationary, i.e. standing still.

  • A curved line tells us that the speed is changing.  The object is accelerating.

Grade C

You can make distance time graphs using:

  • A ticker-timer, a machine that prints 50 dots a second onto a long strip of paper.  It is rather old-fashioned, but dead simple and low-tech.
  • An ultra-sonic motion detector.  This is a more high tech device that sends the data to a computer.

Here is some ticker tape:

The space between each dot is 1/50 second (0.02 s).  Therefore 10 spaces give us 1/5 second (0.2 s)

We can work out the speed:

speed (m/s) = distance (m)

                     time (s)

In this case, speed = 12 cm ÷ 0.2 s = 60 cm/s

 

When we go on a journey, we tend to think of the average speed:

 

average speed (km/h) = total distance (km)

                                       total time (h)

A lorry may take 1 hour to do 70 km, so its average speed is 70 km/h.  However its tachograph will show that its speed might be 0 at some points (waiting for traffic lights), but doing 100 km/h down a dual carriageway road.

Grade A

In a distance time graph, the speed is given by the gradient.  If the distance time graph is a straight line, the speed is constant, so we work out the gradient:

 

Gradient = Rise ÷ run

If the line is curved, the object is accelerating.  We need to take the tangent: