Newton's First Law
Forces and Motion

Measuring constant speeds

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Measuring constant motion.

Pupils gain experience of timing objects moving at a constant speed and recognise that the forces acting add to zero.

What to Prepare

What you need depends on the activity being undertaken – there are three sets of apparatus:

Ball bearings falling through glycerol

  • 1 large measuring cylinder (2 litre) full of glycerol, with marks every 100 millimetre
  • set of small ball bearings (about 5 millimetre diameter)
  • set of stop clocks

All bearings can be removed from the glycerol using a strong magnet positioned on the outside of the measuring cylinder.

Styrocell beads falling through water

  • 1 large measuring cylinder full of water, with marks every 100 millimetre
  • box of styrocell beads
  • set of stop clocks

Styrocell beads need to be projected into the water with a reasonable speed, to ensure that they don't just float on the surface of the water.

Air bubbles travelling upwards in a tube of water

  • one large transparent tube of water clamped in position
  • an air valve connected via rubber tubing to the bung at the bottom of the tube
  • stop clocks

In this experiment, the air bubbles injected into the water will rise upwards.

What Happens During this Activity

In this demonstration the pupils time how long it takes for an object to travel successive distances of 10 centimetre. The key questions to address are:

  • What forces are acting on the object?
  • What can you say about the size of the forces?
  • Why does the object fall (or rise) at a constant rate?


Split the class into groups of three to four and if possible give each pupil a stop watch or clock.

  1. All pupils start their clocks when the ball passes the first line (the starting line).
  2. Group one pupils stop their clocks when the ball passes the second line. They have measured the time for the ball to travel 10 centimetre.
  3. Group two stop timing when the ball has crossed the third line – they have timed for a total distance of 20 centimetre.
  4. Groups continue making timings until the ball bearing has reached the bottom of the measuring cylinder.

The results can be used in at least two ways:

  • An average time can be found from the individual readings of each pupil in a group. Using this average time, the average speed of the ball bearing can be calculated for the distance travelled (e.g. average speed for the first 10 centimetre, average speed for the first 20 centimetre, 30 centimetre, 40 centimetre etc.).
  • Alternatively, by subtracting the time to travel 10 centimetre from the time to travel 20 centimetre, the time to travel the second 10 centimetre can be calculated. All the times for successive 10 cm distances can be compared, and should be very similar. In this case, speed does not need to be calculated since all the distances are the same.

Get pupils to identify the forces acting on the objects in each of the above experiments. Pupils may struggle to understand that the forces acting on the object add to zero. While initially there must be a slightly larger force acting down in order to start the object moving in that direction, the two forces quickly balance each other and so the object travels at a constant speed.

Newton's First Law
formalises Inertia
includes the quantity Force
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