Newton's Second Law
Forces and Motion

Multiflash photographs of accelerated carbon dioxide pucks

Practical Activity for 14-16 PRACTICAL PHYISCS


This demonstration is rather fussy to set up, but produces good results.

Apparatus and Materials

  • Carbon dioxide cylinder (Syphon type)

  • CO2 pucks kit, including glass plate and pucks
  • Dry ice attachment for cylinder
  • Camera and multiflash system
  • Lamp, bright, up to 500W
  • Elastic cord, to accelerate puck

Health & Safety and Technical Notes

When using CO2 and dry ice it is essential to have good ventilation to the room.

Remember to wear heat-insulating gloves when handling dry ice.

Read our standard health & safety guidance

Read this guidance note for general hints and detail of specific methods:

Multiflash Photography

The magnetic puck is a small metal ring magnet, of the kind used as field magnets in television sets. It has a metal or card lid. When it is filled with solid carbon dioxide, the puck floats on the sublimed gas.

A kit is available from scientific suppliers. Four pucks are provided, two magnetic, two non-magnetic and made of brass. They are, however, all of the same size and mass and you can stack them one on top of the other.

Polish the glass plate using a duster and methylated spirit or window cleaning fluid. Carefully level it using the wedges supplied.

The lengths of elastic used with trolleys are not suitable because a smaller force is needed here. Instead, use a longer length, with one end attached to the top of the puck with sticky tape. The stretch on this must be kept uniform. Here is a convenient technique. Hold the end against a half-metre rule. Ensure that it is always the same distance from the puck. With practice you can produce a fairly steady force.

The start of the motion does not necessarily coincide with an exposure or with an image. A high frequency of exposure is required. This reduces any error in identifying the time of the start of the motion, relative to later images.

For further information about making dry ice see the apparatus entry:

carbon dioxide cylinder

Three-quarter length blackout is essential for good photographs.


Making the image

  1. Attach a bright pointer to the centre of a magnetic puck.
  2. Set a camera alongside the glass plate, so that it can photograph the pointer as the puck moves across the plate. Illuminate the pointer with the lamp. Align the stroboscope slit with the camera lens, as shown.
  3. Put 2-3 cm3 of solid CO2 underneath one of the magnetic pucks, and place the puck on the plate.
  4. Attach an elastic cord to the puck and apply a force.
  5. With the multiflash system active, open the camera shutter on the B setting. Release the puck and accelerate it with a small near-constant force.

Analyzing the image

  1. Measure the distances between the puck positions. Use the multiflash frequency (time = 1/frequency) to determine the time between each position.
  2. To find out if the acceleration was uniform, plot distance against (time)2.

Teaching Notes

  • Discussion points arising from the experiment:
    • The time between images is constant.
    • Spacing between images of a body increases with speed.
    • A constant force on a body produces constant acceleration.
  • For constant acceleration, and provided that the starting velocity of the measured motion is zero, the graph will be a straight line passing through the origin.
  • By stacking pucks on top of each other, up to a maximum of four, and applying the same force in each case, you can show that acceleration decreases as mass increases.

This experiment was safety-tested in April 2006

Newton's Second Law
is expressed by the relation F=ma
can be used to derive Kepler's First Law
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