Carbon dioxide puck on a glass plate

Demonstration

To show the link between constant speed in a straight line and constant speed in circular motion.

Apparatus and Materials

• Carbon dioxide puck (ring with cardboard lid)
• Glass plate
• Wedges, 4

• CO2 cylinder (syphon type)

• Dry ice attachment
• String, length about 50cm
• Suction sticker, for central anchor point
• Frame to support multiflash equipment
• Xenon strobe and camera
• Weak spring (OPTIONAL)
• Glass cleaner

Health & Safety and Technical Notes

Xenon strobe: avoid flash frequencies in the range 15-20 Hz, and avoid flickering light, which makes some people can feel unwell. Rarely, some people experience photosensitive epilepsy.

Dry ice is very cold. Handle with thermal gloves, and wear safety spectacles.

Take care when handling the glass plate.

Ethanol (industrial denatured alcohol) is very flammable and harmful.

Read our standard health & safety guidance

Clean the glass plate carefully with methylated spirit or window cleaning fluid. Polish it with a duster and carefully level it with the wedges.

The carbon dioxide cylinder has a pipe inside like a soda-water siphon. Keep it the right way up to release the gas into the dry-ice attachment. Pack solid CO₂ under the puck, on the glass table.

The procedure below is carried out using multiflash photographs and examples are provided (see below).

multiflash photographs

Place the xenon strobe (or substitute lighting} so that its light strikes the glass plate at a glancing angle and does not reflect into the camera. In step 4, use a stopwatch to find the orbital time of the orbiting puck from a ‘standard push’. Decide how many images of the puck you want so that the images are clearly spaced; hence decide the frequency of the strobe.

The photographs should be projected from the negative so that the pucks are actual size and then no scaling factors are needed.

A digital camera attached to a computer or a video camera coupled with a VTR which has frame by frame projection can also be used.

Possible alternatives to the xenon strobe:

• Mechanical strobe near to camera and (1 or 2) bright lamps (up to 500 W)
• Home-made flashing light stuck to puck and camera

Procedure

1. Make a small quantity of dry ice in the usual way using the CO₂ cylinder and dry ice attachment. Fill the puck with dry ice.
2. Give the puck a gentle push.
3. Push it gently again, in the opposite direction, so that students see it travelling both ways and are convinced it is not gliding downhill.
4. Attach the puck to a string at one end and to a centrally placed suction cap at the other end. Give the puck a gentle push; it will follow a circular path around the central point. Point out that its speed remains constant, but its direction constantly changes.
5. Cut the string while the puck is moving in a circle: it will go off on a tangent to its circular path.
6. OPTIONAL Insert the spring into the string connecting the puck to the central point. Give the puck a slightly harder push; it will follow an elliptical path. Get students to observe how the speed of the puck changes along the orbit.

Teaching Notes

• In steps 2 and 3, the puck travels in a straight line at constant velocity. Ask: Is this natural motion? Are any forces necessary?
• The weight of the puck is balanced by the upward contact force of the glass plate and there are no resultant forces on the puck in its direction of motion. It just continues to glide at constant velocity.
• This is an illustration of Newton’s first law. The puck is in equilibrium. This does not always mean staying at rest but rather continuing to move with uniform motion.
• In step 4, ask: "What makes the puck change direction? What forces act on it after the initial push?" The vertical forces are balanced, as in steps 2 and 3, but tension in the string indicates that there is now an unbalanced horizontal force. That force is always at right angles to the velocity of the puck. Point out that this is typical of circular motion.
• Before doing step 5, you might want to ask students to predict how the motion of the puck will change when the string is broken. In any case, link the puck’s change in motion back to what they saw in steps 2 and 3.
• Attached below are some example photos:

Puck A

• Illuminated with a lamp near to the glass plate and a mechanical strobe in front of the camera.
• Note the constant speed of the puck around its orbit. Any deviation from constancy may be due to residual friction or a sloping glass plate.
• Note also that the position of the string indicate constant angular velocity, which can be calculated from the angle and the time between two flashes.

Puck B

• Illuminated with a flashing light on the puck.
• Note the constant speed around the orbit.

Puck C

• Illuminated with a lamp near to the glass plate and a mechanical strobe in front of the camera. The string is burnt through using a taper.
• Note the circular orbit before the string burns and the tangential motion afterwards.

Puck D

• The puck is connected to the suction cap with a spring, so an elliptic orbit ensues.
• Note the change in speed along the orbit.
• Note also the different angles swept out in the same time intervals. Equal areas are swept out in equal times. (Kepler’s law II)