Speed of a rifle pellet measured with a scaler or timer
Practical Activity for 14-16
A direct measurement of the speed of a rifle pellet.
Apparatus and Materials
- Strips of metal, about 5 mm x 5 cm, with a small nick cut in them at their middle
- Mounted air rifle and pellets
- Circuit breakers, 2
- Scaler or timer accurate to 0.001 s
- Safety screens
Health & Safety and Technical Notes
Make absolutely certain the rifle is unloaded before setting up or adjusting the experiment.
A safety screen should be used, with students standing at a distance behind the rifle.
All present should wear eye protection.
Absorbing material should be used to catch stray pellets.
The rifle and target holders must remain fixed to the board at all times. You should check whether a licence is needed. (In the UK you will need a licence if the muzzle energy of the pellet exceeds 16.25 J.)
The rifle is aimed so that the pellet passes through two small strips of metal foil a measured distance apart, breaking each strip in turn. The timer is connected to the strips in such a way that the timer starts counting millisecond pulses when the first strip is broken and stops when the second strip is broken. Thus the timer measures the time taken by the pellet to travel the measured distance between the strips.
Even with a pellet as slow as the pellet from an air rifle, the time to travel one metre will be only a dozen or so milliseconds. The strips should therefore be placed as far apart as is convenient for aiming. The length of the flight is short and so the result will not be very accurate.
circuit breakers can be made with wood or hardboard frames. Each of these should have two terminals with spring clips to hold the aluminium foil strips. Set up two
circuit breakers a metre apart on the board to which the air rifle is mounted.
If thin pencil leads as used for mechanical pencils are available, these serve well instead of strips of foil. They break more sharply.
Since the metal strips must be broken by the pellet, they must be set up very carefully in the line of fire. Making that adjustment might waste a lot of time and spoil the demonstration by a series of misses. Therefore, a scheme such as the following for aligning the metal strips reliably beforehand is essential.
Place a small sheet of paper at each of the positions where metal strips are to be placed. With the rifle clamped in position, fire a pellet through these two sheets of paper. Then position the strips of metal (which are about 5 mm across and 5 cm long with a small nick cut in them at the middle) over the pellet-holes in those pieces of paper on the side away from the rifle in both cases. (A laser beam could also be used for alignment.)
The rifle should be loaded immediately before firing as any delay may produce inconsistent results due to leaking of air or weakening of the spring.
Some teachers find it difficult to aim the pellet and break the foil. An alternative arrangement illustrated above. This uses a card in a stand, which falls over breaking the contact when struck by the pellet. There is then less difficulty in aiming, but there is a danger of great inaccuracy.
- Fire a pellet and note the time taken for the pellet to travel between the two
circuit breakers, t.
- Calculate the pellet velocity, v , using v = s/t where s is the distance between the foil strips.
- This experiment is a direct method of measuring the speed of a rifle pellet. This can be confirmed by using the conservation of momentum to calculate the velocity of the pellet. See the experiment:
- The length of the flight is short and so the timing is only a few milliseconds, which will not be very accurate. The result should, however, give a good order of magnitude value, within about 10%.