Wave speed in a solid

Teachers' notes

This activity is in the A2 unit Build or Bust? and links together the work on earthquake waves, simple harmonic motion and properties of materials. It is immediately preceded by an activity in which a pulse is sent down a row of trolleys, modelling on a large scale, what is happening within the metal bar.

Typical results

Speed of wave by experiment = 2 x 0.700 m/401 µs = 3490 m·s ^-1.

Speed of wave by formula = 5090 ms ^-1.

The differences in values obtained reflect the anisotropy of the rods used, the degree of cold working they underwent in manufacture, variations in E and with temperature, the extent to which shear forces come into play and so present a need to take the Bulk Modulus into account, as well as the uncertainty in the experimental results.

Students may note, especially with ADC-40/42, that the fall in voltage after contact is lost looks somewhat exponential. This is likely to reflect an RC component in the interface and might provide a timely reminder of this type of decay.

Apparatus

• retort stand (2)
• bosses (2)
• clamps (2)
• large elastic bands (2)
• metre rule
• metal rods* (0.7 to 1.3 m long, approximately 15 mm diameter or greater) steel, aluminium, copper etc. Clay Bros. Metal Supplies
• hose clip (to fit rods) DIY
• claw hammer (not ball and pein type) DIY
• hose clip (to fit neck of hammer) DIY
• emery cloth DIY
• 3 V battery
• DrDAQ or ADC-40/42 Pico
• BNC plug to 4 mm plugs Pico MI029 (for ADC-40/42 only)
• PC running PicoScope
• extra flexible connecting wire red JPR 780-080
• extra flexible connecting wire black JPR 780-081
• 4 mm stackable plug red (2) JPR 705-285
• 4 mm stackable plug black JPR 705-286
• in-line 4 mm connectors (2) or make use of stackable plug facility

*For steel and aluminium you may be able to utilize retort stand rods.

Attach a 0.7 m length of red connecting wire to the hammer neck via a hose clip and terminate the other end with a red 4 mm plug.

Attach a 0.7 m length of red connecting wire to the metal rod via a hose clip and terminate the other end with a red 4 mm plug. Ensure that the areas of contact are cleaned with emery cloth first.

If a DrDAQ is being used attach a 0.7 m length of red connecting wire to its V terminal and terminate with a red 4 mm plug. Similarly attach a 0.7 m length of black connecting wire to its GND terminal and terminate with a black 4 mm plug.