RP8: Waves

Question 1

What is the aim of this experiment?

Answer 1

• Measure the frequency, wavelength and speed of waves by observing water waves in a
  ripple tank and waves on a stretched string or elastic cord.

Question 2

Describe how you can measure and observe the water waves in a ripple tank (method)

Answer 2

1. Fill the ripple tank so the water has a depth of approximately 5mm. Place the ripple
   tank on top of a piece of white paper or card.
2. Place a wooden rod on the surface of the water and attach it to the low-voltage
   power supply and motor. Add a lamp to the circuit and hold the lamp above the ripple
   tank.
3. View the wave pattern from the side of the tank, looking through the water.
4. To measure the wavelength, place the metre ruler perpendicular to the wavefronts
   on the page. Measure across as many wavefronts as possible and divide by the
   number of waves.
5. To measure the frequency, count the number of waves passing a particular point in
   the wave tank over a given time (measure 10 or 20 seconds using a stop clock).
6. To calculate the wave speed, multiply the wavelength by the frequency.

Question 3

What is a ripple tank?

Answer 3

• A shallow glass tank with an oscillating paddle to create waves
• It is illuminated from above so the waves can be seen on the surface below the tank

Question 4

Approx how much water should be put in the ripple tank? What would the consequence be of too much water?

Answer 4

• Water should be poured to a depth of around 5mm
• If there is too much water, the rod won’t produce clear waves & they will be less clearly projected onto the card below

Question 5

How should the wooden rod be set up in the ripple tank?

Answer 5

• Placed into the tank so that it just touches the surface of the water

Question 6

Why should you measure across more than one wave?

Answer 6

• To reduce the uncertainty in the measurement and improve the accuracy of the value

Question 7

Describe how you can measure and observe standing wave on a vibrating string (method)

Answer 7

1. Produce a standing wave on the vibrating string by adjusting the frequency or
   the generator, the position of the wooden bridge and the tension in the string
   (by adding or removing masses). A standing wave is created when the wave
   doesn’t appear to move horizontally, instead the string appears to oscillate
   only vertically.
2. To measure the wavelength, use a metre ruler to measure across multiple
   standing waves and divide by the number of total waves.
   The wavelengths of a standing wave is measured
   across two halves, as shown in the diagram to the left.
   Source: slideplayer.org
3. To measure the frequency, use a stopwatch to time wave oscillations over ten
   complete cycles. If the wave is slow enough, time the point at the centre of the
   half-wavelength, starting at equilibrium and counting every other time the
   string passes the equilibrium as a complete cycle. Divide this value by 10 to
   find the time period. Then use the equation,to find the frequency.
4. To calculate the wave speed, multiply the wavelength by the frequency.

Question 8

List some safety precautions

Answer 8

• Take care working with water and electricity in close proximity. Mop up any spillages
  immediately.
• Don’t stand beneath the weight stack, in case weights fall off or the string snaps.
• Wear safety goggles when using the oscillating string. Be cautious that the string
  may snap if at a high frequency and under too much tension. Avoid operating the
  vibration generator at high frequencies.