6.1 Waves in Air, Fluids & Solids

Students in a classroom are trying to calculate the speed of different waves along a string. The equipment provided is shown in Figure 1.

Figure 1

Describe an experiment, using the equipment in Figure 1, that would allow the students to determine the wave speed of different waves along the string.

The students double the frequency each time they perform the experiment and sketch the waves they observe. These sketches are shown in Figure 2.

Figure 2

One of the student makes the conclusion:

"The frequency is doubling so the speed must also be increasing".

Explain why this statement is incorrect and describe how the speed varies.

The students finish early and start a second experiment to study the effect of varying tension force in the string. 

They add mass to the end of the string and vary the frequency until they produce the following wave on the string:

Figure 3

They then add more mass and change the frequency until the pattern from Figure 3 appears again.

They then determine the tension and wave speed at each wave speed.

State the dependent and independent variables in their experiment. 

Name the variable the students must control in this second experiment.

Higher Only

Figure 1 shows an ultrasonic transmitter and receiver being used to detect cracks in steel rails. The transmitted and reflected pulses are shown on an oscilloscope's screen below.

The oscilloscope time base measures 10 microseconds per division on the horizontal axis.

Figure 1

Explain why there are two reflected pulses for transmitter B on the right.

Higher Only

The speed of the ultrasound wave in steel is 6000 m/s.

Calculate the thickness of the steel.   

Higher Only

Calculate the depth of the crack as a percentage of the thickness of the steel.

A student lights a candle and places it in front of a mirror.

The student can see an image of the candle flame in the mirror. This is represented in Figure 6 below.

Figure 6

Complete the ray diagram in Figure 5 above, showing how the image is formed.

Mark the position of the image.

The image is virtual.

What is a virtual image?

Higher Only

When sound waves enter the human ear, they encounter the eardrum and cause it to vibrate.

Describe the process by which the sound waves cause the eardrum to vibrate.

Higher Only

Figure 7 below shows a ship using an echo sounder to measure the depth of the water.

Figure 7

The echo sounder produces short bursts of ultrasound.

What is ultrasound?

Higher Only

The ultrasound pulse travels through the water at a speed of 1500 m/s.

A period of 320 milliseconds passes between a pulse being emitted and received by the echo sounder.

Calculate the depth of the water.

Higher Only

Ultrasound is also used in medicine to scan prenatal babies.

X-rays can also be used to image the body, but are not used to scan prenatal babies.

Explain why.

Higher Only

Earthquakes produce two different types of seismic waves: P-waves and S-waves.

Circle the relevant words below to indicate the nature of both P-waves and S-waves.

P-waves travel with an average speed of about 7000 m/s.

Some P-waves are produced by a minor earthquake and detected a short while later by a seismometer 3000 km away.

Calculate how long it will take the P-waves to reach the seismometer.

Higher Only

Figure 7 below shows S-waves passing through the internal structure of the Earth.

Whilst S-waves can be detected on the side of the Earth nearest to an earthquake, they cannot be detected on the opposite side.

Explain why, and describe what this tells us about the internal structure of the Earth.