Page 314 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
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Magnetic resonance imaging (MRI) is increasingly used in medicine to produce images such as Figure 19.36, showing aspects of a patient’s internal organs. Radio waves having a range of frequencies are used, and particular frequencies are absorbed by particular atomic nuclei. The frequency absorbed depends on the type of nucleus and
on its surroundings. By analysing the absorption of the radio waves, a computer-generated image can be produced. (There is much more about how MRI works in Chapter 32.)
A radio or television also depends on resonance for
its tuning circuitry. The aerial picks up signals of many different frequencies from many transmitters. The
tuner can be adjusted to resonate at the frequency of the transmitting station you are interested in, and the circuit produces a large-amplitude signal for this frequency only.
Big ideas in physics
This study of simple harmonic motion illustrates some important aspects of physics:
■■ Physicists often take a complex problem (such as how
the atoms in a solid vibrate) and reduce it to a simpler, more manageable problem (such as how a mass–spring system vibrates). This is simpler because we know that the spring obeys Hooke’s law, so that force is proportional to displacement.
■■ Physicists generally feel happier if they can write mathematical equations which will give numerical answers to problems. The equation a = −ω2x which describes s.h.m. can be solved to give the sine and cosine equations we have considered above.
■■ Once physicists have solved one problem like this, they look around for other situations where they can use the same ideas all over again. So the mass–spring theory also works well for vibrating atoms and molecules, for objects bobbing up and down in water, and in many other situations.
■■ Physicists also seek to modify the theory to fit a greater range of situations. For example, what happens if the vibrating mass experiences a frictional force as it oscillates? (This is damping, as discussed above.) What happens if the spring doesn’t obey Hooke’s law? (This is a harder question to answer.)
Your A level physics course will help you to build up your appreciation of some of these big ideas – fields (magnetic, electric, gravitational), energy and so on.
QUESTION
24 List three examples of situations where resonance is a problem, and three others where resonance is useful. In each case, state what the oscillating system is and what forces it to resonate.
Figure 19.36 This magnetic resonance imaging (MRI) picture shows a man, a woman and a nine-year-old child. The image has been coloured to show up the bones (white), lungs (dark) and other organs.