2018 MJC P2 Soln

2018 MJC H2 Physics Prelim Exam Paper 2 Suggested solutions 1 (a) (i) When the system oscillates without any external periodic force applied , its frequency is called its natural frequency. Comments: Considering it is a recall question, it is poorly attempted. To get the mark, students are to mention that “it is the frequency when….” (ii) Resonance occurs when the frequency of the driving force (driving frequency) is equal to the natural frequency of the system, giving a maximum amplitude of vibration. Comments: Generally well done. However a handful of students left out the effect of resonance which is giving maximum amplitude. (b) 1) Microwave Cooking In a microwave oven, microwaves with a frequency similar to the natural frequency of vibration of water molecules are used. When food containing water molecules is placed in the oven and radiated by microwave, the water molecules resonate, absorb energy from the microwaves and get heated up. This absorbed energy then spreads through the food and cooks it. The plastic or glass containers do not heat up as much since they do not contain water molecules. 2) Radio Receiver Our air is filled with radio waves of many different frequencies which the aerial (antenna) picks up. The tuner can be adjusted so that the natural frequency of the electrical oscillations in the circuits is the same as that of the radio wave transmitted from a particular station (the desired station). The radio waves of that particular frequency cause much larger oscillations (due to resonance) resonance compared to the radio waves of other frequencies. 3) Magnetic resonance imaging Strong, electromagnetic fields of varying radio frequencies are used to cause oscillations in atomic nuclei. When resonance occurs, energy is absorbed by the molecules. By analysing the pattern of energy absorption, a computer - generated image can be produced. The advantage of MRI scanner is that no ionising radiation (as in the process of producing X-ray images) is involved. 4) Swing The swinging of the legs / the person pushing the swing has to be synchronised and at the same frequency as the natural frequency of the swing so that a large amplitude of oscillation can be obtained.

Meridian Junior College H2 Physics Paper 2 JC2 Preliminary Examinations 2018 2 5) Musical instrument Musical instruments are set into vibrational motion at their natural frequency when a person hits, strikes, strums, plucks the object. Each natural frequency of the object is associated with one of the many s tanding wave patterns by which that object could vibrate. The vibrations of the lips (brass instruments) / vibration of a wooden strip (woodwind instruments) against the mouthpiece matches one of the natural frequencies of the air column inside the brass instrument. This forces the air inside of the column into resonance vibrations. (c) 1) Earthquakes and tidal waves During an earthquak