Question

One end of a plastic foot ruler is held tightly at the edge of a table and the other end is plucked. Name the vibrations produced in the ruler.

Now the ruler is pushed inside partially and plucked again from its free end. State with a reason whether the frequency of vibration increases or decreases.

Two persons A and B are standing in front of a cliff in the same line 170 m apart as shown in the diagram. Person B fires the gun and hears the echo in 3 s. Then the person A standing in front of the person B fires the gun. (The speed of sound in air is 340 m/s.)

Will the position of the centre of gravity change if a hollow sphere is completely filled with mercury? Give a reason for your answer.

Calculate the minimum distance needed in water to hear the echo. (Speed of sound in water is 1500 ms⁻¹. Persistence of hearing is 0.1 s.)