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Exercise 10    If the pendulum is displaced from its lower position, gravity acts to return it to that position. Without gravity there is no force to bring the pendulum back. The energy that drives the pendulum is gravitational potential energy.

Exercise 11    The tension in the violin string provides the elastic force to make it vibrate. The energy that drives the vibration is elastic potential energy.

Exercise 15    The pitch or frequency increases if the string tension is increased or if the mass per unit length is decreased. To raise the pitch you could either increase the tension or decrease the mass per unit length.  For a given string the mass is constant, so to raise the pitch the musician makes the string "shorter" by moving his or her finger down on the string.

Exercise 18    Since the pitch depends on the tension, temperature can affect it. For example, if the temperature increases, the length of the string increases a little bit and this tens to lower the tension. Thus the tension in the string must be adjusted to accommodate temperature changes.

Exercise 22    The pitch or frequency is proportional to the speed of sound for a given organ pipe. Since the speed of sound is higher in helium. the frequency will be higher.