Suppose a certain rope is tied at one end, while the other end is suspended over a pulley. A hanging mass is attached to this end, creating tension in the rope. You begin plucking it and sending transverse pulses down the length of the rope (30 m), which reflect at the tied end and return to you.
You notice that when a 10 kg mass is hung on the free end, the pulses complete a round trip in 3 seconds.
Find the mass of the backpack in kg.
However, when you attach your rather heavy backpack to the free end, the pulses complete their round trip in only 2 seconds.