A fat person is standing on light plank floating on a calm lake. The person walks from one end to the other on the plank. His friend sitting on the shore watches him and finds that the person hardly moves any distance because the plank moves basckward about the same distance as the person moves on the plank. Explain.

A person standing on a platform finds that a train moving with velocity, 0.6c takes one second to pass by him. Find (a) the length of the train as seen by the person and (b) the rest length of the train .

An old person moves on a semi circulasr track of radius 40.0 m during a moirning walk. If he starts at one end of the track and reaches at the other end, find the distance covered and the displacement of the person.

A person writes a letter to four of his friends. He asks each one of them to copy the letter and mail to four different persons with instruction that they move the chain similarly. Assuming that the chain is not broken and that it costs 50 paise to mail one letter. Find the amount spent on the postage when 8th set of letter is mailed.

A string of linear mass density 0.5 g cm^-1 and a total length 30 cm is tied to a fixed wall at one end and to a frictionless ring at the other end. The ring can move on a vertical rod. A wave pulse is produced on the string which moves towards the ring at a speed of 20 cm s^-1 . The pulse is symmetric about its maximum which is located at a distance of 20 cm from the end joined to the ring. (a) Assuming that the wave is reflected from the ends without loss of energy, find the time taken by the string to regain its shape. (b) The shape of the string changes periodically with time. Find this time period. (c) What is the tension in the string ?

A person travels on a spaceship moving at a speed of 5c / 13 . (a) Find the time interval calculated by him between the consecutive birthday celebrations of his friend on the earth . (b) Find the time interval calculated by the friend on the earth between the consecutive birthday celebrations of the traveller.

Two audio speakers are kept some distance apart and are driven by the same amplifier system. A person is sitting at a place 6.0 m from one of the speakers and 6.4 m from the other. If the sound signal is continuously varied from 500 Hz to 5000 Hz, what are the frequencies for which there is a destructive interference at the place of the listener? Speed of sound in air = 320 ms^-1