Take a hollow plastic ball (dia 3 cm to 5 cm) using a needle. Pass a thread of about 105 cm along a diameter of the ball. Tie a knot at one end. Hang the ball from a rigid support so that the ball is free to oscillate. Make a small hole (5 mm dia) in the ball near the top. Fill it with sand and find its time period of oscillation. Empty the ball and fill it with steel balls/iron filings and repeat the experiment. Similarly carry out the experiment with common salt. Compare the time periods obtained in the above three cases. What do you find? Explain your finding.