The String & Bob
Take a piece of thread and tie one end of it with the hook of a metal bob. If the metal bob is not available then you can use any object that has some weight.
Holding the String
After tying one end of the string with the metal bob, hold the other end with your thumb and index finger.
The Circular Motion
As the bob moves in a circular motion, it will be obvious that the more forcefully you rotate the string, the heavier the bob gets. This obeys Newton’s Second Law of Motion.