Physics in daily life # 4
Classical Mechanics
Level
1
Suppose that a ball of mass 2 kg is hanging from a string . You have cut the string . Just the instant after you cut the string , you notice a jerk in the string in the upward direction. What is responsible for this jerk ??
weight of the string
tension in the string
tensile strenght of the string
none of these
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Every material has some elastic property and so does this string. When under tension this string elongates a little and hence can be compared to a stretched spring. When the string is cut, the tension has to be zero in the string so it has to go back to its original length. While going back to its original length, its lower part will acquire some velocity and this velocity is responsible for the jerk which we see.
Since it also has some mass (although very small) it will acquire some momentum and will take some time(although very small) for this momentum (or Kinetic Energy) to vanish by frictional forces in the internal threads of the string and also to the viscous force of the atmosphere.