Answer
See work.
Work Step by Step
With a steady pull on the string, if one lifts the mass upward at constant velocity, the tension in the string is just equal to the weight, because Newton’s first law tells us that the net force on the object is zero.
With a sharp jerk on the string, when one is trying to accelerate the mass upward, the upwardly-directed tension in the string is much greater than the weight, because Newton’s second law tells us that the net force on the object is upward. This causes the string to break.
