When a string is barely strong enough lifts a heavy weight, it can lift the weight by a steady pull; but if you jerk the string, it will break. Explain in terms of Newton’s laws of motion.

The first body exerts a force on the second body, and the second body exerts an equal and opposite force on the first body.

${\overrightarrow{\mathbf{F}}}_{\mathbf{12}}\mathbf{=}\mathbf{-}{\overrightarrow{\mathbf{F}}}_{\mathbf{21}}$

This action and reaction of forces act on different bodies. These forces act on each other as long the objects are in contact.

When the string is pulling the heavy weight, it is exerting a small force resulting in a small acceleration of the weights. By newton’s third law, the weights will exert equal and opposite force on the string. When the string is jerked, a large pulling force is exerted on the weights, and as a result, a large force in equal magnitude and the opposite direction is exerted on the string by the weights. This large reaction force is greater than the tolerance limit of the string, and the string, as a result it breaks.