If the KE of a system changes by 85 Joules, then how much work is done on the system by the net external force?

When net positive work is done on a system then we expect the KE of the system to increase; when positive work is done on the system the net force on the system has a positive component in the direction of motion.

When a system does net positive work (work done BY the system is positive) then we expect that the system is exerting a net force in its direction of motion and that the KE of the system will therefore decrease.

• when the system does positive work (positive work done BY the system) then the net force exerted ON the system is equal and opposite to that exerted BY the system.
• In this case the (equal and opposite) net force exerted on the system is therefore in the direction opposite to the motion of the system and is negative.
• So when the net work done by the system is positive the net work done on the system is negative.

When negative work is done ON a system this implies a decrease in energy.  If negative work is done by the net force then change in the KE of the system is negative--i.e., KE must decrease.  Recall that when negative work is done ON the system the work done BY system is positive.

• The magnitude of the work done by the system is equal to that of the work done on the system.
• Positive work done by the system implies negative work done on the system.

The KE change of a system will therefore the equal to the work done ON the system by the net force acting on the system, and opposite to the net work done BY the system.

In this case the net work done ON the system is 85 J, the net work done BY the system is -85 J, and the KE of the system changes by 85 Joules.