Understanding Work: Kinetic vs. Potential Energy

[dragon-kazooie]

Oh, net work :). That's the big thing here! So look, when you lift something upwards, you're doing work on it, right? BUT it's not changing in kinetic energy! How? Because gravity is doing work that is equal to yours but opposite in sign, so there is zero NET work done on the object, thus its kinetic energy won't change ;).

Hyperphysics is not defining work via the change in kinetic energy, they are describing the work-energy principle. And, as guitarphysics points out, it is the net work on a particle (including all forces acting) that gives the change in kinetic energy.

So, you're saying that work is equal to Δk or -ΔU, but net work is only equal to Δk? (Because in the crane example, there's still a change in potential energy, even though there's no change in kinetic energy and no net work.) That seems really weird.

 

[Doc Al]

So, you're saying that work is equal to Δk or -ΔU, but net work is only equal to Δk? (Because in the crane example, there's still a change in potential energy, even though there's no change in kinetic energy and no net work.) That seems really weird.

It's the net work that gives the change in KE. Of course, if only one force acts then the work it does will equal the change in KE, since that is the net force.

Why weird? -ΔU is just the work done by gravity. The work done by the crane, plus the work done by gravity equals ΔKE:
Work_crane + Work_gravity = ΔKE
Work_crane - ΔU = ΔKE
Work_crane = ΔKE + ΔU

In this case, ΔKE = 0, so the work done by the crane = ΔU.