# Work-Energy Theorem Question

Ken G
Gold Member
Perhaps a lot would fall in place if we told you why the force needs to be 10 N. It's because you said there is no gain in KE-- the mass is lifted at nearly zero speed. If you use a force larger than 10 N, you will accelerate the mass, right? (Assuming, as nasu said, the mass is 1 kg.) To see if you understand, try the problem with an upward force of 20 N for 1 m, and calculate the work done by the 20 N force, the work done by gravity (and only that work can be accounted for as "coming from" the gravitational potential energy, though remember it's negative), and the change in kinetic energy.

By the way, a useful analogy for energy is money. Think of KE like the money the object has "in its wallet", and U like a "bank account" (if negative, that's like taking a loan from the bank), and external work like "money being paid to the object." The work-energy theorem then says the money in your wallet increases by the amount you are paid, minus what is getting deposited in your bank account. Each different force over the distance displaced is like a different way that money is changing hands.

Ah... I see... Well, thank you all for clearly up my conceptuals regarding work!

Mark Harder
Gold Member
The notion of energy dissipation has been mentioned in several posts above. Just to be clear, when we consider heat and other forms of energy lost or gained by a mechanical system, we are in the realm of thermodynamics. There, changes in energy are not equivalent to work done (i.e. force * distance.) If you want to work with energy in a thermodynamic system and assume its energy is conserved, you have to include in the system the environment that absorbs or supplies the energy to the mechanical, work-doing part of the system. Only then is the total energy of the system conserved. That's the essence of the First Law.

Ok! Thanks! All of you helped a lot to allow me to understand a question that had been driving me crazy for a while!