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The answer is this one, and now, I'm not going to go through all of these other four cases.
I'll leave that to you to show that in each of these other four cases, the total work done
when you really include everything--gravity, pushing etc., really must be nonzero
because in each of these, I said, happened at constant speed.
Here, when the object falls, well, that's clearly not constant speed.
We know it accelerates. Now, we're getting somewhere.
This case is so special out of these five because the motion is changing in a very specific way.
When we keep track of the total work which I'll abbreviate with a little T down here,
we find that it causes the object's kinetic energy to change which I will label like this.
This is so important that I'm going to write it. Work causes changes in kinetic energy.
Now, we don't know what kinetic energy is quite yet.
Stay tuned one second but this statement is known as the work energy theory and it's vital to physics.
Now, let's talk about what this whole energy nonsense is.