00:01
Okay, a weightlifter lifts a mass 50 kilograms from the floor to a height of half a meter.
00:06
We want to look at the work done by the lifter on the barbell and the work done by gravity on the barbell.
00:15
So here's our dude lifts this nice big weight.
00:29
We've got some big muscles there.
00:31
So work.
00:32
Let's write down the equation for work.
00:34
So work is equal to...
00:37
In this, we're going to use the dot product form here, so a force dotted with a distance.
00:44
If you haven't seen the dot product form, that's fine.
00:49
I will explain it without that, i guess.
00:52
Basically, work is positive when the force and the displacement in the same direction.
01:08
And work is negative when the force and displacement are in opposite directions.
01:31
Okay, so what does that mean? well, if you have this mass and you lift it up, what is the force, or what direction is the force that you're placing on it? well, since you're lifting it up, the force you place is up.
01:50
And where's the displacement? well, the displacement is up as well, right? because it's moving up.
01:58
So if you lift something up, then the work you do on it is positive.
02:03
Let's think about gravity now...