00:01
In this problem, we're asked to find some information about forces being applied using a simple machine, a movable pulley, and the distances that the forces have to be applied, and then what work is done by the person using the machine and the machine itself.
00:23
So let us take a look at the forces by drawing a quick free body diagram of the canister.
00:31
And pulley system there.
00:34
The force is acting up on the canister in this movable pulley.
00:38
There's a force f going up, and there's another force f going up.
00:42
So there's two of those forces going up, and then it's a force of gravity going down.
00:48
Now, we know we want to lift this at a constant rate.
00:51
Therefore, we know that 2f minus mg had better add up to zero.
00:58
If we lifted at a constant rate, or to say another way, the force must be equal to mg over 2.
01:08
And if you plug in the numbers there, you get the m is 20 kilograms, and g, let's say, g is 10 newtons per kilogram today.
01:17
So this gives us 20 times 10, that's 200 divided by 2.
01:24
That's 100 newtons is the amount of force that we have to apply force f.
01:32
Now, but how about the distance that it has to go? well, if you look at the system carefully, you see that if you pull down a certain distance over here, d, each the ropes has to get shorter by an amount equal to half of d...