everyday-lifenewtonian-mechanicsstatics
I was carrying a couch with my flatmate yesterday, and I started thinking about this.
Often when carrying objects like this, one person will be taller and has thus lifted his end of the couch higher than the other person's. Additionally, one can intentionally lift his end of the couch higher, for comfort.
My question is, in situation is one person carrying more weight than the other?
Just don't start writing equations without a complete Free-Body-Diagram(may be rough diagram) according to the frame of reference..
Now . Newtons law. $$\sum \vec{F_{ext}}=\frac{dp}{dt}=ma(for\ constant \ mass)$$
So, $$N-mg=\sum \vec{F_{ext}}=ma$$ Where system is boy in ground's frame.
Here we equate the net external force to the acceleration of body . $ma$ is not a force it is the measure of net force which causes body to accelerate.
here we have to add a fictious force $-m\vec{a}$ where $\vec a$ is the acceleration of frame .
Here too $$N-mg-ma=\sum \vec{F_{ext}}=0(as \ in \ frame \ of \ elevator \ boy \ is \ at \ rest)$$ Again you get $$N-mg=ma$$
The black machine is a weight lifting machine. It is self contained with no power source.
If it can lift an external weight and return to its original state as shown below, it is a perpetual motion machine. Suppose the blue weight is water. We could add a water wheel and generator on the right. You start at the top and work your way to the bottom. Then you pour the water onto the water wheel and generate electricity, This leaves you back at the top illustration, except that you have generated free power in between.
Feynman is assuming that a perpetual motion machine is impossible.
Feynman is defining a reversible machine as one like the black machine with the blue weight left out. Without a power source, it can lower 3 weights as it raise one weight 3 times as far. Likewise it can run the other way without a power source.
A reversible machine is also impossible, but only just. You can make a machine where it will lower 1 weight with a very tiny blue weight to overcome friction. But now it will only run one way.
He assumes for the sake of argument that we do manage to make a reversible machine like the black one, He uses it to show that no machine without a power source can outperform a reversible machine.
For example, the illustration above shows a proposed machine that does outperform a reversible machine. It lifts more weight. Because it lifts more weight, it is a perpetual motion machine, and perpetual motion machines cannot exist.
Another proposed machine might lift 1 weight farther than a reversible machine. Any machine like this without a power source has the same problem. Because it outperforms a reversible machine, it is a perpetual motion machine.
This proposed black and blue machine has no power source. It has a jack underneath one weight that lifts it higher than the black reversible part can do alone. You can get free power by lowering the weight back where it was. So the black and blue machine is a perpetual motion machine.
Best Answer
I believe the following diagram tells you everything you need to know: when the center of mass is above the support point, then the lower point will carry more of the weight since it is acting closer to the center of mass ($x_1 < x_2$) and torque balance requires that $F_1 x_1 = F_2 x_2$. Conversely, if the center of mass is below the support, the higher point will carry more of the weight when the object is tilted.
Understanding this diagram really helps when you are moving large objects up or down the stairs: the lower person should grab the object higher up (say the arms of the couch) if you want to achieve more equal weight distribution.
ADDENDUM
In the second case, if the person on the right pushes against the couch, it is possible for the person on the left to end up carrying the entire weight of the couch. In the case of carrying things up stairs that can easily happen... Which is why movers often use webbing straps to permit/ensure a more equal distribution of effort (and to allow lifting with shoulders/back/knees and not just arms).