Please clear up this confusion for me:
I just watched the video on khan academy @7:30 where the guy explains newton's 3rd law. He explains that for a box on a table, the forces equal out so it's at rest. I also understand that $F=ma$ so an object with less mass may have more acceleration for an equal force.
But then he said if you put the table on a rocket ship, the force of gravity is not a partner force to the force moving upward. I'm having a very hard time reconcilling the terminology of what's happening here because I can calculate the force of objects moving sideways along the ground, but am getting lost somewhere in how gravity is not a partner force to the force on a body moving upward.
I'm also confused about a force "pushing down" but at the same time that means a force upward? So then is the force down or up? I know there is an equal and opposite force somewhere in this mess but also that something has to be exceeding gravity or the rocket wouldn't be able to escape?
I'm looking at other stack threads and they're saying it has something to do with different objects relative to each other but that's even more confusing.
Best Answer
Newton 3rd law forces, what your link refers to as "partner forces", are equal and opposite forces that act on different objects. The motion of an object is due to the net force acting on that object, per Newton's 2nd law. It is sometimes difficult to distinguish these forces without the aid of free body diagrams.
FIG 1 below shows the forces acting on the box, table and Earth for the "stationary" system. The Newton 3rd law pairs are circled in blue. The forces responsible for the motion of the box and table are circled in red.
Note that the normal reaction force of the table acting up on the box is simply the force of gravity acting down on the box. The net force acting up on the block is the normal reaction force of the table minus the downward force of gravity of the box, for a net force of zero. The same applies to the table except that the force acting down on the table is the sum of the gravitational forces acting downward on the box and table.
FIG 2 shows the rocket with the table and box as its only contents. I didn't actually view the link, but when its says "the force of gravity is not a partner force to the force moving upward" what it probable means is the upward force on the box is no longer simply the force of gravity acting down on the box as in FIG 1. As shown in FIG 2 the upward reaction force of the table acting on the box minus the force of gravity acting downward on the box must equal the net upward force on the box responsible for its acceleration.
Hope this helps.