You learnt before that going up the lift causes you to feel 'heavier' and going down makes you feel 'lighter'. Why is that so? First, lets analyze the Newton's Third Law of Motion:
So, when you are standing up, there are two forces acting upon you, the gravity, mg (downwards) and the normal reaction force, R (upwards).
How about the formula thingie? Well, here is the correct technique to write for the SPM...
"Why the formula R-mg=ma? What is R? What is mg? What is ma?"
mg is mass(m) multiply with gravitational acceleration(g) which results in weight, which in this case it goes downward. Feel the weight of all objects around you. It goes downward on your hand, right? That's mg.
R is the reaction force opposing the mg. It's a normal phenomenon to all objects. That's why it is called normal reaction force. In this case, the floor/ground is 'pushing' your feet upward. It also causes the reading on the weighing scale.
ma is the resultant force. It is the result from the 'clash' between the opposing R and mg (like cats and dogs, R and mg fight each other!). So, the result can be felt in the acceleration (a) in ma.
In this case of standing up, no acceleration is felt. That's why you are stationary. The R cancelling mg when added together, causing the resultant force to be zero.
Just like a war with no winning side...
In the seminar, we shall discuss the case of resultant force in the elevator.