Homework Help: Elevator problem

1. Jun 26, 2008

devanlevin

in a problem where there is a 100kg bag placed on the floor of a 1000kg elevator, going up with an acceleration of 0.8m/s^2, what is the normal force applied on the bag by the elevator.

i thought that the normal force would be mg(using the mass 1100kg)-F(the force lifting the elevator) but its not, its the mg(using only the 100kg) + ma(again using 100kg)

therefore
N=980+80=1060

can anyone explain the logic in this, if it is correct.

2. Jun 26, 2008

alphysicist

Hi devanlevin,

It is correct. From the equation in your post, I think the answer you're looking for is this:

When we draw a force diagram for an object (such as the 100kg bag here), we only include those forces that are actually acting on the object itself. The force lifting the elevator (from the cable) is acting on the elevator, and so is not included in the force diagram for the bag. (It obviously has an effect on the bag, but it does not directly act on it.)