# Newtons third law and elevators

1. Oct 3, 2011

### kthejohnster

If two objects are stacked in an accelerating elevator what is the force equal and opposite to the normal force on the top object?

I was thinking that it has to be either the force of gravity or contact force between the two objects could it be anything else?

2. Oct 3, 2011

### harrylin

Hi welcome to physicforums.

Here you are dealing with four (or even five) objects: the earth, the elevator (and the cable that holds it up) and two objects inside the elevator. The force of gravity, using classical physics, is pulling on he elevator and the two objects.

Newton's third law applies to all force pairs. In this case you can just focus on the force pair [top object] <-> [bottom object]. The force that is felt by the top object is the contact force from the bottom object; inversely the bottom object feels the same force (but of course in opposite direction) from the top object at its top surface.

The magnitude is equal to the force from gravitation plus the force from acceleration.