Solving the Elevator's Acceleration: Examining Newton's Laws

[Vipercobra]

Homework Statement

An object is placed on a scale in an elevator which reads 8kg at rest. This scale shows 9.2kg when the elevator accelerates. What is the direction and magnitude of the elevator's acceleration.

Homework Equations

F=MA, F=MG, Fnormal=m(a-g)

The Attempt at a Solution

Can't really work out how to approach this equation as normally acceleration is given while in this case the change in weight is given. Tried substituing 8 into F=MG which gave 78.4 but I don't know how to process from here. I also tried using the formula Fnormal=m(a-g) but acceleration is unkown so I don't know how to proceed.

I also find Newton's third law confusing when applied to an accelerating lift. From what I have read each force has an action reaction pair in which both are equal. When an elevator accelerates upwards the upward force of the elevator is larger than the downwards force of an object, MG. What is the action reaction pair in this instance? The force upwards on the object isn't equal to the downwards force because if they were the object wouldn't be accelerating upwards.

 

[yster]

Lets work from the bottom up

The action reaction pair.
Action: The force due to gravity and the force that cause acceleration
Reaction: the force from the scale keeping the object stationary

This action reaction (equal and opposite reaction) theory only applies to objects that are not accelerating i.e they are either stationary or traveling at constant velocity.

Now for your problem

A scale measure weight, which is a force, not mass.
I assume you know how to convert from mass to weight for calculation purpose.
I also assume you know that gravity is a force that causes acceleration when the is nothing to react to the force like when it is placed on a scale.

Given this information all you have to do is apply logic to determine what direction the lift accelerate for the object to press harder on the scale's surface. then you can determine the actual acceleration.

Relevant equations

F= mg
F=ma

 

[StaceyPurcher]

So then does that mean that the formula for apparent weight is simply m(a+g)??