Acceleration in an elevator and Physical Laws

[Tomdoml]

Homework Statement

I've gone out and measured my changing weight in an elevator using a bathroom scale, taking measurements each second, with the elevator ascending 4 storeys. I am wondering if it is possible to work out the acceleration of the elevator from these results, and if so how they relate to Newton's 2st and 3rd law.
e.g.1 mass = 75kg
mass inside elevator = 81kg
e.g.2 mass = 75kg
mass inside elevator = 70kg

Homework Equations

F=ma
g=9.8
Those should break down to give these?
F = m * (g + a)
a = F / m – g

The Attempt at a Solution

e.g.1 a = (81 * 9.8)N / 75kg – 9.8ms-2
a = 0.784ms-2 Upwards

e.g.2 a = (70 * 9.8)N / 75kg – 9.8ms-2
a = -0.653ms-2 Upwards

 

[rl.bhat]

HI Tomdoml, welcome to PF.
When the elevator is not moving the weight is
F1 = m*g. = 75 kg.wt.
When the lift is moving up, the weight is
F2 = m(g+a) = 81 kg.wt.
Now take the ratio and find the acceleration.

 

[Tomdoml]

So would my attempted solutions be correct in determining the acceleration in the elevator?

 

[rl.bhat]

So would my attempted solutions be correct in determining the acceleration in the elevator?

Correct.

 

[Tomdoml]

The one things confusing me in regards to this experiment is the effect that Newtons 3rd law has on the results. Since the elevator pushes me up, i push back down with equal force which is how i figure out the acceleration of the elevator, but if the forces were equal wouldn't that just result in the elevator not moving at all?

 

[rl.bhat]

The one think confusing me in regards to this experiment is the effect that Newtons 3rd law has on the results. Since the elevator pushes me up, i push back down with equal force which is how i figure out the acceleration of the elevator, but if the forces were equal wouldn't that just result in the elevator not moving at all?

The action and reactions equal and opposite, but they act on different objects