Acceleration in an elevator and Physical Laws

AI Thread Summary
Measurements of weight changes in an elevator can be used to calculate its acceleration, relating to Newton's laws. When the elevator ascends, the apparent weight increases, indicating positive acceleration, while a decrease suggests negative acceleration. The calculations provided show that an increase in measured weight corresponds to upward acceleration, while a decrease indicates downward acceleration. Newton's third law explains that while the forces are equal and opposite, they act on different objects, allowing for movement. Understanding these principles clarifies how to determine the elevator's acceleration from weight measurements.
Tomdoml
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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
 
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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.
 
So would my attempted solutions be correct in determining the acceleration in the elevator?
 
Tomdoml said:
So would my attempted solutions be correct in determining the acceleration in the elevator?
Correct.
 
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?
 
Last edited:
Tomdoml said:
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
 
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