- #1
Aysce
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Homework Statement
A mass of 2kg is sitting on a dial scale balance on the floor of a lift. What is the reading on the balance when the lift is moving downward with acceleration of 3 m/s2
Homework Equations
F = ma
F = mg where g = 10 m/s2 in this question.
The Attempt at a Solution
Consider the mass first. Mass = 2kg and under gravity, the force is F = mg = 20N.
Consider the reading on the scale, let's say x. Under gravity, F = mg = 10xN.
It is given that the lift is accelerating downwards at 3 m/s2 and let's establish that moving downwards is negative. Using F = ma, we get F = ma = -6N.
Since the scale and the mass are experiencing a net downwards force, we equate them to -6N.
This is where I get stuck. The solutions give the equation:
10x - 20 = -6.
Firstly, why is there a negative sign on the left hand side? Shouldn't it be positive since both objects are experiencing a downwards motion? And why does 10x go first as opposed to 20 being first? I'm not understanding the logic behind the left hand side.
Thanks to those who answer!