Solving Part (a) of Lamp Mass in Descending Elevator w/ Deceleration of 1.4 m/s2

[shell4987]

Homework Statement

A lamp hangs vertically from a cord in a descending elevator that decelerates at 1.4 m/s2. (a) If the tension in the cord is 56 N, what is the lamp's mass? (b) What is the cord's tension when the elevator ascends with an upward acceleration of 1.4 m/s2?

Homework Equations

T-mg=ma

The Attempt at a Solution

I got part (b) to be 56 N and that was correct but then I used the formula... but I don't know how to solve for mass for part (a) when there are two masses in the equation above, I know T to be 56N and g to be 9.8m/s squared also a to be 1.4m/s squared... Can anyone help me out? I'm confused.

 

[Doc Al]

Just rearrange your initial equation:
T-mg=ma

Thus:
T = mg + ma = m(g +a)

You take it from here.