Trouble with FBD: Is There an Upward Force?

[Physicsstudent2]

Homework Statement

Two blocks (m1 = 15 kg and m2 = 25 kg) are placed side by side on an elevator
accelerating at 2.2 m/s2
[up]. The coefficients of friction between the blocks and the
floor are µs = 0.8 and µk = 0.6. If a horizontal force of 400 N is applied to the first block, find the acceleration of the system.

Relevant Equations

F = ma
Fg = mg

I am having trouble coming up with a FBD. I know there is a Fg, Fn, friction, and applied force on the masses, but would there be another force going upwards because of the acceleration?

 

[PeroK]

What is the effect of the upwards acceleration? In a nutshell?

 

[Physicsstudent2]

What is the effect of the upwards acceleration? In a nutshell?

won't the upwards force have to be greater than the force of gravity so that the elevator can accelerate upwards

 

[PeroK]

won't the upwards force have to be greater than the force of gravity so that the elevator can accelerate upwards

Okay, but that doesn't help you much. Put it another way: what does it feel like in the elevator? Does it still feel like planet Earth?

 

[Physicsstudent2]

no it doesnt. depending on which direction you are moving you will feel you're a different weight

 

[PeroK]

no it doesnt. depending on which direction you are moving you will feel you're a different weight

So, can we just take the "effective" gravity to be greater than the usual ##g##, but otherwise proceed as normal?

 

[Doc Al]

but would there be another force going upwards because of the acceleration?

The upward force acting on the blocks is the normal force Fn. To find the normal force on each block, analyze the vertical forces (apply Newton's 2nd law).