Three masses in an elevator accelerating downwards

[Korrie]

Homework Statement

Three masses (m1=3.2, m2=9.6, and m3=6.4) hang from three identical springs in a elevator with a spring constant of 349N/m. The elevator is moving downwards at -3.7m/s and accelerating downwards at
a= -2.5m/s^2.

What is the magnitude of the net force on the middle mass?

Homework Equations

Force(spring)=-kx
F-mg= -ma
F= mg-ma

The Attempt at a Solution

I tried 9.6(9.81-2.5) and got 70.176 but that is wrong.
So I tried (9.6+6.4)X(9.81-2.5)= 116.96 but that is also wrong.

What am I doing wrong?

 

[berkeman]

Looks like you are neglecting the spring so far... What force does it exert, and how does that affect the acceleration?

 

[Korrie]

Looks like you are neglecting the spring so far... What force does it exert, and how does that affect the acceleration?

The force by the spring on the middle mass would the the sum of the bottom two masses time gravity.
The force by the spring would equal (9.6+6.4)(9.81)=156.96N

So to find the magnitude of the force on the middle mass I would take the 70.176N I gotten originally and add them?

 

[berkeman]

The force by the spring on the middle mass would the the sum of the bottom two masses time gravity.
The force by the spring would equal (9.6+6.4)(9.81)=156.96N

So to find the magnitude of the force on the middle mass I would take the 70.176N I gotten originally and add them?

Oh, I misunderstood the problem. You need to post a diagram of the setup please. Use the UPLOAD button in the lower right up upload a PDF or JPG of the setup. Thanks. Also, if you could show your FBD for each of the 3 masses on that diagram, it would be a help.

 

[Korrie]

Oh, I misunderstood the problem. You need to post a diagram of the setup please. Use the UPLOAD button in the lower right up upload a PDF or JPG of the setup. Thanks. Also, if you could show your FBD for each of the 3 masses on that diagram, it would be a help.

I'm unsure of my FBD for each of the masses but this is what I drew.

 

[jbriggs444]

Do we know the acceleration of the middle block?
Do we know the mass of the middle block?
What is the unknown that we want to calculate?
Is there an equation that relates those three quantities?

 

[Korrie]

Do we know the acceleration of the middle block?
Do we know the mass of the middle block?
What is the unknown that we want to calculate?
Is there an equation tat relates those three quantities?

I tried the acceleration times the middle mass and that was wrong.
F=ma
(9.6kg)(-2.5m/s^2)= -24 N

 

[jbriggs444]

I tried the acceleration times the middle mass and that was wrong.
F=ma
(9.6kg)(-2.5m/s^2)= -24 N

That answer is correct.

 

[Korrie]

That answer is correct.

Thank you, turns out the program wanted positive 24 N.