Question

A single mass m1 = 3.4 kg hangs from a spring in a motionless elevator. The spring is extended x = 14 cm from its unstretched length.

I found the spring constant to be 2.38Nm

but then I come across this question:
a) Now, three masses m1 = 3.4 kg, m2 = 10.2 kg and m3 = 6.8 kg hang from three identical springs in a motionless elevator. The springs all have the same spring constant that you just calculated above.
What is the force the top spring exerts on the top mass?

I tried the answer 33.3 which is wrong because the top mass holds the other two, but just adding the masses and plugging it in

Answer 1

into the equation doesn't work

Answer 2

So I skipped the question and came to this:
Now the elevator is moving downward with a velocity of v = -2.1 m/s but accelerating upward with an acceleration of a = 5.5 m/s2. (Note: an upward acceleration when the elevator is moving down means the elevator is slowing down.)
What is the force the bottom spring exerts on the bottom mass?

I found this to be simple until I saw velocity and I couldnt figure out how to put velocity into a force formula without the time (momentum).
It WOULD be something like this
ma-(X formula)=kx

Final question:
What is the distance the upper spring is extended from its unstretched length?

I tried to add the masses and plug into the formula, didn't work. I also tried doing each respectively and adding them, this was also incorrect.

I know this is a large question but if anyone could help that would be great, thanks!

Answer 3

Hey matty going back to question are all three spring combined into one?

Answer 4

Yeah sorry, probably should have cleared them up, the picture shows a spring hanging from the top of the elevator connected to a mass, which is connected to another spring etc. 3 times.

So, yeah they should all be combined into one if thats what you meant.

Answer 5

ok and you know that k constant for combines is equal to
Kequ=(1/k1+1/k2+1/k3)^-1 ASSUMING THEY ARE IN SERIES!
Anyway I get : 0.79N*m <-- Please verify.

Answer 6

That's incorrect, but that may be because they are asking for the force the top spring exerts on the top mass.

Answer 7

Ok, Can you draw a picture because I have the wrong idea >.<?

Answer 8

Attached it actually =)

Answer 9

You tried something like this?