Now, three masses (m1 = 3.1 kg, m2 = 9.3 kg and m3 = 6.2) hang from three identical springs in a motionless elevator. The springs all have the same spring constant given above.
Now the elevator is moving downward with a velocity of v = -2.9 m/s but accelerating upward at an acceleration of a = 3.6 m/s2. (Note: an upward acceleration when the elevator is moving down means the elevator is slowing down.)
What is the magnitude of the net force on the middle mass?
Newton wins another round .
the masses are in series so m1 is on the top and m2 in the middle then m3 is on the bottom
ok
now the net force on middle is the mass times the acceleration, period.
I care not what velocity is. acceleration up interests me
total mass = 18.6 kg
all we did here was change gravity
g = 9.81 + 3.6 = 13.4 meters/ second^2
spring force up on bottom mass = 6.2 * 13.4 = 83.1 N
spring force up on middle mass = 83.1 + 9.3*13.4 = 208 N
spring force up on top mass = 208 + 3.1*13.4 = 250 N
if you do 208 - 83.1 - 9.81*9.3 = 33.7 = net force up on middle
that is what we said to start with 9.3*3.6 = 33.5 close enough
I had originally done that and it was wrong
I also tried the negative answer since the velocity is negative, but that also marked it wrong, I'm assuming it might be a different formula but I have no idea which one it could be
Sorry, it is right ;)
You sure they did not say net SPRING force up on middle?
208 - 83.1