What is the Force Exerted by a Spring in a Moving Elevator?

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    Elevator Spring
AI Thread Summary
The discussion revolves around calculating the force exerted by a spring on a mass in a downward-moving elevator that is accelerating upward. Participants clarify that the spring constant, k, remains unchanged regardless of the elevator's motion, as it is a property of the spring itself. Confusion arises regarding the terminology of the elevator being "motionless" while it is moving, which is clarified to mean that the masses are not bouncing around relative to the elevator. A free-body diagram is recommended as a crucial step in solving the problem. Understanding these concepts is essential for accurately determining the spring's force on the mass.
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Homework Statement



Three masses m1 = 3.1 kg, m2 = 9.3 kg and m3 = 6.2 kg hang from three identical springs in a motionless elevator. the elevator is moving downward with a velocity of v = -2.3 m/s but accelerating upward with an acceleration of a = 4.8 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?

Homework Equations



f=-kx

The Attempt at a Solution



Some of my confusion lies with k. Since k is a constant is it the same value as if the elevator was motionless or does k change when the elevator moves?
 
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mrshappy0 said:
Some of my confusion lies with k. Since k is a constant is it the same value as if the elevator was motionless or does k change when the elevator moves?
The spring constant k is constant in the elastic region of the spring. Assume this for your solution.
 
Still not sure how to solve this I guess
 
Draw your Free-Body Diagram first.
 
How is it this possible "... a motionless elevator. the elevator is moving " ?

Follow flyingpig's advice !
 
SammyS said:
How is it this possible "... a motionless elevator. the elevator is moving " ?

Follow flyingpig's advice !

I think he means the masses are motionless wrt the lift and not bouncing around - but yes free body diagram.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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