
#1
Nov2412, 10:35 PM

P: 25

Can any one briefly explain to me when using conservation of energy to calculate for vertically mounted spring questions, why gravitational potential energy is neglected?




#2
Nov2512, 01:14 AM

Homework
Sci Advisor
HW Helper
Thanks ∞
P: 9,154

Do you mean as it pertains to the mass of the spring? Usually you are told the spring is light and some other mass is affected by both spring and gravity. Sure, you could allow for the spring's weight too, and probably would in a real situation, but it usually doesn't add anything very interesting to the algebra.




#3
Nov2512, 07:46 AM

P: 123

a) F = k z V = [itex]\frac{1}{2}[/itex] k z^{2} b) F = k z  m g = k (z+z_{0}) , where z_{0} = mg/k you see that the mass balances now at z = z_{0} , a little longer than before. So you can study the problem using the new variable ζ = z + z_{0} . The equation of motion will be: F = k ζ which is equivalent to a) case. V = [itex]\frac{1}{2}[/itex] k z^{2} + m g z = [itex]\frac{1}{2}[/itex] k (z+z_{0})^{2}  [itex]\frac{1}{2}[/itex] k z_{0}^{2} Since constant terms in potential energy have no physical significance, you can drop them and define the equivalent potential function: V^{*} = [itex]\frac{1}{2}[/itex] k ζ^{2} Compare the equation of motion and the potential function in a) and b) cases, and get your answer! 



#4
Nov2512, 05:51 PM

Sci Advisor
PF Gold
P: 11,352

Vertical Mounted Spring
The mass of the spring will, however, have an effect on the period of oscillation because it's part of the oscillating system.




#5
Nov2512, 09:15 PM

Sci Advisor
HW Helper
Thanks
PF Gold
P: 4,420

If the mass of the spring is regarded as a distributed parameter along its length, then including the spring mass can add significant complexity to the analysis. Sometimes what people do to take this into account is to approximate the spring as a sequence of masses joined by massless springs.




#6
Nov2612, 08:15 AM

P: 1,902

If you are talking about harmonic motion, maybe. If you are looking at a problem like a bloc falls from 2 m on top of a spring, then no, you don't neglect it. 


Register to reply 
Related Discussions  
Find spring constant, unknown projectile mass, vertical spring gun.  Introductory Physics Homework  6  
Horizontally mounted compression spring  General Engineering  1  
Transmitted force to the base of the spring mounted system  Mechanical Engineering  0  
Vertical Spring  Introductory Physics Homework  2  
Vertical spring  Introductory Physics Homework  1 