Potential spring energy

In summary, a 2.0kg block is suspended from a cord with a spring constant of 100N/m. The length of the unstretched cord is 0.5m. When the block is released, the cord reaches its maximum elongation. To determine this length, one can use either forces or energy conservation. Using conservation of energy, the initial energy is calculated to be 0, while the final energy is a combination of kinetic and potential spring energy. Solving for the maximum displacement from equilibrium gives a final length of 0.464327m for the cord. Using forces in the energy conservation set up is possible, but not necessary and can lead to additional work.
  • #1
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a cord with a spring constant of 100N/m has a 2.0kg block suspended from it.
the length of the cord when it is unstretched is .5m.
the block is released. determine the length of the cord when it is at the maximum length of elongation.

my attempt:
first determine the kinetic energy of the block just before the block reaches the point where
the cord begins to elongate beyond the length of .5m
it is after this point that kinetic energy then begins to be converted into potential spring energy in the cord.
it is this energy combined with the weight of the block that are going to determine the maximum length of elongation of the cord. this is because when all of the kinetic energy of the block is turned into potential spring energy the only remaining force that would cause the cord to further elongate would be the weight of the block pulling down on the cord.
First, to determine the extension of the spring due to the weight of the block
use f=kx
9.8 x 2 = 100x
x=0.196
then to determine the energy that this causes to be stored in the cord use
E=1/2kx^2
1/2(100)(.196)^2=.98 J
this energy combined with the energy that the kinetic energy provides should determine the total length of elongation of the cord
.98 + (2 x 9.8 x .5) = 1/2(100)x^2
x=.464327m
can someone tell me where I am going wrong?
 
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  • #2
Don't use forces in an energy conservation set up. Start with initial energy and then do the final energy where the mass is stretched to its farthest...

KE = kinetic energy
GPE = gravitation potential energy
SPE = spring potential energy

Initial Energy: KE = 0 (because it starts from rest), GPE = mgh (starts from height 'h'), and SPE = 0 (spring is starting at equilibrium)

Final Energy: KE = 0 (reached max distance so speed is 0), GPE = 0 (fell as far as it will go), SPE = (1/2)*k*h^2 (now it is a distance 'h' from the equilibrium point)

You equate the energies due to energy conservation and solve for the longest displacement from equilibrium. Then you need to remember that the question is asking for the full length of the string when it is stretched this much.
 
  • #3
what's wrong with using forces in an energy conservation set up? why doesn't doing it that way yield the correct answer?
 
  • #4
You can use forces to solve for it or use conservation of energy. Both will get you the right answer. But there is no use mixing them together. One deals with vectors and the other uses scalars. You would just be doing more work than necessary.
 

1. What is potential spring energy?

Potential spring energy is the energy stored in a spring when it is compressed or stretched from its resting position. This energy is proportional to the amount the spring is compressed or stretched and can be released to do work.

2. How is potential spring energy calculated?

The formula for calculating potential spring energy is E = 1/2 kx², where E is the potential energy, k is the spring constant, and x is the displacement from the resting position.

3. What affects the amount of potential spring energy?

The amount of potential spring energy is affected by the spring constant, which is determined by the material and shape of the spring, and the displacement from the resting position. The greater the displacement or the stiffer the spring, the more potential energy is stored.

4. In what situations is potential spring energy important?

Potential spring energy is important in many situations, such as in mechanical devices like springs in watches or car suspensions, in sports equipment like trampolines or pogo sticks, and in natural phenomena like earthquakes or the movement of tectonic plates.

5. How is potential spring energy related to kinetic energy?

Potential spring energy and kinetic energy are two forms of mechanical energy that are interrelated. When a spring is released, the potential energy is converted into kinetic energy as the spring returns to its resting position. This is described by the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred from one form to another.

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