Conservation of energy - spring problem

In summary, a 0.500 kg mass attached to a spring with a spring constant of 45 N/m is pulled by a lab partner to a certain distance from the equilibrium position and then released. When it passes the equilibrium position, its speed is 3.375 m/s. Using the equations for elastic potential energy and kinetic energy, the distance from the equilibrium position is calculated to be 0.36 m. However, the textbook's answer of 0.19 m is incorrect.
  • #1
shawli
78
0

Homework Statement



A 0.500 kg mass resting on a frictionless surface is attached to a horizontal spring with a spring constant of 45 N/m. When you are not looking, your lab partner pulls the mass to oneside and then releases it. When it passes the equilibrium position, its speed is 3.375 m/s. How far from the equilibrium position did your lab partner pull the mass before releasing it?


Homework Equations



Ee = 0.5 * k * x^2

Ek = 0.5 * m * v^2

Energy is conserved within the system: Einitial = Efinal


The Attempt at a Solution



At the equilibrium position, only kinetic energy is present.
At the "unknown" position, there is elastic potential energy because the mass has strayed from the equilibrium ...but since we don't know if the mass has been pulled to the maximum distance, I'm not sure whether or not there is kinetic energy. I'm not going to include it in my calculation because otherwise I'll have two unknowns ("x" and "v2")

Ek = Ee
0.5*m*v^2 = 0.5 * k * x^2
x = SQRT [ (m*v^2) / k ]
x = SQRT [ (0.500*3.375^2) / 45) ]
x = 0.36m

The answer is actually 0.19 m. I think I probably set up my equation incorrectly. Could someone look it over please?
 
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  • #2
shawli said:

Homework Statement



A 0.500 kg mass resting on a frictionless surface is attached to a horizontal spring with a spring constant of 45 N/m. When you are not looking, your lab partner pulls the mass to oneside and then releases it. When it passes the equilibrium position, its speed is 3.375 m/s. How far from the equilibrium position did your lab partner pull the mass before releasing it?


Homework Equations



Ee = 0.5 * k * x^2

Ek = 0.5 * m * v^2

Energy is conserved within the system: Einitial = Efinal


The Attempt at a Solution



At the equilibrium position, only kinetic energy is present.
At the "unknown" position, there is elastic potential energy because the mass has strayed from the equilibrium ...but since we don't know if the mass has been pulled to the maximum distance, I'm not sure whether or not there is kinetic energy. I'm not going to include it in my calculation because otherwise I'll have two unknowns ("x" and "v2")

Ek = Ee
0.5*m*v^2 = 0.5 * k * x^2
x = SQRT [ (m*v^2) / k ]
x = SQRT [ (0.500*3.375^2) / 45) ]
x = 0.36m

The answer is actually 0.19 m. I think I probably set up my equation incorrectly. Could someone look it over please?

Well no matter how far your lab partner displaces the mass, this is the max amount of energy in the system for that displacement. So the total kinetic energy has to be equal to total elastic potential energy at any moment that there is no elastic pot energy (all ke) or at any moment when there is only elastic potential energy (thus no kinetic energy so no motion). And the red above proves you know when the system only has kinetic energy.

So if you use the equation you already have above, you only have one unknown, x.
 
  • #3
Okay I think I understand what you're saying, that at the equilibrium position there is only kinetic energy and that this magnitude of kinetic energy should be equal to the energy at any other point in the spring ...For example a point where there is all elastic potential energy only (in other words, the maximum amplitude of the spring).

But if the spring is pulled to any point between the equilibrium, is there only elastic potential energy or is there a mixture of elastic potential energy and kinetic potential energy?

Because with my equation of Ek = Ee ...my answer is wrong, and it's really confusing me :|.
 
  • #4
In this case Ek SHOULD equal to Ee even if the spring isn't pulled all the way to the max positions since when my partner pulled the spring, it still didnt have Ek (like the spring wasn't already going)?
 
  • #5
shawli said:
Okay I think I understand what you're saying, that at the equilibrium position there is only kinetic energy and that this magnitude of kinetic energy should be equal to the energy at any other point in the spring ...For example a point where there is all elastic potential energy only (in other words, the maximum amplitude of the spring).

But if the spring is pulled to any point between the equilibrium, is there only elastic potential energy or is there a mixture of elastic potential energy and kinetic potential energy?

Because with my equation of Ek = Ee ...my answer is wrong, and it's really confusing me :|.

If the mass is pulled a little bit away from the equilibrium position, you put a small amount of total mechanical energy into the system. If the mass is pulled far away from the equilibrium position you put a larger amount of energy into the system and this is calculated as 1/2*k*x^2. If you pull the mass away from the equlibrium position and just hold it there does it have elastic potential energy, Kinetic energy, or both?

When you let the mass go and it is moving on its way to the equilibrium position does it have elastic potential energy, Kinetic energy, or both?

When you let the mass go and it is moving and at is located right at the equilibrium position does it have elastic potential energy, Kinetic energy, or both?
 
  • #6
Just kinetic energy?

And depending on how far you pull the mass away, the speed at the equilibrium will vary.

So Ek = Ee ...and the back of the textbook must be wrong ?
 
  • #7
shawli said:
Just kinetic energy?

And depending on how far you pull the mass away, the speed at the equilibrium will vary.

So Ek = Ee ...and the back of the textbook must be wrong ?

Sorry, I totally just got kicked off the internet. I get 0.356 meters. And I did not even see you had attempted the problem before, sorry.
 
  • #8
Don't worry about it, your explanations helped me understand what I was doing. And I'm sure that IS the correct answer and not 0.19m, like the book says. Thanks for your help!
 

1. What is conservation of energy and how does it apply to spring problems?

Conservation of energy is a fundamental law of physics that states that energy cannot be created or destroyed, but can only be converted from one form to another. In the context of spring problems, it means that the total energy of a spring system (potential energy + kinetic energy) remains constant as the spring oscillates back and forth.

2. How is potential energy related to the spring constant and displacement?

Potential energy in a spring system is directly proportional to both the spring constant (k) and the square of the displacement (x) from the equilibrium position. This relationship is described by the equation PE = 1/2 * k * x^2.

3. Can you explain the concept of mechanical energy in relation to spring problems?

Mechanical energy is the sum of an object's potential and kinetic energy. In the case of a spring, the total mechanical energy remains constant as the spring oscillates, with potential energy being at its maximum when the spring is at its most stretched position, and kinetic energy being at its maximum when the spring is at its equilibrium position.

4. How does the mass of an object affect its energy in a spring problem?

The mass of an object does not affect its potential energy in a spring system, but it does affect its kinetic energy. Heavier objects will have a greater kinetic energy at the equilibrium position compared to lighter objects, as they require more energy to be moved at the same displacement.

5. What are some real-life applications of the conservation of energy in spring problems?

The conservation of energy in spring problems has many real-life applications, such as in the design of car suspensions, bungee jumping, and pogo sticks. It is also used in the study of earthquakes, as the energy released during an earthquake is related to the spring constant and displacement of the fault.

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