Need help on a energy question: Hockey puck sliding across the ice

AI Thread Summary
The discussion centers on the confusion regarding energy conservation in a physics problem involving a hockey puck sliding on ice after being propelled by a spring. The initial approach incorrectly included energy loss due to friction, while the answer key simplified the equation to Ein = Eout, leading to misunderstandings about energy conservation. It is clarified that while total energy is conserved, mechanical energy is not, as friction dissipates energy. The key takeaway is that the energy initially stored in the spring is converted entirely into work against friction, resulting in the puck coming to a stop. Understanding the distinction between total energy conservation and mechanical energy conservation is crucial in solving such problems.
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Homework Statement
A 0.18 kg hokey puck is placed against a horizontal spring compressing it 15cm. The spring has a spring constant k = 37 and shoots the puck sideways along the ice. If the friction force between the puck and ice is 1.77N how far will the puck travel after leaving the spring before coming to a stop?
Relevant Equations
Ein = Eout
Uspring = Wspring
(1/2)kx^2 = Ffriction * x
I first attempted to do Ein = Eloss + Eout because the equation had friction in it but in the answer key they set it as the energy was conserved by doing Ein = Eout, why is it conserved when the puck comes to a stop which means energy was lost? I thought the equation should have been Uspring = Wspring + Ffriction because energy wasn't conserved.
 
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sky said:
Homework Statement: A 0.18 kg hokey puck is placed against a horizontal spring compressing it 15cm. The spring has a spring constant k = 37 and shoots the puck sideways along the ice. If the friction force between the puck and ice is 1.77N how far will the puck travel after leaving the spring before coming to a stop?
Relevant Equations: Ein = Eout
Uspring = Wspring
(1/2)kx^2 = Ffriction * x

I first attempted to do Ein = Eloss + Eout because the equation had friction in it but in the answer key they set it as the energy was conserved by doing Ein = Eout, why is it conserved when the puck comes to a stop which means energy was lost? I thought the equation should have been Uspring = Wspring + Ffriction because energy wasn't conserved.
Please show your calculation. Mechanical energy is not conserved, so you must be misunderstanding the answer key.
 
Here are the files (white is answer key):
 

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PeroK said:
Please show your calculation. Mechanical energy is not conserved, so you must be misunderstanding the answer key.
I don't know why the images didn't load the first time but hopefully you can see this:
IMG_2746.jpg
IMG_2748.jpg
 
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In your solution, you have a force of 1.77 (N) in the middle of an energy equation!
 
The answer keys says "use energy to solve". This does not mean that the mechanical energy of the puck is conserved.
 
PeroK said:
The answer keys says "use energy to solve". This does not mean that the mechanical energy of the puck is conserved.
By setting Ein to Eout and making the equation Us = Wf why doesn't that imply conservation of energy? I always thought you had to include an E loss to the equation as well to show that the mechanical energy wasn't conserved. Where did the Eloss go in this case?
 
sky said:
Where did the Eloss go in this case?
Mechanical energy was lost to friction.

You perhaps need to learn the difference between conservation of energy and conservation of mechanical energy. Total energy is always conserved.
 
Ah that makes sense, thank you!
 
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Note that at the start all the energy is in the elastic PE of the spring. And at the end all that energy has been lost to friction. So, in terms of magnitudes, these two must be equal.
 
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