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

In summary, the question involves analyzing the energy dynamics of a hockey puck sliding across the ice, focusing on concepts such as kinetic energy, frictional forces, and the impact of ice conditions on the puck's motion. The discussion may explore how energy is conserved, how friction affects the puck's speed, and the role of external factors like temperature and surface texture in influencing the puck's glide.
  • #1
sky
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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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  • #2
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.
 
  • #3
Here are the files (white is answer key):
 

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  • #4
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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  • #5
In your solution, you have a force of 1.77 (N) in the middle of an energy equation!
 
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  • #6
The answer keys says "use energy to solve". This does not mean that the mechanical energy of the puck is conserved.
 
  • #7
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?
 
  • #8
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.
 
  • #9
Ah that makes sense, thank you!
 
  • #10
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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FAQ: Need help on a energy question: Hockey puck sliding across the ice

What forces act on a hockey puck sliding across the ice?

The primary forces acting on a hockey puck sliding across the ice are gravity, the normal force from the ice, and friction. Gravity pulls the puck downward, the normal force from the ice pushes upward, and friction opposes the puck's motion.

How does friction affect the motion of a hockey puck on ice?

Friction between the puck and the ice surface slows down the puck over time. Ice has a relatively low coefficient of friction, so the puck can slide a considerable distance before coming to a stop, but it will eventually lose its kinetic energy due to this frictional force.

Why does a hockey puck slide so easily on ice compared to other surfaces?

Ice provides a very smooth surface with a low coefficient of friction, which minimizes the resistance against the puck's motion. Additionally, a thin layer of water on the ice surface can act as a lubricant, further reducing friction and allowing the puck to slide more easily.

How can you calculate the deceleration of a hockey puck due to friction?

The deceleration of a hockey puck due to friction can be calculated using the formula: \( a = \mu g \), where \( a \) is the deceleration, \( \mu \) is the coefficient of friction between the puck and the ice, and \( g \) is the acceleration due to gravity (approximately 9.8 m/s²).

What role does the mass of the hockey puck play in its motion across the ice?

The mass of the hockey puck affects its inertia, or resistance to changes in motion. A more massive puck requires more force to accelerate or decelerate. However, for a given force of friction, the deceleration due to friction is independent of the puck's mass because both the frictional force and the gravitational force scale with mass.

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