Final velocity of this hockey puck

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Homework Help Overview

The discussion revolves around a physics problem involving the final velocity of a hockey puck that is struck towards an empty net. The puck's initial speed and the distance to the net are provided, along with its mass and the coefficient of kinetic friction.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the calculations related to forces acting on the puck, including friction and acceleration. Questions arise regarding the direction of acceleration and the impact of friction on the puck's velocity.

Discussion Status

Some participants have pointed out potential errors in the original poster's calculations, particularly concerning the direction of acceleration and the calculation of frictional force. There is an acknowledgment of the small effect of acceleration on the puck's final velocity, suggesting that the change may be negligible.

Contextual Notes

Participants discuss the implications of significant figures in the calculations and the potential for rounding errors affecting the results. The original poster expresses confusion about their findings, indicating a need for clarification on the physics concepts involved.

Ace.
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Homework Statement



In the last second of a hockey game, a player slaps the puck in the empty net 32.5 m away. The puck leaves the stick with the speed of 41.5 m/s. Calculate the final velocity of the puck if its mass is 0.17 kg.

Homework Equations



μK = 0.005
μK = FK / FN
F = ma
v22 = v12 + 2ad

The Attempt at a Solution

Force normal of puck:
FN = Fg = m x g
= 0.17 kg x 9.8 m/s2
= 1.67 N​
μK = FK / FN
FK = μK / FN
= 0.005 / 1.67 N​
= 0.00835 N​

a = (Funbalance/ m)
= (0.00835 N / 0.17 kg​
)
0.049 m/s2

Finally, I solve for v2:
v2 = √(v12 + 2ad)
= √(41.52 + 2(0.049)(32.5)​
= 41.5 m/s2
My problem is that my final velocity is the same as my initial velocity? Where did I go wrong?
 
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Well, you did do something wrong, but it won't affect your answer that much.

What direction is the acceleration going in?
 
You went wrong twice: first, your algebra error when calculating friction force, which should be uk(N). Then , watch your signage for acceleration when using the kinematic equations.
 
I feel dumb, thanks.
 
You have two problems.
The acceleration is so small that you have lost the change in velocity in the rounding error. On the other hand, if you show more digits in your answer then you will be guilty of gaining precision: you'll have more significant digits in the answer than in the provided data. So the short answer is the change in speed is negligible.
Your second problem is that you have acceleration going the wrong way. The dynamic friction seems to be making your puck go faster.
 

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