Dunk on the basketball net

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  • #1
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Homework Statement


A basketball player dunks the ball and momentarily hangs from the rm of the basket. Assume that the player can be considered as a 95.0kg point mass at a height of 2.0m above the floor. If the basket rim has a spring constant of 7.4 x 10^3 N/m, by how much does the player displace the rim from the horizontal position?


Homework Equations


Conservation of Energy


The Attempt at a Solution


I set the initial instance to be the bottom, where Eg and Ee would be 0. The final instance to be at the top where he is holding onto the rim. There Eg and Ee are present.

0 = Eg' + Ee'
0 = (m)(g)(h) + 1/2(k)(x)^2
-(m)(g)(h)*2 / k = x^2

Woops, I ran into a negative square root. What am I doing wrong here?
 

Answers and Replies

  • #2

Homework Statement


A basketball player dunks the ball and momentarily hangs from the rm of the basket. Assume that the player can be considered as a 95.0kg point mass at a height of 2.0m above the floor. If the basket rim has a spring constant of 7.4 x 10^3 N/m, by how much does the player displace the rim from the horizontal position?


Homework Equations


Conservation of Energy


The Attempt at a Solution


I set the initial instance to be the bottom, where Eg and Ee would be 0. The final instance to be at the top where he is holding onto the rim. There Eg and Ee are present.

0 = Eg' + Ee'
0 = (m)(g)(h) + 1/2(k)(x)^2
-(m)(g)(h)*2 / k = x^2

Woops, I ran into a negative square root. What am I doing wrong here?
To me, this looks like a static problem of Hooke's Law (spring displacement proportional to load) not one of conservation of energy. What does it matter how high above the floor the ring is?
 
  • #3
Andrew Mason
Science Advisor
Homework Helper
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To me, this looks like a static problem of Hooke's Law (spring displacement proportional to load) not one of conservation of energy. What does it matter how high above the floor the ring is?
The spring has to stretch not only the distance required to balance mg but it has to also stop the player's downward motion. So the approach taken is correct. The negative sign is fixed if one uses g = -9.8 m/sec^2 or if one uses a + sign and says that the energy stored in the spring has to equal the loss of potential energy of the player:

[tex]mgh = \frac{1}{2}kx^2[/tex]

Having said all that, it is not all that clear how the spring is supposed to work. I think you just have to assume that the relationship between horizontal displacement of the hoop edge and the force is F = -kx. So h = x.

AM
 
  • #4
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Ok I tried to take two approaches to answer this problem.

[1] 0 = Eg + Ee
0 = (m)(-9.81)(h) + 1/2(k)(x)^2
sqrt[(95)(+9.81)(2)(2) / (7.4*10^3) ] = x
x = 0.7097m
x ≈ 0.71m

[2] F = kx
In this case, I assumed that the gravitational force from the player is the force that is pulling on the spring to stretch it, thus (m)(g)

(95)(9.81) / (7.4*10^3) = x
x = 0.1259m
x ≈ 0.13m

The answer at the back is 0.15m..
 

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