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Conservation of energy and momentum

  • Thread starter henry3369
  • Start date
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1. Homework Statement
A girl of mass m1=60 kilograms springs from a trampoline with an initial upward velocity of vi=8.0meters per second. At height h=2.0 meters above the trampoline, the girl grabs a box of mass m2=15 kilograms.

What is the speed of the girl after she grabs the box?

2. Homework Equations
Conservation of momentum

3. The Attempt at a Solution
In order to find this, I have to use conservation of energy to find the velocity right before she grabs the box, then use conservation of momentum. My question is where does the momentum go as the girl is rising? Isn't momentum supposed to be conserved? I know that internal forces conserve momentum, and the only force that acts on the girl is gravity. Is gravity not an internal force?
 
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Gravity is only internal if you consider the other agent in your system, namely the Earth. Since you're not, it's external
 

Suraj M

Gold Member
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What is the speed of the girl after she grabs the box?
I'm having a hard time finding out the exact time at which they are asking us to find the velocity.
Gravity is only internal if you consider the other agent in your system, namely the Earth. Since you're not, it's external
Does this mean that we cannot use conservation of angular momentum?
 

jbriggs444

Science Advisor
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I'm having a hard time finding out the exact time at which they are asking us to find the velocity.
Immediately after the girl grabs the box.

Does this mean that we cannot use conservation of angular momentum?
One could use conservation of angular momentum and arrive at the correct answer. But that would be a needlessly complicated approach. The problem can be solved in one dimension. Why invoke two?
 

Suraj M

Gold Member
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Would you get an accurate answer? because when she grabs it, she's at almost 5m/s, so after grabbing she would have moved by a certain height, shoúld we consider that too?or negligible?
 

jbriggs444

Science Advisor
Homework Helper
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Negligible. Almost all textbook problems are involve simplifying assumptions. Collisions take negligible time, ropes are massless, springs are perfect. In this case, "grabbing the box" amounts to an inelastic collision. Part of the what is being tested is the ability to recognize it as such.
 

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