Conservation of energy and momentum

In summary, the conversation discussed the problem of finding the speed of a girl after she grabs a box while jumping on a trampoline. The solution involves using conservation of energy to find the velocity before the grab and then using conservation of momentum to find the final velocity. The question of whether gravity is an internal or external force was also addressed, with the conclusion that it is external in this scenario. The use of conservation of angular momentum was also considered, but determined to be a needlessly complicated approach. Lastly, the topic of simplifying assumptions in textbook problems was mentioned, with the reminder that they should be recognized and accounted for when solving problems.
  • #1
henry3369
194
0

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?

Homework Equations


Conservation of momentum

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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  • #2
Gravity is only internal if you consider the other agent in your system, namely the Earth. Since you're not, it's external
 
  • #3
henry3369 said:
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.
Brian T said:
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?
 
  • #4
Suraj M said:
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?
 
  • #5
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?
 
  • #6
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.
 

What is the law of conservation of energy and momentum?

The law of conservation of energy and momentum states that the total amount of energy and momentum in a closed system remains constant over time. This means that energy and momentum cannot be created or destroyed, but can only be transferred or transformed from one form to another.

What is the difference between energy and momentum?

Energy is the ability to do work, while momentum is the measure of an object's motion. Both are important concepts in physics and are related to each other through the laws of conservation of energy and momentum.

Why is conservation of energy and momentum important?

Conservation of energy and momentum is important because it is a fundamental principle in physics that helps us understand and predict the behavior of objects and systems in the universe. It allows us to make accurate calculations and predictions about the outcome of various physical processes.

What are some real-life examples of conservation of energy and momentum?

Some examples of conservation of energy and momentum in everyday life include a ball rolling down a hill (energy and momentum are conserved as the ball gains speed and potential energy is converted into kinetic energy), a pendulum swinging (energy is conserved as it swings back and forth, repeatedly converting between potential and kinetic energy), and a car colliding with another car (momentum is conserved as the total momentum of the two cars before and after the collision remains the same).

Can conservation of energy and momentum be violated?

No, conservation of energy and momentum is a fundamental law of physics and has been observed to hold true in all physical processes. If it appears to be violated, it is likely due to an error in measurement or incomplete understanding of the system. However, there are some processes, such as nuclear reactions, where energy and momentum may seem to be lost, but in reality they are simply being transformed into other forms of energy and momentum that are not being measured.

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