Momentum & Energy Homework: Isolated System at Rest

In summary: So if the bowling ball has momentum when it's thrown, it has momentum when it's caught, right?Yes, if the bowling ball has momentum when it's thrown, it has momentum when it's caught.
  • #1
clarkandlarry
20
1

Homework Statement


An isolated system is initially at rest. Is it possible for parts of the system to be in motion at some later time? Explain.


Homework Equations


N/A


The Attempt at a Solution



My guess is no because if the system is at rest, the only way the system could get into motion is if some outside force acts on it. Can anyone tell me if I am right??
 
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  • #2
What would happen if I took a glass of icewater and put it in an well-insulated box full of warm air? The system would be isolated, but you'd have quite a hard time convincing me that the glass would be exactly the same if I took it out after a few hours.
 
  • #3
I'm not entirely sure what you mean by that. Are you saying that parts of the system are in motion at some later time?
 
  • #4
Okay, consider the picture below:
diagram_before.jpg


There is a block of some material placed atop a chunk of ice that prevents the block from falling into the cup. The temperature of the air in the box is much higher than the melting point of ice. We assume the box is well-insulated, so there is no heat transfer with the surroundings.

What happens to the ice as time goes on?
 
  • #5
CrazyIvan said:
What would happen if I took a glass of icewater and put it in an well-insulated box full of warm air? The system would be isolated, but you'd have quite a hard time convincing me that the glass would be exactly the same if I took it out after a few hours.

That's kind of an exotic example. But I see why you suggested it. Just take a stretched spring and put it far away from any forces. Now let it go. Initially it's at rest. Later?
 
  • #6
So even though the spring is at rest, when you release it, the spring is no longer at rest, therefore the answer to the question is yes it is possible for parts of the system to be in motion at some later time. Is this right?
 
  • #7
When you release it, the spring is at rest. After the release, parts of it start to move, yes. I suspect this question is concerning conservation of momentum. The center of mass of the whole system can't move. But the parts can move.
 
  • #8
Indeed. Imagine, say, a person floating in space with a bowling ball in his hands. Now he can throw the bowling ball and will begin moving! No external forces. In essence, this question sets the basis for momentum conservation on a type of problem called explosion, where initial momentum is zero.
 

1. What is an isolated system in terms of momentum and energy?

An isolated system is a physical system that does not interact with its surroundings in terms of momentum and energy. This means that the total momentum and energy of the system remains constant and does not change over time.

2. How does an isolated system at rest conserve momentum and energy?

An isolated system at rest conserves momentum and energy through the principle of conservation of momentum and energy. This means that the total momentum and energy before and after any interaction within the system remains the same, as long as there are no external forces acting on the system.

3. What is the equation for calculating momentum?

The equation for calculating momentum is p = m x v, where p is momentum, m is mass, and v is velocity. Momentum is a vector quantity, meaning it has both magnitude and direction.

4. How does the law of conservation of energy apply to an isolated system at rest?

The law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed. In an isolated system at rest, there is no external energy being added or removed from the system, so the total energy remains constant.

5. Can an isolated system at rest have changes in momentum and energy?

Yes, an isolated system at rest can still have changes in momentum and energy as long as those changes occur within the system and do not involve any external factors. For example, if two objects within the system collide and exchange momentum and energy, the total momentum and energy of the system will remain the same.

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