Newton's First Law problem -- Conceptual question about KE

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Discussion Overview

The discussion revolves around a conceptual question related to Newton's First Law of Motion and the nature of kinetic energy in a vacuum. Participants explore the implications of throwing an object into space and the energy dynamics involved, including the motion of celestial bodies like the moon.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the source of kinetic energy for a thrown object in a void, suggesting it cannot solely come from the initial throw.
  • Another participant asserts that the kinetic energy does indeed come from the initial throw, emphasizing that momentum is conserved and the thrower experiences a recoil.
  • A participant challenges the idea that it takes more energy for the ball to travel indefinitely than to throw it, asking for clarification on the calculations involved.
  • Some participants note that in the absence of friction or opposing forces in space, an object will maintain its kinetic energy indefinitely.
  • There is a discussion about the energy dynamics of objects in orbit, with one participant explaining that gravitational forces do not deplete energy but rather maintain constant speed and kinetic energy.
  • Another participant acknowledges a misunderstanding regarding the need for acceleration to calculate energy, recognizing that there is no acceleration in the scenario described.
  • A humorous remark is made about the potential interference from hypothetical aliens affecting the motion of the ball.

Areas of Agreement / Disagreement

Participants express differing views on the source of kinetic energy and the implications of Newton's First Law. While some agree that the initial throw provides the necessary energy, others question this perspective, leading to an unresolved discussion.

Contextual Notes

Some participants highlight the absence of friction and opposing forces in space as a critical factor in understanding the motion of objects, but the discussion does not resolve the complexities of energy conservation in this context.

Lunct
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I have come up with a bit of a problem in my head regarding Newton's first law. If any of you could help me solve it that would be much appreciated.

So if you throw a tennis ball (or any object really), and you throw it into an infinite expanse of nothing but void, it will travel forever, or at least a very long time. So where would the kinetic energy from the movement of the ball come from? The law of the conservation of energy would say it cannot just be created, it would have to come from somewhere. Surely it couldn't just be the initial throw from your arm.

Also side question that is related: Where does the energy for the movement of an object in orbit, like the moon, come from? I know it comes from the gravity of the Earth (for the moon), but does that mean it is using up energy from the earth?

HELP
 
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Lunct said:
Surely it couldn't just be the initial throw from your arm.
Yes it is. And momentum is conserved, so you and your spaceship recoil backwards a bit as you throw the ball out into space.

And please don't call me Shirley. :smile:
 
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berkeman said:
Yes it is. And momentum is conserved, so you and your spaceship recoil backwards a bit as you throw the ball out into space.

And please don't call me Shirley. :smile:
How?
If you do the math(s) it takes less energy to throw a ball than it does for that ball to travel forever.
Can you explain.
 
No, there is no friction in space (sort of), so there is nothing to affect the ball's trajectory or speed.

EDIT/ADD -- And do what math? Can you show us?
 
Lunct said:
Surely it couldn't just be the initial throw from your arm
Why not? That's exactly where it comes from. As long as a body continues in uniform motion without being acted on by a force, it maintains its kinetic energy. It doesn't need to be continually supplied with energy to keep it moving, because it isn't losing energy. The analogy from common experience, where we need to supply energy to keep moving, is because the motion is opposed by forces (friction, air resistance etc.) and energy is lost in resisting those forces. In your scenario there are no forces to resist, and no loss of energy. Ideally it will keep moving for ever.
Similarly in the case of an object in circular orbit, this time there is a force (the gravity of the parent body), but the force is at right angles to the velocity, so the direction of motion changes, but the speed, and kinetic energy, remain constant. (There is an exchange of energy due to tidal forces, but this is not relevant to your question.)
 
mjc123 said:
Why not? That's exactly where it comes from. As long as a body continues in uniform motion without being acted on by a force, it maintains its kinetic energy. It doesn't need to be continually supplied with energy to keep it moving, because it isn't losing energy. The analogy from common experience, where we need to supply energy to keep moving, is because the motion is opposed by forces (friction, air resistance etc.) and energy is lost in resisting those forces. In your scenario there are no forces to resist, and no loss of energy. Ideally it will keep moving for ever.
Similarly in the case of an object in circular orbit, this time there is a force (the gravity of the parent body), but the force is at right angles to the velocity, so the direction of motion changes, but the speed, and kinetic energy, remain constant. (There is an exchange of energy due to tidal forces, but this is not relevant to your question.)
That makes so much more sense and I understand everything. I am enlightened with your knowledge.
Thanks.
 
berkeman said:
No, there is no friction in space (sort of), so there is nothing to affect the ball's trajectory or speed.

EDIT/ADD -- And do what math? Can you show us?
I realize that was stupid. To work out how much energy it takes to move something you need acceleration, and there is no acceleration. I didn't think that one through.
 
Lunct said:
I have come up with a bit of a problem in my head regarding Newton's first law. If any of you could help me solve it that would be much appreciated.

So if you throw a tennis ball (or any object really), and you throw it into an infinite expanse of nothing but void, it will travel forever, or at least a very long time. So where would the kinetic energy from the movement of the ball come from? The law of the conservation of energy would say it cannot just be created, it would have to come from somewhere. Surely it couldn't just be the initial throw from your arm.

Also side question that is related: Where does the energy for the movement of an object in orbit, like the moon, come from? I know it comes from the gravity of the Earth (for the moon), but does that mean it is using up energy from the earth?

HELP
Newton's 1st law says ...unless acted by external force. So in space there is no opposing force i.e gravity and air resistance. The kinetic energy will be constant all the time till it acted by external force. So the ball will go on and on and on... unless some fooking Alien touches it that we human race known of.
 

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