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

[Lunct]

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

 

[berkeman]

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.

 

[Lunct]

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.

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.

 

[berkeman]

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?

 

[mjc123]

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.)

 

[Lunct]

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.

 

[Lunct]

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.

 

[ArunMaurya]

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.