2007-03-21 17:52:30 · 7 answers · asked by kevin p 3 in Science & Mathematics ➔ Physics
Air friction, surface friction, all sorts of fun friction, gravitational forces don't allow for the conservation of upward momentum or downward momentum, it's all kinda wonky.
2007-03-21 17:56:09 · answer #1 · answered by Archangel 4 · 0⤊ 0⤋
Momentum is a function of force and resistance. Momentum can be perpetual in a vacuum where there is no resistance.
However on Earth, outside of a vacuum, resistance comes in the form of counterforces. Air causes resistance. Gravity exerts additional force against momentum. And finally, depending on how the object is moving, additional resistance can come in the form of surface friction.
Assuming you had an arrow being fired from a bow, air acts against the arrow by forcing it to slow. Gravity tries to pull the arrow from midflight to the ground. And finally, surface friction of the air passing along the length of the arrow shaft slows the flight of the arrow.
To compensate and add more momentum to an arrow, fletches are added to put both a spin on the arrow (make it rotate in flight) and deflect air resistance or friction along the shaft. Add to that the front of the shaft piercing through air, the air itself will exert counterforce against the arrow head.
So these are at least three sources of resistance that prevent momentum from being perpetual.
Hence, objects set in motion on Earth cannot stay in motion based upon counterforces forming resistance to motion.
However, all of that said, everything on Earth is actually in motion as the Earth rotates on its axis AND the Earth orbits around the sun.
So in reality, let's just say, we are all in perpetual motion. However, relative to the earth as a fixed object, things do not stay in motion relative to the terra firma stuff we have to deal with.
Hope that answers your question and adds a brain teaser to the answer.
2007-03-21 19:01:03 · answer #2 · answered by krollohare2 7 · 0⤊ 0⤋
Actually, people in the old times believe that there is such a thing as perpetual motion, that is, there can be such a machine that can work endlessly by just giving it an initial energy. Many tried in inventing such a machine but they all failed because it can never exists.
All bodies in motion will eventually come to rest because the energy in that body will be used up, other's will be converted to heat and that will be released to the surroundings.
The conservation of energy explains this. Just like people need to eat for them to be able to continuously work so as all other things, they must gain energy to move. And if all those energy are used up they will stop moving.
The gravitational pull, air resistance and friction are basically the forces counterreacting the motion of bodies that makes all the energy to be converted to other forms of energy that is not anymore applicable to be used up like heat energy.
2007-03-21 18:22:19 · answer #3 · answered by activista 2 · 0⤊ 0⤋
because there are counteracting forces such as gravity, air friction, objects exerting normal forces, etc to stop them. that's why in space where there is no air friction, objects in motion stay in motion
2007-03-21 18:00:40 · answer #4 · answered by JizZ E. Jizzy 2 · 0⤊ 0⤋
Everything on earth is in motion, orbiting around the sun, which is orbiting around a black hole in the MilkyWay and on and on. Standing still is only relative to the position of the observer.
2007-03-21 18:40:15 · answer #5 · answered by stedyedy 5 · 0⤊ 0⤋
heat friction and gravity
2007-03-21 18:00:10 · answer #6 · answered by Sherlock Holmes 2 · 0⤊ 0⤋
in a word (ok, two words): usually friction
2007-03-21 17:56:36 · answer #7 · answered by Jim S 5 · 0⤊ 0⤋