Say you had a 10 pound weight on the ground & you pulled upward with a force of exactly 10 lbs so that the weight was in perfect equilibium with no net force on it (10 lbs down & 10 lbs up). It would be motionless & no work would be being done.
Why couldn't you now just touch it & with no effort, using your finger just push it up with almost no effort, and in this way push it up to 10 feet in the air thereby obtaining all this potential energy "for free" (ie, with almost no effort)?
2007-06-23 12:46:48 · 7 answers · asked by R.D.W. 2 in Science & Mathematics ➔ Physics
Well, you sort of CAN do this -- but you don't get any free energy out of it.
As another responder pointed out, you couldn't move the weight up & down with your finger unless somehow you were able to maintain 10 lbs. of tension in the rope at all times. The simplest way I can think of to do this, would be to swing the rope around a pulley, and attach a second 10 lb. weight to the other end.
In that case, it would require essentially no work to move the first weight up and down all day. This is actually pretty useful (for example, that's why they use counterweights in elevators.)
But you're still not getting energy for free; because the catch is, when you move the 10 lb. weight up on your side (gaining PE), the 10 lb. counterweight moves DOWN on the other side (losing PE). No free energy.
Other alternatives suffer from the same problem, when you think about the details of how to implement them. For example, you could suspend the weight from a big spring (instead of a rope); that way the tension wouldn't "disappear" as soon as you pushed up on the weight. In fact, if you made the spring really long (say 1000 feet), then you could push the weight upward a good 10 feet or so with essentially no effort. But there's still a catch: The spring has potential energy by virtue of being stretched; and by "unstretching" it (when you lift the weight), the spring loses some of that potential energy.
You just can't win.
2007-06-23 13:39:56 · answer #1 · answered by RickB 7 · 0⤊ 0⤋
The instant you push it upwards, the rope is no longer stretched tight, and is no longer exerting a 10-pound force (in fact, it's probably exerting 0 pounds). As you lifted, the amount of force your finger would have to supply would jump very, very quickly up to 10 pounds as the rope went slack.
If you kept 10 pounds of force on the rope, your finger would do no work as you lifted. However, the rope must be lifted to accomplish this. Rope force * rope distance = work done by the rope.
There's no escaping the inherent nature of conservative forces. You cannot move an object against gravity without doing work on it.
2007-06-23 19:56:04 · answer #2 · answered by lithiumdeuteride 7 · 0⤊ 0⤋
Essentially that is what we do every time we lift an object. Part of the force we apply to a weight we lift is used to overcome gravity and anything else is extra energy that is stored as gravitational potential energy. That extra energy is ahceived when the weight is lifted to any hight, but it is not free energy. It comes from your muscles. According to the laws of thermodynamics, energy cannot be created or destroyed, only changed from one form to another. In this case you've changed chemical energy in your body into potential energy buy lifting the weight.
2007-06-23 20:03:02 · answer #3 · answered by Anonymous · 0⤊ 0⤋
What is gravity......?..you can see its effect and if this force accelerates an object towards the earth at 10 m/s per s it defines weight.For example on the moon this force is less and thus weight is relative to gravity ,which is mass multiplied by acceleration.So to balance your falling object you would need to accelerate your 10lbs up ,which is why rockets use so much fuel.Alternatively using a fulcrum with 10lbs on each side at a distance of 2 feet from centre gravity is acting on each side canceling itself .You cannot get energy from this ....
2007-06-24 00:44:06 · answer #4 · answered by stephen g 2 · 0⤊ 0⤋
The concept of this application is used quite frequently in elevators, old-type windows, pulleys, and so on in industrial settings. If we knew the exact weight of each person on an elevator, then you could adjust the counter weights and lift the elevator by hand.
2007-06-23 20:20:00 · answer #5 · answered by rnl_enterprises 1 · 0⤊ 0⤋
Gravity always wins.
2007-06-23 20:03:53 · answer #6 · answered by stork5100 4 · 0⤊ 0⤋
No.
Have you ever heard of Newton?
He already figured this all out.
2007-06-23 19:54:57 · answer #7 · answered by William R 7 · 0⤊ 0⤋