Most recent answer: 10/22/2007
(published on 10/22/2007)
Follow-Up #1: work and motion
- Muhammed (age 17)
Sir George Monoux College, London
If you hold an apple up in certain ways, it feels like you're doing work but in other ways it doesn't. If you lie on the ground with an apple on your belly (or hand) it doesn't feel like you're doing work. If you hold the apple out in your hand, it does feel like you're doing work. There must be some difference.
The feeling of doing work is a way of registering what's going on in your muscles. If they're using up chemical fuel to make one muscle protein (actin) move with respect to another (myosin) then you sense that you're doing work. Yet if the apple is just staying at a fixed height, no work is being done on it, in the physics sense. Why do you need to use energy in your muscles just to keep the apple up?
If you're using your muscles to hold up the apple, it would slip a little because the actin and the myosin don't stick perfectly. You have to use up some energy to make up for that slippage.
Here's an analogy. Say you have a car parked on a slope, with very high friction between the tires and the road. The car doesn't need to do any work to stay up. You can leave the engine off. Now say that the tires slipped a little on the road. The car will fall unless you turn on the engine and get the wheels turning, expending fuel. Keeping the car in place will feel a lot like accelerating it, since the motor will vibrate, fuel will be used up, exhaust products made, etc.
Now back to the magnet. There aren't any little molecules slipping when the magnet sticks well. It's like the car with high-friction tires. So no energy has to be expended.
(published on 10/22/2007)
Follow-Up #2: magnetic work
- Heather (age 25)
This is a really nice question. There are a couple ways in which work is done, i.e. in which the energy stored in the magnetic field is converted to kinetic energy. One is that particles such as electrons are not simply charged particles whose magnetic fields come only from their motion. They also have intrinsic spin magnetism even when they are at rest. So there's a term in the potential energy which depends on the product of this intrinsic magnetism and the magnetic field. If the electrons move the right way in the field, that potential energy drops and work is done on the electrons.
Even in the case of magnetism due to currents flowing, not spin, work can be done indirectly.
For a fuller discussion, see
(published on 07/23/2008)
Follow-Up #3: magnetic perpetual motion machine?
- Tim McCreary (age 46)
Roaring Spring, PA USA
(published on 04/18/2013)
Follow-Up #4: magnets sticking forever?
- Paul (age 73)
Our arguments above about how no work is done apply exactly to this situation.
Will the magnet stick forever? Oddly, energy is required for something here- for the magnet to come loose. It sticks because there's no path for it to come off that doesn't require increasing the energy some on the way. Once it's loose, that extra energy and more can convert to kinetic and ultimately thermal, after it lands. First, though, it would have to pick up that extra energy to get loose. If it's a big enough magnet to see, and if the only energy around is ordinary room-temperature thermal energy, the time required to accidentally get enough energy to knock the magnet loose is, for all practical purposes, infinite.
(published on 10/17/2014)
Follow-Up #5: magnets, glue, and energy
Yes, that's a nice way to think of it. One of your points, however, is incorrect. You write "the pull of gravity, by the third law of Newton, is generating a reaction of the same magnitude, but opposite direction." That the glue force on the weight is equal and opposite to the gravitational force is not a consequence of Newton's 3d law. The law is entirely general, so if it accounted for that equality it would be impossible to have a case where the glue force was smaller and the object fell. Anybody who has tried home repairs knows that it's quite possible for that to happen. The force pairs that Newton's 3d law says are equal but opposite are:
1. The gravitational force of the weight on the earth is equal and opposite to the gravitational force of the earth on the weight.
2. The force of the weight on the glue is equal and opposite to the force of the glue on the weight.
Whether the glue force and the gravity force are equal depends on details, not general laws.
(published on 03/09/2017)
Follow-Up #6: forces in static arrangements
Thanks for the clarification.
(published on 03/13/2017)