Third law

it'd be easier to answer that with more context - eg how many pints of what? what colour and shape were the pills? did you inhale...

Because it's easier to hold something still than to move it.

If you turned the sander off and rubbed the wood against it, the sander wouldn't be using any energy.

It's a bit like cycling - easier than walking, but actually less efficient in a purely energy sense because there's the additional weight of the bicylce to move.

Owain

Mass.

If the sander were made of polystyrene you would be more in agreement with Newton.

AB

Because you only have to steady it a little and aim it at the work, the inertia of the weight of the sander, will help you to keep the sander steady, forcing the plate with the sandpaper to move.

Depends rather on if you turn the sander on... if you hold it against

But you are sitting, or even recumbent on a bicycle. In physics standing around doesn't use energy, but it does in people.

Thomas Prufer

I would have thought it was obvious. All your energy is concentrated in mostly one direction. The friction against the job face only has to be resisted,not generated as well. Brian

So you need the sanding equivalent of freewheeling downhill. Brian

Yes inertia does have a roll to play, if it did no then hammers would be useless for banging nails in. Brian

because in theory holding something in place does no work at all

Work = force times distance.

All force, no distance

An infintely large infinitely strong helicopter could hover with no power input.

Not so. It's having to generate lift with the rotating rotor blades. The work is proportional to the torque on the rotor shaft and the number of revolutions.

An infintely large rotor turns at zero RPM

There are no revolutions.

an infinitely large rotor will at best generate a finite lift since there is only a finite amount of air in which to perform. That lift is less than required to overcome the infinite gravitational attraction to the Earth.

if it is also an infinitely heavy helicopter I'd like to see the proof

if it isn't infinitely heavy why bother with rotors when you could just use cavorite?

The point is that it takes no energy to hover since no potential energy is lost or gained.

It takes energy to accelerate an air mass downwards to generate the momentum change that provides the lift.

But energy is 1/2mV^2 whereas momentum is mV so as m tends to infinity V tends to zero for the same lift

That is, an infinitely large rotor.

If you google the man powered helicopter you will see it has a rotor the size of a tennis court

With infinite gravity so it wouldn't hover.

Its OK TNP lies about understanding physics.

Consider a helicopter with rotors with radius r=10m turning at 0 rpm. Lift generated is zero. OK?

Double the radius to 20m. Lift generated is still zero. OK?

Let the radius r tend to infinity. The lift if rpm is zero is still zero. Oh dear!

No. It isnt

No, it isn't

Think 'infinite parachute'