# What kind of force can a rock exert which a motor cannot?

Imagine a dam with two doors. We have two cases:

First case: there is a rock heavy enough to stop the doors from opening.

Second case: there are are two motors or kind of machines (not sure if it can be motors) applying force on the two doors enough to keep them closed.

In the first case we do not consume energy (I guess!!) but in the second case we consume energy. How is that possible? What kind of force does the rock use to keep the doors closed, which the motors/machines cannot use?

PS: I am 15 years old and I only know the basics of physics so keep it simple,please.

• A motor would not necessarily consume energy (in any significant amount) when not turning. Eg, a piston engine driven by compressed air would not consume any air in full stall. Any energy consumed is representative of inefficiency in the system, and is not inherent. – Hot Licks Mar 9 '17 at 21:27
• Note that a person holding the door shut against an opening force would certainly feel, and feel correctly, that they are consuming energy, although in essence they are doing nothing different than the rock. Once realizing that, they might just sit down in front of it, playing rock, and stop exhausting themselves ;-). It is perhaps noteworthy that all movement and force a body exerts are created by molecules held together through electromagnetic forces, so that an organism can be regarded as an (intricate) electric motor. – Peter A. Schneider Mar 10 '17 at 4:24
• I immediately had to think of CNC machines with stepper motors, which can (and regularly do) indeed function in "hold" mode. When they are not actively turning their shaft, they hold it with very considerable force. They certainly do use electrical energy to do that, and it is not just an issue that is caused by waste heat. I don't feel this warants its own answer, and don't want to repeat what everybody wrote alread. Maybe someone who has a high-voted answer might want to elaborate on that, in 15yo language. ;) – AnoE Mar 10 '17 at 12:33
• note that you still have to brace your machine (motor, solenoid, whatever) against something which wont move. Say for example a nice big heavy rock. Otherwise the machine will remain in its "hold" configuration but simply be pushed out of the way by the force trying to open the door. – Joseph Rogers Mar 10 '17 at 14:03

In both examples, the motor/rock does no work against the door. We know this because there is no movement in the direction of the force.

The difference is that the motor is using an electromagnetic field to generate that force, and that field is being generated by passing electricity through a bunch of wound wires. The electricity flowing through these wires dissipates energy due to the resistance of the wires. The energy consumed by the motor is thus dissipated as heat within the motor.

In fact, many motors can burn out if you operate them this way. Many motors rely in their own rotation to provide airflow to cool the wiring inside the motor.

• That's why I said not sure if it can be a motor may be another machine – abdelrahman taher Mar 9 '17 at 18:26
• The answer varies by machine. If you are using a pneumatic piston to hold the door shut, you'll find that it takes no energy at all. On the other hand, if the piston is leaky, and thus you must constantly pressurize the cylinder with more air, you'll find that you expend energy -- exactly equal to the energy lost through the leaky piston. Same thought process, just a differnet machine. – Cort Ammon Mar 9 '17 at 19:01
• Not only does the rotation provide air flow, in many motors it also significantly reduces the current (Lenz's Law). Over-current detection is used to diagnose stalled motors -- e.g. when determining whether a mechanical door is closed. Otherwise the proper answer. If we reduce the energy loss through electric "friction", e.g. by using a supra conductor, the magnetic field could be sustained without expending any energy beyond that necessary to establish it. A permanent magnet holding the door in place is an example for that, if one will. – Peter A. Schneider Mar 10 '17 at 4:18
• Comments are not for extended discussion; this conversation has been moved to chat. – ACuriousMind Mar 14 '17 at 13:49

The force is friction. When a heavy rock is sitting on the ground, it takes a lot of force to overcome the static friction force to get the rock to slide.

Compare this to a motor. The only part of a motor attached to the door would be a gear. The whole idea behind motors is being efficient, and so they are designed to have as little friction as possible in them. This means we need to apply a force to hold things in place.

If you had a motor, with a big, heavy gear and a big, heavy drive shaft and it had some bad bearings and so on, the friction in the motor system would be larger then the force applied by the door and so we wouldn't have to apply anything else. It would be the same as a big, heavy rock.

• Does this mean we can get energy from static energy I mean in the case of something that needs to apply force continously it needs enegy (I guess) so does it go the other way also. – abdelrahman taher Mar 9 '17 at 18:29
• @abdelrahmantaher the rock (nor friction) doesn't do any work here: it just lies still. Work is defined as $\int \vec F d\vec l$, and energy is the work spent/gained. – Ruslan Mar 9 '17 at 18:43
• But don't confuse gravity forces and energy. These all have very precise definitions. I am using them informally to keep it a bit simpler. – tpg2114 Mar 9 '17 at 19:00
• @Octopus Inertia (as a property that corresponds to mass) only affects the rate of acceleration from a force. It doesn't prevent acceleration. The rock would eventually move without the friction force. In technical contexts, you probably wouldn't even use the term that way; it would just refer to the law of inertia (that objects do not accelerate in the absence of forces). – jpmc26 Mar 9 '17 at 21:23
• I'm downvoting this answer because friction has nothing to do with it. – immibis Mar 10 '17 at 7:57

It is not a different kind of force which enables the rock to hold the door closed, but a different kind of mechanism for providing that force.

The contact force from the rock presses against the door while friction (another contact force) with the ground prevents the rock from being pushed away by the door. Contact forces are electro-static reaction forces which prevent the electrons in two objects from being made to occupy the same space.

Machines and motors can also exert passive contact forces against the door. Bolts - which exert contact forces against the ground - prevent them from moving. If the machines contains a piston, cogs and a ratchet, the ratchet can prevent the piston from being pushed backwards after it has pushed the door closed.

If the piston is pushed by a steam or combustion engine, the pressure of the steam or combustion gases can provide a constant force to keep the door closed, as long as there are no leaks and the cylinder does not cool down. The exhaust vent must be locked in place to prevent the gases escaping. This acts like a ratchet.

If the machine has no ratchet-like mechanism to prevent the piston from moving backwards, then a constant supply of energy is needed to maintain an active force against the door. Water wheels require a constant flow of water to maintain torque. Electric motors require constant electric current to maintain a constant torque on the armature. The coil of the motor has some resistance, however small, so the current dissipates energy as heat in the coil.

This is in some way like the active forces provided by humans. It takes no energy to exert a passive force by leaning against the door, but we get tired when actively exerting a constant push against a door, or when holding things up, even though no work is being done in a physics sense. Our muscles consume energy just to keep them in tension. See Why does holding something up cost energy while no work is being done?.

Your are correct, you do not need to consume energy to keep the door close.

In the case of the stone the door is hold inplace by the contact normal force from the rock, however the rock has to also experience enough static friction from the floor in order to keep it from moving, without friction (for instance, if the rock is held above the floor by a large balloon, the door will open, regardless of the rock's mass, but it will move slower the larger the mass).

But you can use other less efficient processes to keep the door closed (a bad and unnecessary choice), in which case you can spend an arbitrarily large amount of energy. For instance, if you move the rock some distance away from the door, and attach to the rock a motor with an "arm" oriented towards the wall which needs to consume energy from the motor in order to avoid being compressed, then you will spend internal energy in order to keep the arm from compressing. But this energy is not really necessary, is a result of using a non-efficient method to hold the door.

The electrical energy "consumed" by the motor can transfer into two forms of energy, namely heat and work. Their sum equals the supplied electrical energy (1st law of thermodynamics).

Work is definded as displacement (in meter) times force (in Newton). Since the door does not move, all electrical energy is transfered into heat.

Energy is not consumed by motor straining against a force but not moving. If you look at any definition of work, you will find that no work is being done in the situation that you describe. No force is being exerted against a resistive force for a certain distance.

If you connect an electrical motor to a gear and then brake that gear with a larger force or torque than the motor can produce, motor will not draw any additional current vice a similar arrangement of wires (don't forget to include the motor windings). If you attempt to brake an internal combustion engine or motor so that you stop its rotation, it dies, ending the consumption of fuel.

• Your statement that a stalled internal combustion engine uses no energy is right, but for a motor it is completely wrong, except in the case of electrostatic motors (which are generally only found in nanotechnology). Most (standard electromagnetic) motors draw the highest current when stalled (and all the energy is wasted in the motor's resistance.) The current drawn by a motor is proportional to the force it produces, and it produces most force when stalled. A en.wikipedia.org/wiki/Francis_turbine would be a good analogy as there is still significant flow when stalled. – Level River St Mar 9 '17 at 19:42

## protected by Qmechanic♦Mar 10 '17 at 13:24

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