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Physics Question - ma = mg

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TimFox:
My favorite descriptions of Einstein's equivalence between a uniform gravitational field and a uniformly-accelerated reference frame take place in elevators.  This is a good way to show the deflection of a light beam by gravitation, since the entrance and exit of the beam on the two walls of the elevator depends on the acceleration.

RJSV:
...A good thread going on, so please don't misunderstand, my mild distress, faced with recent BULK (simply stated), I tend to skim past. My reasons being medically based. Various ailments having associated 'Chronic Fatique' limits on time / energy.

So, my take on movements and expending 'WORK' in formal defined terms includes the non-linear real world of friction. In casual terms, I learned in basic physics, about how friction expresses stronger incremental increases, as you go up the scale of speed. Maybe not increasing as the SQUARE of applied speed; certainly not linearly, usually somewhere in between. It's not a clean business, for example the parachute jump, with all the attendant air resistance and limiting velocity of the jumper (some 120 mph I believe).
   But I don't dedicate all needed time, on friction or WORK debates, and there is a reason:
   The blog posts, on eevblog generally, I've been using as I speculate on improving personal JOB positioning.
And so it makes reasonable sense, to skip over some details (here),...especially while having time / energy limits.
Heck, all creatures operate within limits, huh ?
Thank you. - Rick

TimFox:
Friction is complicated.  In freshman physics, it is normally approximated as a constant value of force, independent of speed, opposite in direction to the velocity, proportional to the "normal" component of force between the object and base.  (Here, "normal" means perpendicular to the surface.)  A block sliding on an inclined plane is a good homework exercise, where you need to find the normal force as a component of the gravitational force, and then subtract the frictional force from the gravitational force component in the direction of motion.  One can find tables of friction co-efficients, including "waxed hickory on snow".  To be careful, one distinguishes static friction (before the sliding starts) and dynamic friction (while the object moves).
Later, one learns of viscous friction, also opposite in direction to the velocity, whose value is roughly proportional to the velocity.  The "dashpot" (piston moving through a fluid, like an automobile's shock absorber) is used to provide damping in the basic model of forced, damped harmonic oscillator (mass, spring, dashpot, and possibly gravity), which is the model for the R-L-C electrical resonant circuit.

CatalinaWOW:

--- Quote from: TimFox on July 04, 2021, 05:46:05 pm ---Friction is complicated.  In freshman physics, it is normally approximated as a constant value of force, independent of speed, opposite in direction to the velocity, proportional to the "normal" component of force between the object and base.  (Here, "normal" means perpendicular to the surface.)  A block sliding on an inclined plane is a good homework exercise, where you need to find the normal force as a component of the gravitational force, and then subtract the frictional force from the gravitational force component in the direction of motion.  One can find tables of friction co-efficients, including "waxed hickory on snow".  To be careful, one distinguishes static friction (before the sliding starts) and dynamic friction (while the object moves).
Later, one learns of viscous friction, also opposite in direction to the velocity, whose value is roughly proportional to the velocity.  The "dashpot" (piston moving through a fluid, like an automobile's shock absorber) is used to provide damping in the basic model of forced, damped harmonic oscillator (mass, spring, dashpot, and possibly gravity), which is the model for the R-L-C electrical resonant circuit.

--- End quote ---

Friction IS complicated.  Some folks refer to pressure drag as a form of viscous friction, with some justification.  But this type of friction is "sort of" proportional to the square of velocity.  With "sort of" meaning over a limited range of speed, pressure and a few other variables.

When examining the force required to move and keep a car in motion you can observe at least four classes of friction.  A breakaway force, a constant force at low speeds, a force linearly increasing with speed and one proportional to the square of speed.  If the dynamic friction is large and the drag is large the viscous term may not be observed without very careful measurements as the pressure drag may become large before viscous drag causes significant growth in the total drag.

The simple concepts and models of physics are extremely useful.  But many real world problems require a great deal of observation and thought to determine which simple models apply and how they are used.

Nominal Animal:
(I'll try to be brief!)

--- Quote from: RJHayward on July 04, 2021, 05:34:56 pm ---friction expresses stronger incremental increases, as you go up the scale of speed

--- End quote ---
Yes.

Friction itself can be described as a force opposite to velocity.
Usually, we choose a coordinate system where our object is stationary, and use the velocity of the surrounding fluid (for drag equations) or the velocity on the surface the object is rubbing against.

Velocity is not the only thing involved in determining its magnitude.  Pressure of the fluid affects its drag coefficient, and the force pushing the object to the surface it slides on affects the coefficient of friction.  As mentioned above by Tim Fox, CatalinaWOW and others, its magnitude can be proportional to the square of the velocity, to the magnitude of the velocity, or something in between or only slightly different, depending on the situation.  The proportionality is useful for us humans to estimate the effect, but arises from shapes and flows so there is no clear formulae, except for idealised shapes and fluids and surfaces.

The 'work' done by those forces describes exactly the amount of energy transferred due to drag and friction.
That energy is usually lost in heat and deformation.

If you have ever done lathe work or milling, "chip breaking" is important because the tools rely on the removed particles (chips) to carry off the extra heat generated by friction.  With good tools and proper setup – surface speed of the cutting bit and the depth of each cut –, your work piece can remain relatively cool, while the metal shavings flying off are so hot they rapidly oxidize and change color.  (I love machining videos.  Knowing the funky physics that occur there just makes them even more enjoyable.)

In his 1980 Science Fiction book Sundiver, David Brin describes a craft skimming in the chromosphere of our Sun, that uses a high-tech version of that to remain cool: a very, very powerful refrigeration laser.  The "chips" are then photons instead of matter, with energies corresponding to crazy high temperatures, but the core principle is the same.  As with lathe and mill chip breaking, to work, those chips have to be hotter than the ambient and hotter than the object you are trying to keep cool(er).

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