Why is Velocity Squared?
Glenn, thanks for your answers. I am still thinking about kinetic energy. In the 17th century, experiments by Willem Jacob 's Gravesande showed that the striking of a ball in clay was proportional to squared velocity and later on a French physician showed it was proportional to 'mass times squared velocity'. What is it about, is my question? How to get the formulae not by math manipulation, but from experimental considerations? By the way, I explained to a friend the idea of matter-motion by using examples and despite he is a dummy in math and physics, the matter-motion explanation seems to be more appropriate to grasp physics even at his age of 75.
Thanks again for the comment. Before the Gravesande experiment, one would have thought that doubling the velocity of a microcosm would result in double the impact. Not so. As you mentioned, the doubling of velocity causes four times the impact. In other words, a microcosm impacting a soft, wet clay at velocity 2v will create a crater 4 times as deep as a microcosm impacting the clay at velocity v; an auto crashing into a wall at 40 mph will suffer 4 times the damage as one crashing into a wall at 20 mph. Your question: What gives?
I am afraid that this is one occasion in which I will not be able to avoid math, as much as I would like too. The simple reason that velocity is squared is the fact that there is a macrocosm.
This was explained by Newton’s First Law of Motion (The velocity of a body remains constant unless the body is acted upon by an external force). This observation, the law of the universe, makes Newton the most brilliant scientist who ever lived. Thus, a microcosm moving through “empty space” at 2 m/s will move two meters in one second. At a velocity of 1 m/s it will move one meter in one second. In other words, if Gravesande’s wet clay was really “empty space,” doubling the velocity would have produced a “hole” twice as deep. Of course, there would be no “hole” and the microcosm would not stop either, because empty space offers no resistance.
Motion With a Macrocosm
This was explained by Newton’s Second Law of Motion (The acceleration a of a body is parallel and directly proportional to the net force F and inversely proportional to the mass m, i.e., F = ma.). While the First Law is just an astute observation concerning inertia, this Second Law describes causality. A cause produces acceleration, that is, a change in velocity. Here, Newton is describing a cause as a “force.” Of course, “forces” do not exist, only microcosms exist. The force concept is a handy, necessary mathematization. It is especially useful when we really do not know the actual cause. The true cause of the acceleration of any particular microcosm must be at least one other microcosm that collides with it. Incidentally, this is why determinists deny the possibility of ESP. “Extra Sensory Perception” is the indeterministic hypothesis that something or some motion might be perceived without microcosms colliding with a sensory organ.
Back to velocity squared… As generally explained, the Second Law is all about increasing the velocity of microcosms (acceleration). The collider hits the collidee. The same equations explain the opposite result (deceleration) when the situation is reversed and the collidee becomes the collider and the collider becomes the collidee. In either case, we are recognizing the effect of the macrocosm—the presence of something other than empty space. The explanation below, from http://hyperphysics.phy-astr.gsu.edu/hbase/ke.html#c3, gives the standard mathematics showing how a microcosm gets its motion:
So what does all this mean? First, remember that, in neomechanics, there is no such thing as “energy” or “energy of motion.” Energy is neither matter nor motion. Instead, we define energy as a calculation: the multiplication of a term for matter times a term for motion. Nevertheless, the calculation or “energy concept” admitted above is a handy way to help us understand matter in motion. The illustration above shows how the microcosm gets its motion even before it collides with the clay. The somewhat mysterious “force” in the illustration is simply the push provided by some other microcosm. Incidentally, indeterminists who believe in finity often speculate about where the “first” push came from. Of course, this question becomes moot for an infinite universe—there is always yet another microcosm to do the pushing.
The gist of the KE explanation is the equation: Work = KE = Fd = mad = ½mv2. If you have ever pushed a car out of the ditch, you will have some practical feel for this. It takes a lot of work (Fd, force over a distance) to get a vehicle from zero velocity to any velocity at all. The heavier the vehicle (m), the harder it is; the farther you have to push it (d), the harder it is. Once, the vehicle is moving, it is just as hard to stop it (as our daughter learned when her friend’s formerly stuck car rolled driverless and brakeless into the neighbor’s garage).
From the car example, we learn that the velocity of a microcosm cannot be increased or decreased instantaneously. The increase or decrease must occur over some distance and take some time. In the KE = ½mv2 equation, we get the distance by multiplying the average velocity ( ½ v) times the time it takes to reach the final velocity, vf. For instance, to accelerate a car from 0 to 60 mph in 10 seconds would take a distance of 440 feet (30 mph X 10 s or 44 ft/s X 10). That gives us the “d” in the Fd = mad equation. The mass, m, is assumed to be constant. Acceleration is the change in velocity. A change in velocity from 0 to 60 mph over 10 seconds is an acceleration of (0 + 60)/10 = 6 mph/s. That is, we increased the velocity by 6 mph for each second that we held the pedal to the metal.
Stopping a microcosm in wet clay involves the same process in reverse. Velocity must drop to zero as the microcosm transfers its motion to the wet clay over the period in which it decelerates. As shown by the math manipulation done in the KE illustration above, time cancels out and velocity appears twice. I find the KE = mad equation to be a bit more intuitive. Kinetic energy then becomes what happens to a mass as it accelerates or decelerates over distance.