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Founded January 1970
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2019-2020: CXXVI
THE DAILY ASTRONOMER
Wednesday, April 15, 2020
Remote Planetarium 13: Newton's Weighty Laws
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Isaac Newton (1643-1727)
Considered by many to have been the world's greatest scientist, Isaac Newton devised three fundamental motion laws, the Universal Law of Gravitation and invented the Calculus (along with Liebniz). A full account of his life and work would require volumes.
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Why are we taking a break from astronomy to study physics? Well, in the real Universe, they are inextricably linked. One cannot know astronomy without knowing physics, the branch of science devoted to understanding how the cosmos works. Physics enables us to explain actions as simple as you walking down to the street to a phenomenon as grand as galactic mergers and collisions. Today, we discuss Newton's three motion laws and, of course, his Universal Law of Gravitation. It will all lead naturally to the important theorem in all of astronomy.
NEWTON'S FIRST LAW: An object remains in a constant state of motion unless acted upon by an external force.
Regard the following object:
At this very moment, the Voyager 1 craft is leaving the solar system at the breakneck speed of 35,290 miles per hour! At a current distance of nearly 13.8 billion miles, Voyager 1 is the most distant human made object from Earth. Provided it doesn't collide with any other object, Voyager 1 will continue moving inexorably through outer space for millions or even billions of years. One might wonder: what keeps Voyager 1 moving? The answer is: itself. After its launch and a few swings around planets, this spacecraft has attained a rapid speed that it can maintain indefinitely as long as no external force is applied to it. Newton's First Law explains this "perpetual motion" quite nicely. Voyager 1's environment is outer space, a region that closely approximates a perfect vacuum. Nothing impedes it, apart from dust particles which won't slow its progress in the slightest. When we drive on Earth, we have to constantly press on the accelerator to counteract the retarding effects of wind and friction between the road and tires. (Let's also not forget the little matter of the planet's gravity field which we certainly feel when driving uphill.) A driven car constantly experiences external forces. A spacecraft escaping the solar system experiences none, apart from the meager gravitational pull the distant Sun still exerts.
Planetary motion provides another example of Newton's First Law. If we could snap off the Sun's gravity, the planets would all start moving along a line tangent to their orbits. They would all continue traveling along this tangent until some other large body ensnared them. The planets constantly experience the Sun's external force and so are constantly moving along their present orbits.
NEWTON'S SECOND LAW: Force equals mass times acceleration
Let's talk about a common misconception. It is believed that one could easily push an orbiting satellite around because, as it is in space, it is "weightless." The problem is that all material objects are massive. Mass measures a body's inertia. The greater the object's mass, the more it resists changes in its motion. We know on Earth that pebbles move more easily than boulders. Place those boulders and pebbles in outer space. One would find that it would be much easier to alter the motion of the latter than the former.
The amount of force one applies to a resting object such as this drop dead gorgeous soccer ball determines its acceleration. If you apply a strong force, it will travel quite far until Earth's gravity pulls it down. If you apply a weak force, it will hardly travel at all.
As another example: try kicking a car! An automobile is far more massive than a soccer ball. In order to apply sufficient force to accelerate a vehicle, you need to press on the accelerator. The harder you press, the greater the force and also the acceleration.
NEWTON'S THIRD LAW: Every action has an equal and opposite reaction
A perfect example:
What better way to demonstrate Newton's third law than with a beautiful rocket launch. By issuing violent expulsions in one direction, the rocket is propelled quickly in the other. Newton's "action/reaction" law is the fundamental principle on which rocketry is based. We wouldn't have sent humans to the moon without it.
Whitewater kayaking offers a more Earthbound example. When the kayaker pushes his/her oar backward, the kayak moves forward. The swift currents help immensely with the speed, hence the thrill seeker's partiality for white water kayaking.
NEWTON'S UNIVERSAL LAW OF GRAVITATION: The magnitude of the force exerted between any two massive objects is proportional to the masses of both and inversely proportional to the square of their separation distance.
Regard two asteroids:
They exert a gravitational force on each other. That force depends on the asteroids' masses and their separation distances. If you doubled the mass of one of the asteroids, the gravitational force it exerts on the other is doubled. However, if you double the distance between them, the magnitude of that force is reduced to one quarter of its original value. Triple the distance and the force is reduced to one ninth of its initial value.
Theoretically, gravity's range is infinite. Even the Andromeda Galaxy is tugging on us a bit. However, as the Andromeda Galaxy is immensely far away, the force is negligible. Scientists now know however, that Newton's Universal Law of Gravitation is indeed Universal.
That notion brings us naturally to the
FUNDAMENTAL THEOREM OF ASTROPHYSICS: the physical laws that govern Earth are applicable throughout the Universe.
Without the fundamental theorem, astrophysics would never have gotten off the ground. (pause for mood improving laugh.) We know that a dropped ball on a planet somewhere in the Perseus Supercluster is experiencing the same forces a ball experiences on Earth. Even though the force might be weaker or stronger depending on the other planet's surface gravity, the physical principles governing that ball's motion will be the same.
Knowing that physics is Universal will prove very helpful to us as we continue our excursions through the cosmos.
Tomorrow, right ascensions and starry declinations.
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