NOT AS OBVIOUS AS IT SEEMS GRAVITY NOT AS OBVIOUS AS IT SEEMS Misconceptions

GRAVITY IS A WEAK FORCE Electromagnetic force > force of earth’s gravity Really? Really! Otherwise, we’d be a puddle the EM force keeps atoms together, despite gravity wanting to tug them all to the earth More on this in a bit Gravity, the weakest of the four forces, is about 10-36 times the strength of the strong force. This weakness is easily demonstrable - on a dry day, rub a comb across your shirt to give it static electricity, then hold it over a piece of paper on a desk. If you were successful, the piece of paper lifts off the desk. It takes an entire planet to keep the paper on the desk, but this force is easily overcome with everyday materials employing the electromagnetic force. However, the range of gravity is unlimited - every object in the universe exerts a gravitational force on everything else. The effects of gravity depend on two things: the mass of two bodies and the distance between them. The electromagnetic force is actually second in effective strength only to the strong force, but it is listed out of order here because it, like gravity, is more familiar to most people. Its strength is less than 1% of that of the strong force, but it, like gravity, has infinite range. However, unlike gravity, the electromagnetism has both attractive and repulsive properties that can combine or cancel each other out. Whereas gravity is always attractive, electromagnetism comes in two charges: positive and negative. Two positive or two negative things will repel each other, but one positive and one negative attract each other. This can be neatly illustrated by magnets: two of the same "pole" will repel each other, but two opposite poles attract each other. The weak nuclear force operates only on the extremely short distance scales found in an atomic nucleus. The weak force is responsible for radioactive decay. In actuality, it is stronger than electromagnetism, but its messenger particles (W and Z bosons) are so massive and sluggish that they do not faithfully transmit its intrinsic strength. The strong nuclear forces range is limited to subatomic distances. Its "duties" are keeping quarks together inside protons and neutrons, and keeping protons and neutrons inside atomic nuclei. Another way to think about it is this: a proton has both charge and mass. If I hold another proton a centimeter away, how strong is the gravitational attraction? It's about10−57newtons. How strong is the electric repulsion? It's about10−24newtons. How much stronger is the electric force than the gravitational? We find that it's1033times stronger, as in 1,000,000,000,000,000,000,000,000,000,000,000 times more powerful! http://www.youtube.com/watch?v=BQA5VDXE7ts

Aristotle: reasoning-based Galileo: empirical evidence TWO SCHOOLS OF THOUGHT Aristotle: reasoning-based Galileo: empirical evidence Objects fall at different rates due to their different masses Neglected to consider other variables that affect the rate of fall Objects fall at the same rate so long as air resistance is equal Air resistance establishes terminal velocity – max rate of fall http://www.youtube.com/watch?v=AYz_K3mwq6A

WHY NOT MASS? Apply Newton’s laws of Motion: 2 m/s = 2N = 20N #2: Force = mass x acceleration aka a = F/m Where: mass is the falling object the force is gravity between object and Earth the fall is the acceleration • #1: More mass has greater inertia, requires more force to move it • a proportional relationship between force and mass • therefore objects fall at same rate 2 m/s = 2N = 20N (rate of fall) 1 Kg 10Kg smaller bigger mass mass

NEWTON, ROUND 2 Gravity is not restricted to objects falling to earth Every object in the universe exerts a gravitational force on everything else aka Universal Gravitation Isaac Newton MinutePhysics

THE G-FORCE IS VARIABLE • Gravitational force depends on: - masses of the objects - the distance between them • Mass is proportional to force of gravity; greater masses have a greater g-force between them • Distance is inversely proportional to force of gravity; at greater distances, force between masses is lower

THEREFORE: FORCE CHANGES AS OBJECTS FALL TO EARTH Distance is reduced as an object falls Force increases proportionately Causes constantly increasing acceleration of fall Use: a=F/m a = 10N/2kg vs a= 100N/2kg greater less distance distance apart apart

HOW FAST? Terminal velocity Free fall Max rate with air resistance When resistance = g-force of fall; no addl. acceleration Increase resistance causes a slow down Without air resistance No variables affect rate http://www.youtube.com/watch?v=KDp1tiUsZw8 http://www.youtube.com/watch?v=5cPFXagNCe4

FREE FALL With no resistance object falls faster and faster Acceleration of 9.8m/sec/sec increasing distance each second http://www.youtube.com/watch?v=xQ4znShlK5A http://www.youtube.com/watch?v=OtxlQTmx1LE

SO WHAT? The relationship has two dependent variables: How much area is the force (gravity, spray paint…) spread out over? What is the power/intensity of the force (light, radiation…) per area unit? At increasing distance, the effect of the force (paint, sound, gravity…) is spread, much more thinly, over a much larger area

INVERSE SQUARE LAW The relationship between these variables is the inverse square law Governs G force as well as: : Sound http://auditoryneuroscience.com/acoustics/inverse_square_law Light http://www.youtube.com/watch?v=fF74ghL8txw http://bcs.whfreeman.com/universe7e/content/ch19/1903002.html Radiation http://zebu.uoregon.edu/images/cowbomb.mpg Paint http://www.mrwaynesclass.com/teacher/Gravity/InverseSquare/ Butter? http://www.youtube.com/watch?v=JW3tT0L2gpc

As the paper is moved away from the candle; what happens to the number of squares illuminated? What happens to the brightness on each square? How many squares will be lit up at 4 feet? Bonus; What is the percentage of light on any one square at 4 feet?