1. NOT AS OBVIOUS AS IT SEEMS
GRAVITY
NOT AS OBVIOUS AS IT SEEMS
Misconceptions

2. 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 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!

3. 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

4. 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 = N = 20N
(rate of fall) Kg Kg
smaller bigger
mass mass

5. 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

6. 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

7. 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

8. 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

9. FREE FALL With no resistance object falls faster and faster
Acceleration of 9.8m/sec/sec
increasing distance each second

11. 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

12. INVERSE SQUARE LAW
The relationship between these variables is the inverse square law
Governs G force as well as: :
Sound
Light
Radiation
Paint
Butter?

13. 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?