Chapter 2 Notes
The Ordered Universe
Physical events are predictable and quantifiable
The Birth of Modern Astronomy:
Ptolemy vs. Copernicus
Geocentric Model of Solar System – earth-centered model with other planets and sun orbiting the earth.
Heliocentric Model of Solar System – sun-centered model with the planets orbiting the sun.
Tycho Brahe and Johannes Kepler
Motion:
Mechanics – the study of the motion of objects
Galileo – the Father of Experimental Science
Speed – the distance an object travels in a certain amount of time.
Speed = distance traveled/time to travel that distance
s = d/t
Example: How fast does an object travel if it travels 500 m in 10 minutes?
SI Units – the International System of Units – a subset of the Metric System of Units.
scalar quantity – has only magnitude
Example: If an object travels 20 m, 50m and 30 m, how far does the object travel from where it started?
Velocity – the distance an object travels in a certain direction in a certain amount of time.
Vector quantity – has both magnitude and direction.
Example: If an object travels 20 m due west, 50m due east and 30 m due east, how far does the object travel from where it started?
Velocity = distance/time
v = d/t
Example: How fast does an object travel if it travels 500 m in 10 minutes?
Acceleration – the change in velocity of an object
Acceleration = change in velocity/change in time
a = (vf – vi)/t
Example: An object is traveling at 20 m/s, what is the acceleration of the object if the velocity increases to 70 m/s in 30 seconds?
The object “accelerates” – increasing velocity - + sign on acceleration
The object “decelerates” – decreasing velocity - - sign on acceleration
Example: An object traveling at 100 m/s stops in 45 seconds. What is the acceleration of the object?
Example: An object traveling at an initial velocity of 40 m/s has an acceleration of -2.0 m/s2. How fast is the object traveling after 10 seconds?
Laws of Motion:
Sir Isaac Newton – developed the Laws of Motion
First Law of Motion – a moving or stationary object will continue to move in a straight line at a constant velocity or remain stationary until a force acts on the object.
Second Law of Motion – the acceleration given to an object by an applied force is proportional to the magnitude of the applied force and is dependant upon the mass of the object.
Force Applied = mass of the object X acceleration of object
F = ma
The units of force are Newtons (N).
Example: A 40-kg object, moving at 100m/s, has a force of 20 N applied to it in the opposite direction of the motion of the object. What is the acceleration of the object? What is the velocity of the object after the force has been applied for 1 minute?
Third Law of Motion – for every action (force) there is a equal and opposite reaction (force).
Inertia – resistance to change.
Momentum = mass X velocity
p = mv
What is the momentum of a 0.50-kg object moving at 100 m/s?
Angular momentum – momentum applied to spinning objects
Angular Momentum = mass X velocity X radius of object
L = mvr
NOTE: v is in angular velocity units of revolutions/second or radians/second.
What is the angular momentum of a 10-kg spherical object of radius 0.10 m spinning at 10 revolutions/sec?
Torque – the twisting force which causes angular motion.
The Force of Gravity:
Universal Gravitational Force between any two objects having mass is given by the equation
Gravitational Force = (Gravitational Constant X object 1 mass X object 2 mass)/distance between object 1 and object 2 squared
FG = (Gm1m2)/d2
G = 6.67 X 10-11
What is the gravitational pull between the earth and a 1-kg object? The mass of the earth is 6.0 X 1024 kg and the radius of the earth is 6.5 X 106 m.
Weight is a special type of gravitational force for objects located on the surface of the earth.
Weight = mass X acceleration of gravity
W = mg
G = 9.8 m/s2
Mass - the amount of matter in an object.
What is the weight of a 1.0-kg object?
Why does an object on the moon have a “weight” of one-sixth the weight of the same object on the earth?
OUTSIDE ASSIGNMENTS:
Homework (p. 58-59): Review Questions 2, 3, 4, 5, 8, 9, 10; Discussion Questions 1, 2, and 7; Problems 2, 3, 5, and 7