52 terms

Laws of Motion

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Early Ideas about motion from Aristotle
The earth is the center of the universe
Close things fall toward the earth and celestial bodies naturally orbit the earth
It is "the nature of things" to be at rest
It requires a force to make something move
It requires a force to keep it moving
Two types of motion from Aristotle
Natural and Violent
Natural Motion from Aristotle
Vertical motion that occurs "naturally"
Heavy things fall toward earth (rocks of clumsy people)
Light things rise from earth (smoke or clouds)
Violent Motion
Horizontal motion imposed by force
a horse pulls a cart, an athlete kicks a ball
Copernicus and Motion
published (on his death bed) a work demonstrating the heliocentric universe - the earth and other planets ( wanderers) are in orbit around the sun
Galileo and Motion (cosmic level)
out spoken proponent of the Copernican system (got him "in trouble)
discovered sun spots
discovered moons of Jupiter
Galileo and Motion (terrestrial level)
performed famous ramp experiment to test how things move
decided you should test nature with nature (not philosophical reasoning)
discovered objects at rest will stay at rest unless force to move
discovered objects in motion will stay in motion unless forced to stop
Newton First Law of Motion (in a word)
The Law of Inertia
Newton's First Law of Motion (formally stated)
Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it.
Newton's First Law of Motion (simply stated)
resistance to change in motion
What property of matter accounts for its inertia
mass
relationship between mass and inertia
intimately related - you can't have one without the other - they are directly related - the greater the mass the greater the inertia
mass (definition)
a measure of the amount of matter of an object
mass (units of)
grams; kilogram
weight (definition)
a measure of the pull of gravity on an object
weight (units of)
newton; pounds
volume (definition)
a measure of the amount of space an object occupies
volume (units of)
cubic meters; cubic decimeters (liters); cubic centimeters (milliliters)
Force (definition)
A push or pull on an object; a physical quantity that can effect the motion of an object (can cause it to accelerate)
Net Force
The combination of all forces acting on an object
Force equilibrium
when net forces acting on an object combine to equal zero
the object therefore is not accelerating
Force Equilibrium (examples)
a book on the table. a person sitting in a chair, a car driving down a straight section of the freeway (at the speed limit)
Are all objects at equilibrium at rest?
no, they are merely not accelerating; they could be moving at a constant velocity like a skydiver at terminal velocity
types of forces
weight, support, normal, tension, friction, many others
support force
force that completely balances the weight of an object at rest
normal force
a force that acts on a surface in a direction perpendicular to the surface
tension force
force on a "rope" pulling or supporting an object.
friction force
force that opposes motion of an object when two surfaces are in contact
Newton's Second Law of Motion (Two Words)
Force -> Acceleration
Newton's Second Law of Motion (Formally Stated)
The acceleration produced by a net force on an object is directly proportional to the magnitude of that force in the same direction as that force and is inversely proportional to the mass of the object.
Newton's Second Law of Motion (Simply Stated)
Acceleration is caused by net forces
Newton's Second Law Equation
F = ma
Acceleration Definition Equation
a = change in v/t
Acceleration Causal Equation
a = F/m
relationship between force and acceleration
the greater the force the greater the acceleration
relationship between mass and acceleration
the greater the mass the lesser the acceleration
from this free body diagram describe the motion of the box
the box may be moving but it is not accelerating - it is in equilibrium - the horizontal arrows are the same size and the vertical arrows are the same size
from this free body diagram describe the motion of the bag
the bag is falling downward- it is also accelerating downward - the down arrow is larger than the up arrow
why does a light object fall as fast as a heavy object in the absence of air resistance?
gravity pulls harder on the heavier object than the light object thus their ratios (a = F/m) are the same (9.8m/s^2)
what happens to the rate of acceleration as an object falls in air? why?
the rate of acceleration decreases - the downward force of weight is constant but the upward force of air resistance increases until it equals the weight of the object at which point it stops accelerating (a = 0)
what is the velocity and acceleration of a skydiver at terminal velocity?
v ~ 120 mph
a = 0
what does it mean to say that a force is actually an interaction.
when an object gets pushed, it pushes back
Newton's Third Law (Two Words)
action/reaction
Newton's Third Law (Formally Stated)
Whenever one object exerts a force on a second object the second object exerts an equal and opposite force on the first object
Newton's Third Law (Simply Stated)
for every action there is an equal and opposite reaction
what part of the interaction is equal?
the magnitude of the force is equal
what part of the interaction is opposite
the direction of the force is opposite
what part of the interaction is usually unequal?
the result of the force
what is the reaction force to you pushing on the ground when you walk?
the ground pushes on you
what is the reaction force to a rocket pushing the exploding gases out?
the exploding gases push the rocket
when a bug hits your car's windshield who hits harder?
they hit with the same force
what is the action / reaction pair when you fall?
earth pulls on you / you pull on the earth