Transcript File - Dr. Wall`s Science

Unit 6 Chapter 6
Forces and Motion
Gravity and Motion
• 400 BC
• Aristotle
– Thought that the rate at which an object falls
depends on its mass
– Heavier things fall faster
– Never tested this theory
Gravity and Motion
• 1589
• Galileo Galilei
– Argued that Aristotle was wrong
– Galileo said that mass doesn’t affect the rate of
objects falling
– Proved argument with the Leaning Tower of Pisa
experiment
• Stairwell demo
Gravity and Motion
• Acceleration due to gravity
– Acceleration is the change in velocity over time
– All objects on Earth accelerate toward Earth at a
rate of 9.8 m/s2
– For every second you are in free fall, you speed up
at a velocity of 9.8 m/s
– We use the letter g as an abbreviation for this (9.8
m/s2)
Gravity and Motion
• Δv = g x t
• Change in velocity equals acceleration due to
gravity multiplied by time
• To find velocity in free fall, multiply 9.8 times
the number of seconds from start to finish
Gravity and Motion
• Air resistance
– Opposes motion of objects falling
– Flat paper vs crumpled, which falls faster?
• Terminal velocity
– As velocity increases, air resistance increases
– Upward force of air resistance increases until it is
equal to g
– At this point, object stops accelerating, and falls at a
constant velocity – terminal velocity
Gravity and Motion
• Terminal velocity is what keeps small things
from hitting the ground extremely fast
• Hail would normally fall at 500 mph without
terminal velocity
• Free fall
– Motion of an object with only the force of gravity
acting upon it
Gravity and Motion
• Feather vs lead
• In normal circumstance, feather falls slower
due to air resistance
• In a vacuum, they fall at the same rate, since
there is no air resistance
• Free fall is how astronauts feel in outer space,
because the force of gravity is very small
• Are the astronauts weightless when floating?
Gravity and Motion
• Orbiting – an object traveling around another
object in space
• The moon orbits the Earth
• The Earth orbits the Sun
• The sun orbits around the galaxy
Gravity and Motion
• Most orbits are circular in motion
• Objects in circular motion are always changing
direction
• Force that causes objects to move in circular
motion is called centripetal force
• Always moving toward the center of what you
are orbiting
Gravity and Motion
• Projectile motion
– Curved path an object follows when thrown
– Has two components
• Horizontal - outward straight
• Vertical – downward straight
– When two motions are combined, forms a curve
Newton’s Laws of Motion
• 1686
• Sir Isaac Newton
– Came up with explanations of why things move
the way the do
– Three laws of motion
Newton’s Laws of Motion
• Newton’s 1st Law
– An object at rest remains at rest, and an object in
motion remains in motion at constant speed and
in a straight line unless acted upon by an outside
force
– Something moving will move in a straight line
forever
– Something sitting still will stay sitting still forever
– Unless some force stops or starts it moving
Newton’s Laws of Motion
• Slide a book across your desk
• Does it stay moving forever?
• What stops the book from moving?
• Friction is an unbalanced force that opposes
motion
• Causes moving objects to slow down
Newton’s Laws of Motion
• Inertia
• Newton’s 1st Law sometimes called Law of
Inertia
• Inertia is tendency to resist any CHANGE in
motion
• Larger the mass, the larger the inertia
• Object at rest stays at rest because of inertia
• Object moving stays moving because of inertia
Newton’s Laws of Motion
• Newton’s 2nd Law of Motion
– The acceleration of an object depends on the
mass of the object and the amount of the force
applied
– Acceleration depends on mass
• Larger things accelerate slower
– Acceleration depends on force
• Larger force, larger acceleration
– Written as F = ma
Newton’s Laws of Motion
• a = F/m
• F = ma
• Find the force of a 10 kg object being accelerated at
100 m/s2
• Find the acceleration of an object that was pushed with
a force of 100 N and has a mass of 20 kg
• How do you find the mass of an object if you know the
force and the acceleration?
Newton’s Laws of Motion
• Newton’s 3rd Law
– Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first
– For every action there is an equal and opposite
reaction
– All forces act in pairs
– Your butt pushes down on the chair, the Earth is
pushing back up on your butt with the same force
Newton’s Laws of Motion
• Forces do not always act on the same object
– Tires and road
• Can sometimes be hard to see
– A ball thrown through the air
Momentum
• Momentum
– Depends on mass and velocity
– Larger the mass, more momentum
– Larger the velocity, more momentum
• p = mv
• Momentum = Mass x Velocity
Momentum
• What has more momentum, a 1300 kg car
traveling at 25 m/s, or a 5000 kg truck
traveling at 10 m/s?
• p = mv
Momentum
• Law of Conservation of Momentum
– Any time objects collide, the total amount of
momentum stays the same
– This applies whether the objects bounce off each
other or stick together
– Cars sticking together after a wreck, cue ball shot
at 8 ball when playing billiards