Transcript Forces and Motion

Forces and Motion
Chapter 2 – Gravity, Motion, and Light
Focus activity
• You are an astronaut working on the
outside of the international space
station. You do not have a rocket
back pack. Your line breaks and you
begin to float away from the space
station. All you have is your tool
box which is loaded with tools. Are
you able to get back to the station. If
so how?
Matter
• Anything that has shape and takes up
space.
• All matter is composed of Atoms
• Atoms have several subatomic
particles.
– _________&_________ – In the nucleus
– __________ – Orbit around the nucleus
• All matter, and subsequently atoms,
are subject to four forces of Nature.
Forces of Nature
•
– The force of attraction that is
between two bodies and is generated by
their masses.
– More later
•
– The force arising between
electrically charged particles or between
charges of a magnetic field. (holds
electrons to the nucleus)
– More later
•
force – Only important on a
subatomic scale. Holds the protons to
the neutrons in the nucleus.
•
force – Only important at an atomic
scale and is responsible for radioactive
decay.
Gravity
• _______gives the universe its structure.
• It acts on all objects.
• Every particle is drawn toward every
other particle by its pull.
•
– The tendency for an object
at rest to remain at rest, and a body in
motion to remain in motion in a straight
line at a constant speed.
• Inertia was first demonstrated by
Galileo, but describe in to the law we
know today by Isaac Newton.
Newton’s first law of motion
• The law of inertia
• Objects at rest remain at rest unless
a force is acted upon it. Objects in
uniform motion continue moving in a
straight line unless other forces act
upon it.
• Newton’s first law explains how
planets move around the sun.
• Video
Activity
• Use your stop watch to time how long it takes
the ball to hit the ground when thrown.
• Use your stop watch to time how long it takes
the ball to hit the ground when dropped.
• What was the vertical height of the ball when
thrown?
• What was the vertical height of the ball when
dropped?
• What forces were acting on the dropped ball?
• What forces were acting on the thrown ball?
• Repeat the experiment using your lab table
and car. (Use a jacket to make a cushion for
your car.)
• Are there any similarities in your results?
Determining the rate of drop
• Velocity is how fast an object is moving
in one direction.
• Velocity =
• V=d/t so
• T = d/v
• On Earth gravity causes objects to fall at
9.8 m/s2
• Notice that an objects mass has little to
do with its rate of fall in this example.
• However, an objects mass has a lot to do
with how quickly an object can accelerate
Newton’s second law of motion
Newton’s second law deals with a change in
direction or the acceleration of an object.
•
– A change in an object’s
velocity. (a deviation from uniform
velocity).
– Acceleration can be positive - speeding up
– Acceleration can be negative - slowing down
– A change in direction is also a change in
acceleration.
•
– The amount of
acceleration that a force can produce,
depends on the mass of the object being
accelerated.
The math behind Newton’s 2nd law
• __________ = object’s mass x acceleration.
• F = ma
• Force is measured in Newtons, mass in
grams, and acceleration in meters/sec.
• Shortly you will measure the amount of
force necessary to move two objects
across two different surfaces.
• Newton was able to deduce the law of
gravity using the moon’s motion and
applying his second law of motion.
The law of gravity
• Every mass exerts a force of
attraction on every other mass. The
strength of the force is directly
proportional to the product of the
masses divided by the square of
their separation.
• OR simply : Gravity = GMm/r2
• G is a constant = 6.67x10-11
• We will not be calculating the force
of gravity between two objects
The effects of Gravity
• Gravity is the force that keeps us
“attached” to the Earth.
• It also keeps the moon orbiting
around the Earth.
• The moon’s gravitational pull is what
causes the tides.
– We will discuss the tides later in the
semester.
• Newton then used is first two laws to
state his third law – action-reaction.
Newton’s third law of motion
• When two bodies interact, they
create equal and opposite force on
each other. See fig 2.8 in book.
• When I asked how would you return
to the space station with only a tool
box in your possession?
• We relied on Newton’s third law. The
force you applied to the tool box was
equal and opposite to the force that
the tool box applied on you, thus it
would force you to move in the
opposite direction.
Pulling g’s
• The term pulling g’s refers to the
gravitational attraction at a planet’s surface.
• Otherwise known as surface gravity.
• Surface gravity (g) is VERY important.
– It determines your: weight
– The celestial body’s shape
– The presence &/or the components of the
atmosphere
• The formula for determining surface gravity
is: g = GM/R2.
– G = constant 6.7x10-11
– M = The mass of the attracting body
– R = Radius of the attracting body
Determine your weight on the Moon
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Mass of the moon = 7.3x1022
Radius of the moon = 1.7x106
G= 6.7x10-11
Formula for g = GM/R2
The g on earth = 9.8
divide 9.8 by gmoon and that is how
many times different your weight is
on the moon than on earth
• Multiply your weight by that number
to get your lunar weight.
Escape velocity
•
• Escape velocity = √2GM/R
– G = constatnt (6.7x10-11)
– M= Mass of the body be escaped from
– R= Radius of the body to be escaped from
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The escape velocity from the moon is 2.4 km/second
The escape velocity from the Earth is 11 km/second.
Why is it easier to escape the moon than Earth?
Does escape velocity have an effect on the ability of
a planet to sustain life?