Transcript A Force - Cloudfront.net

Chapter 6
Forces and Motion
Section 1
Gravity and Motion
Essential Questions for 6-1
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What is a force?
What are the effects of gravity and air
resistance on falling objects?
Why are objects in orbit in free fall?
Why do they appear to be weightless?
How is projectile motion affected by
gravity?
Forces and Fields
• A Force is a push or a pull
• A Force Field is an area in which a
force is experienced.
Vectors and Scalars
• Vectors are quantities that have both
magnitude and direction.
– For example: Velocity: 45mph West
• Scalars are quantities that just have
magnitude
– For example: Speed 45mph
Why use vectors
• They show the forces acting on an object.
We can add them or subtract them to
determine the Net force acting on an
object.
This is Galileo Galilei (15641642)
Galileo went to the Leaning Tower of
Pisa…
… dropped two cannon balls of
different masses to the
ground…
…and this is what happened.
Force of Gravity
• Gravity = A force that pulls objects
together.
• On Earth, gravity makes any object of
any mass accelerate toward Earth at 9.8
m/s2.
FG  mass acceleration
2
FG  mass  9.8 m/s
Relationships in FG = m x 9.8 m/s2
• Heavier objects do not fall faster than
lighter objects.
• Heavier objects hit the ground with a
Stronger force, but they accelerate toward
Earth at the same rate.
Gravity and Falling Objects
• Objects fall to ground at the same rate because
acceleration due to gravity is the same for ALL
objects
• Why? Acceleration depends on
both force and mass.
– A heavier object experiences a
greater gravitational force, BUT it
is also harder to accelerate.
– Galileo was a genius.
Acceleration and Velocity
• Acceleration is the rate velocity changes
over time. The acceleration of an object
affected by gravity is 9.8 m/s.
• Change in velocity of falling objects can be
measured by the following equation:
∆v = g x t
OR 9.8 m/s times the number of seconds
an object falls…
Formula for Force of G
where:
•F is the force between the masses,
•G is the gravitational constant,
•m1 is the first mass,
•m2 is the second mass, and
•r is the distance between the centers of the masses.
The force of gravity we experience on Earth is primarily due
to the pull of the sun on the Earth. Although, we are far from
it, its’ mass is so great it creates the strongest gravitational
pull in our solar system
Which of these objects will fall at the
fastest rate when dropped?
A. The ball with a mass
of 75 kg
B. The ball with a mass
of 25 kg
C. The ball with a mass
of 10 kg
D. They all fall at the
same rate.
Mr. I
Earth
Moon
a = 9.8 m/s2
m = 70 kg
a = 1.6 m/s2
m = 70 kg
Fgravity = ma
Fgravity = ma
Fgravity = 80 kg  9.8 m/s 2
Fgravity = 784 N
Fgravity = 80 kg  1.6 m/s 2
Fgravity = 128 N

Calculate Your Weight
Earth
Moon
a = 9.8 m/s2
m = your mass in kg
a = 1.6 m/s2
m = your mass in kg
Fgravity = ma
Fgravity = ma
Fgravity = (your mass)  9.8 m/s 2
Fgravity = (your mass)  1.6 m/s 2

Air Resistance
• Air resistance – the force that
opposes the motion of objects
through air
– Amount of air resistance depends
on the size, shape, and speed of
the object
– A place where all the particles have been sucked
out is called a vacuum. There is no Air resistance
in a vacuum, because there is nothing to cause
friction with.
Discussion
• Why would a feather dropped from the
same height as an acorn fall to Earth more
slowly than the acorn?
Discussion
• How can a sky diver benefit from air
resistance?
Terminal Velocity
• Terminal Velocity – when air resistance =
gravity
– 0 N or a balanced force!
– It’s the fastest an object will fall
– It’s also why rain drops don’t kill
you… think about it…
Free Fall
• Free fall - when gravity is the only force acting
upon an object
– Can only happen where there is NO air resistance:
SPACE (or a vacuum)
•
- Orbiting objects are in
free fall. Orbit is caused
by two motions:
- Orbiting objects
move forward, but are
also in free fall – so are
the astronauts
Discussion
• How does free fall in an orbiting spacecraft
cause the appearance of weightlessness?
Orbiting and Centripetal Force
• Orbit is caused by two
things that create an
unbalanced force
– Gravity provides
centripetal force to
the orbiting object
– Centripetal force – the
unbalanced force that
causes an object to
move in a circular path
– Ex: planets around the
sun or the moon around
the Earth
Discussion
• What does free fall in an orbit look like?
• Which is
– The path of the orbiting spacecraft?
– The path of free fall for the
spacecraft?
– The path the spacecraft
would follow if there was
gravity?
– The curve of the Earth’s
surface?
no
Projectile Motion
• The curved path an object follows when it is
thrown near the surface of the Earth
• Composed of horizontal motion and vertical
motion
– Horizontal motion (like throwing a ball) causes the
object to go forward
– Vertical motion (gravity) causes the object to fall
• Together they create a curved path: BUT still hits the
ground the same time as a dropped object
– This is why you always have to aim above a target
when trying to hit it…
An example of projectile motion
Quiz 6-1: 4 Questions
1. If a baseball and a cannonball are
dropped from the same height at the same
time, and there is no air resistance, which
ball will hit the ground first?
a. The cannonball lands first.
b. The baseball lands first.
c. The balls land at the same time.
d. The ball with the larger volume lands
first.
2. What feature of an object does not affect
air resistance?
a. its size
b. its chemical properties
c. its shape
d. its speed
3. Why do astronauts in space appear
weightless?
a. There is no gravity in space.
b. Air resistance opposes gravity.
c. They have no mass in space.
d. They are in free fall.
4. In what directions is a projectile
accelerated?
a. both vertically and horizontally
b. vertically downward
c. vertically upward
d. horizontally forward
Chapter 6
Forces and Motion
Section 2
Newton’s Laws of Motion
Essential Questions for 6-2
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What is Newton's first law of motion?
How does it relate to objects at rest and
objects in motion?
What is Newton's second law of motion?
What is the relationship between force,
mass, and acceleration?
What is Newton's third law of motion?
What are some examples of force pairs?
1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.
1st Law
• Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
motionless.
These pumpkins will not move unless acted on
by an unbalanced force.
1st Law
• Once moving,
unless acted
on by an
unbalanced
force (gravity
and air – fluid
friction), an
object would
never stop!
1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
• Ex. So far…
Discussion 1:
• You are a passenger in a car that is
moving rapidly down a straight road. As
the driver makes a sharp left turn, you are
pressed against the right side of the car.
Explain why this happens.
Why then, do we observe
every day objects in motion
slowing down and becoming
motionless seemingly without
an outside force?
It’s a force we sometimes cannot see –
friction.
What is this unbalanced force that acts on an object in motion?
• There are four main types of friction:
– Sliding friction: ice skating
–
Rolling friction: skateboarding
– Fluid friction (air or liquid): air or water resistance
– Static friction: initial friction when moving an object
Newton’s 2nd Law
2nd Law
The net force of an object is
equal to the product of its
mass and acceleration, or
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate. (Gravity = 9.8
m/s)
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
F = ma
98 N = 10 kg x 9.8 m/s/s
9.8 N = 1 kg x 9.8 m/s/s
Discussion 2:
• How does Newton’s second law explain
why it is easier to push a bicycle than to
push a car with the same acceleration?
3rd Law
• For every action, there is an
equal and opposite reaction.
3rd Law
There are two forces
resulting from this
interaction - a force
on the chair and a
force on your body.
These two forces are
called action and
reaction forces.
Newton’s 3rd Law in Nature
• Consider the propulsion
of a fish through the
water. A fish uses its fins
to push water backwards.
In turn, the water reacts
by pushing the fish
forwards, propelling the
fish through the water.
3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.
Other examples of Newton’s Third
Law
• The baseball forces
the bat to the left (an
action); the bat forces
the ball to the right
(the reaction).
3rd Law
• Consider the motion
of a car on the way to
school. A car is
equipped with wheels
which spin forward.
As the wheels spin
forward, they grip the
road and push the
road backwards.
3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.
So the third law says…
Discussion 3:
• How does Newton’s third law explain how
a rocket takes off?
Quiz 6-2: 3 Questions
1. The reaction force of a chair you
are sitting on
a. is greater than your weight.
b. is equal to your weight.
c. is determined by many factors.
d. varies.
2. Which of the following is the
equation for Newton’s second law of
motion?
a. F= m × a
b. m=F × a
c. a=m ÷ F
d. F=m ÷ a
3. Which is a common unbalanced force
acting on objects in motion?
a. inertia
b. acceleration
c. friction
d. speed
Chapter 6
Forces and Motion
Section 3
Momentum
Essential Questions for 6-3
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How is the momentum of a moving object
calculated?
How would you explain the law of
conservation of momentum?
Momentum
• Dependent on the objects mass and
velocity
• More momentum = harder to stop or
change direction of the object
• Momentum (p) = mass in kilograms times
velocity in m/s.
p=mxv
• Momentum must have a direction, like
velocity
Law of Conservation of Momentum
• When objects collide, the total amount of
momentum stays the same
• Momentum before collision always =
momentum after collision
– This is called a transfer of momentum
– Sometimes it is split between the objects
– Sometimes one object stops and transfers all
momentum to the object that was struck
Discussion
• Use Newton’s third law to explain how a
person hammering a nail into a block of
wood is demonstrating conservation of
momentum.
Problem
• A 0.151 kg baseball is pitched with a
velocity of 43.1 m/s. Ignoring air
resistance, what is the baseball’s
momentum after being pitched? Show
your work.
END
Quiz 6-3: 2 Questions
1. When a train increases its velocity, its
momentum
a. decreases.
b. remains unchanged.
c. increases.
d. is transferred to it surroundings.
2. The law of conservation of momentum
states that when two objects collide, their
combined momentum
a. increases after the collision.
b. remains the same after the collision.
c. decreases after the collision.
d. cannot be determined after the collision.