Download CHAPTER 12 POWERPOINT

Chapter 12 – Forces and Motion
12.1 Forces
What is a Force?
Force – A push or pull that acts on an object
12.1 Forces
What effect does a force have on an object?
1. Can cause an object to speed up or slow down
2. Can cause an object to change direction
In other words a force causes an object to accelerate
12.1 Forces
The SI Unit for measuring force is the newton (N)
Named in honor of Sir Isaac Newton
A newton is the force that causes a 1 kg mass to accelerate at
a rate of 1 m/s2
1 N = 1 kg · m/s2
Force is a vector – has both magnitude and direction
12.1 Forces
Since force is a vector, it can be represented with an
arrow
The direction of the arrow
represents the direction of
the force
The length of the arrow
represents the strength, or,
the magnitude of the force
12.1 Forces
Forces can be combined by vector addition
Forces in the same
direction add together
Forces in the opposite
direction subtract
The net force is the overall force after all forces have
been combined
12.1 Forces
force of man
on toy
=
force of dog
on toy
The dog and
man pull with
the same
amount of
force, but in
opposite
directions
When the forces on an object are balanced, the
net force is zero and there is no change in the
object’s motion
12.1 Forces
force of stick
on ball
When an unbalanced force acts on
an object, the object accelerates
12.1 Forces
What is friction?
friction – a force that opposes the motion of objects that
touch as they move past each other
friction acts at the surface where objects are in contact
4 main types of friction
1. Static friction
3. Rolling friction
2. Sliding friction
4. Fluid friction
12.1 Forces
static friction – the friction force that acts on objects that
are not moving
static friction
on crate
=
pushing force
on crate
The crate won’t move unless the pushing
force overcomes the maximum static
frictional force
12.1 Forces
sliding (kinetic) friction – a force that opposes the
direction of motion of an object as it slides over a surface
sliding friction
on player
friction always acts in a direction opposite the
motion of an object
12.1 Forces
12.1 Forces
sliding friction is always less than static friction
12.1 Forces
static and sliding friction have two primary causes
friction always acts in a direction opposite the
motion of an object
12.1 Forces
rolling friction – caused by the change in shape at the
point of rolling contact
friction always acts in a direction opposite the
motion of an object
12.1 Forces
fluid friction – opposes the motion of an object moving
through a fluid
friction between
submarine and water
air resistance is a type of
fluid friction
friction always acts in a direction opposite the
motion of an object
12.1 Forces
gravity – a force that acts between any two masses
an attractive force – pulls objects
together
12.1 Forces
12.1 Forces
gravity – a force that acts between any two masses
Earth’s gravity acts downward
toward the center of the Earth
A field force that can act over
large distances
12.1 Forces
normal force
normal force
gravitational
force
gravitational
force
An upward force balances the downward force
of gravity (normal force)
12.1 Forces
Gravity (a force) causes objects to accelerate downward
air resistance
gravitational
force
as objects fall to the ground
they accelerate
air resistance opposes the
motion
with greater speed, comes
greater air resistance
What is the direction of the net force on the squirrel?
Why is the squirrel able to increase the distance covered
when it jumps if the air resistance increases?
12.1 Forces
Gravity (a force) causes objects to accelerate downward
air resistance
gravitational
force
as objects fall to the ground
they accelerate
air resistance opposes the
motion
with greater speed, comes
greater air resistance
terminal velocity – the constant velocity of a falling object
when the force of air resistance equals the force of gravity
12.1 Forces
Terminal Speed
1. What is the net force on the skydiver
just before stepping out of the
plane?
2. How do the force of gravity and air
resistance compare as the skydiver
is falling and gaining speed?
3. How does the force of air resistance
change as the skydiver gains speed
while falling?
4. What happens to the skydiver’s
speed when the force of air
resistance becomes equal to the
force of gravity?
5. How does the force of air resistance
change when the parachute opens?
12.1 Forces
12.1 Forces
12.1 Forces
Projectile Motion
projectile motion – the motion of a
falling object (projectile) after it is
given an initial forward velocity
a projectile follows a curved path
despite an initial forward velocity,
both projectiles hit the ground at
the same time
12.1 Forces
12.1 Forces
When a sufficiently massive star reaches the end of its lifetime, it explodes
in a violent event called a supernova. The core of the star collapses
suddenly, releasing vast quantities of particles and energy that blow the star
apart.
As shown in this animation, an expanding shell of gas called a supernova
remnant spreads away from such a stellar cataclysm. (The animation ends
with an X-ray image of the supernova remnant Cassiopeia A.)
12.1 Forces
The first part of this time-lapse video
shows how stars near the very center
of the galaxy moved from 1992 through
1998. The area shown in these infrared
images is about 0.12 parsec (0.38 lightyears, or 24,000 AU) on a side. The
second part of the video zooms in on
the motion of one particular star called
S2, and shows how this star is
expected to move through 2006.
The stars' motions indicate that they
are orbiting around an unseen object at
the position marked by the yellow
cross. Using Newton's form of Kepler's
third law, astronomers calculate that
the mass of this object is about 3.7
million solar masses. This compact,
invisible object is almost certainly a
supermassive black hole.
12.1 Forces
12.2 Newton’s 1st and 2nd Laws of Motion
Aristotle (384 B.C. – 322 B.C.)
Proposed that force is required to
keep an object moving at constant
speed
While right about many things he
was INCORRECT about this
Where else did we hear about Aristotle?
12.2 Newton’s 1st and 2nd Laws of Motion
Galileo (1564 - 1642)
Discovered that gravity
produces a constant
acceleration
Concluded that moving object not subjected to a force
would continue to move indefinitely
12.2 Newton’s 1st and 2nd Laws of Motion
Isaac Newton (1642-1727)
Built on the work of Galileo
Defined mass and force
Introduced his laws of motion
His important work in which all of
this is published is called Principia
12.2 Newton’s 1st and 2nd Laws of Motion
12.2 Newton’s 1st and 2nd Laws of Motion
Newton’s 1st Law of Motion
The Law of Inertia – The state of motion of an object does
not change as long as the net force acting on the object is
zero.
Inertia – The tendency of an object to resist a change in its
motion.
An object at rest remains at rest, and an object in motion continues in a
straight line at a constant speed, unless acted upon by an outside force.
12.2 Newton’s 1st and 2nd Laws of Motion
12.2 Newton’s 1st and 2nd Laws of Motion
12.2 Newton’s 1st and 2nd Laws of Motion
12.2 Newton’s 1st and 2nd Laws of Motion
Newton’s 2nd Law of Motion
Mass – A measure of the inertia of an object. Depends
on the amount of matter the object contains.
The Law of Acceleration - The force needed to give an object
a given acceleration is equal to the object’s mass times the
acceleration.
F = ma
kg · m/s2
N
12.2 Newton’s 1st and 2nd Laws of Motion
Newton’s 2nd Law of Motion
The acceleration of an object is always in the direction
of the net force.
12.2 Newton’s 1st and 2nd Laws of Motion
Newton’s 2nd Law of Motion
The acceleration of an object is always in the direction
of the net force.
The acceleration is inversely proportional to the mass.
12.2 Newton’s 1st and 2nd Laws of Motion
Using Newton’s Second Law
F = ma
Example 1
A 20.0 N net force acts on an object with a mass of 2.0 kg.
What is the object’s acceleration?
Example 2
A box has a mass of 150 kg. If a net force of 3000 N acts on
the box, what is the box’s acceleration?
Example 3
What is the acceleration of a 1000 kg car subject to a 500 N
net force?
Page 367 Math Skills and Math Practice
12.2 Newton’s 1st and 2nd Laws of Motion
1.
You are stuck going to the grocery store...your job is to
push the cart. If the cart has a mass of 15 kg and you
can push with a force of 20 N, what acceleration can
you give to the cart?
2.
Just before leaving the grocery store, the cart is now
loaded with 30 kg of stuff. Now, what acceleration can
you give to the cart (with the same amount of force)?
3.
Why did your answer for #2 change like it did?
Explain.
12.2 Newton’s 1st and 2nd Laws of Motion
mass and weight are NOT the same thing
Weight – The force of gravity acting on an object.
Weight is the product of the object’s mass and acceleration due to gravity
W = mg
kg · m/s2
g = 9.80 m/s2
N
mass is a measure of the inertia of an object, weight is a
measure of the force of gravity acting on an object
12.2 Newton’s 1st and 2nd Laws of Motion
mass is measured with a
balance
weight is measured with
a scale
12.3 Newton’s 3rd Law of Motion and Momentum
girl’s car on
boy’s car
=
boy’s car on
girl’s car
A force CANNOT
exist alone. Forces
ALWAYS exist in
pairs.
12.3 Newton’s 3rd Law of Motion and Momentum
Newton’s 3rd Law of Motion
The Law of Reaction – Whenever one object exerts a force
on a second object, the second object exerts an equal and
opposite force on the first object.
One of these forces is
called the action force and
the other is called the
reaction force
12.3 Newton’s 3rd Law of Motion and Momentum
Force of nail on hammer
Force of hammer on nail
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
Force gas on rocket
Force of rocket on gas
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
Ronald pushes on bar
Bar pushes on Ronald
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
Force of bat on ball
Force of ball on bat
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
Table on Child
Child on table
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
Ball pulls on Earth
Earth pulls on ball
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
Donkey
pulls cart
Cart pulls
donkey
Friction on
wheel
If the cart pulls back on the donkey, why does the cart move???
action – reaction forces DO NOT
act on the same object!!!!
12.3 Newton’s 3rd Law of Motion and Momentum
12.3 Newton’s 3rd Law of Motion and Momentum
12.3 Newton’s 3rd Law of Motion and Momentum
If a cement truck and a car have a head-on collision,
which vehicle will experience the greater impact force?
A. The cement truck
B. The car
C. Both the same
D. ….it depends on other factors
12.3 Newton’s 3rd Law of Motion and Momentum
If a cement truck and a car have a head-on collision,
which vehicle will experience the greater impact force?
A. The cement truck
B. The car
C. Both the same
D. ….it depends on other factors
12.3 Newton’s 3rd Law of Motion and Momentum
If both vehicles are going the same speed, which
would be harder to stop?
12.3 Newton’s 3rd Law of Motion and Momentum
Momentum – The product of an object’s mass and its
velocity.
Momentum = mass x velocity
p = mv
kg · m/s
12.3 Newton’s 3rd Law of Motion and Momentum
Which would be harder to stop….a .25 kg softball thrown
at you at 40 m/s or a 7 kg bowling ball rolled at 1 m/s?
It would be harder to stop the softball; it has more momentum.
12.3 Newton’s 3rd Law of Motion and Momentum
Law of Conservation of Momentum – If no net force acts
on a system the total momentum of the system doesn’t
change.
12.3 Newton’s 3rd Law of Motion and Momentum
300,000 kg · m/s
300,000 kg · m/s
momentum is conserved
12.3 Newton’s 3rd Law of Motion and Momentum
300,000 kg · m/s
300,000 kg · m/s
momentum is conserved
12.3 Newton’s 3rd Law of Motion and Momentum
Data analysis on page 377
12.3 Newton’s 3rd Law of Motion and Momentum