Download The Force! - Cobb Learning

Catalyst
1.Calculate the speed of an object traveling 550km in 12
hours. Use correct units!!
45.8 km/hr
2.Explain the difference between speed and velocity. Use
Speed is distance
divided by time; Velocity is
the appropriate
vocabulary.
a vector and requires a direction!
3.The police are tracking an escaped prisoner. Would it
be more helpful
to know–her
speedmoving
or her velocity?
Velocity
is she
towards
WHY?
you??
4. Velocity of 5 m/s forward to 5 m/s to the right. Is this
Acceleration – changing direction!
Acceleration, Deceleration, or Neither
Forces
So, what causes acceleration?
Accelerate to the
Dark Side, Luke!
The Force!
No, not that force,
but it is FORCE that
causes acceleration.
FORCE
A push or a pull on an
object; an interaction
The Measurement of
Force
 m
1 Newton  1kg1 2 
 s 
• 1 Newton is the amount of force
required to give a 1 kg mass an
acceleration of 1 m/s2
• Must have a direction!!!
(yup, it is a vector)
Newton
Newton
UNITS
What are the units on
force?
What is Measured
Unit(s)
Symbol(s)
Force
N or
kg*m/s2
F
FOUR TYPES OF
FORCES
GRAVITATIONAL
FORCE
• An attractive force
between two bodies
• The weakest of all forces
ELECTROMAGNETIC
FORCE
• Charged particles at rest exert
electric forces on each other
• Charged particles in motion
exert magnetic forces on each
other
STRONG NUCLEAR
FORCE
• Holds particles of the
nucleus together
• The strongest of all
forces
WEAK FORCE
• Form of an
electromagnetic force
•Happens when some
nuclei radioactively
decay
GRAVITY FORCE
(Gravity Force) = (mass)
x (gravity)
Mass is measured in grams or
kilograms (kg). Gravity is
2
ALWAYS = 9.8m/s
Calculate your
gravitational force!
F=m*g
(1lb = 0.454 kg)
(100lb = 100 x 0.454 kg) = 45.4kg
F=
2
(45.4kg)(9.8m/s )
= 444.92N
What is the force weight of a 45
kg desk?
441 N
What is the force weight of a 12 kg
dog?
117.6 N
What is the mass of a 35 N cat?
3.57 kg (÷9.8)
NEWTON’S
FIRST
LAW
Newton’s First Law
An object at rest tends to stay
at rest and an object in
motion tends to stay in
motion with the same speed
and in the same direction,
unless acted upon by
unbalanced forces
Possibilities of
st
Newton’s 1
Newton’s First
Law
Also known as
“The Law of Inertia”
WEARING YOUR
SEATBELT IS THE LAW…
…THE LAW OF INERTIA!
Objects at rest tend to stay at
rest, objects in motion tend to
stay in motion, unless acted
on by an unbalanced force
INERTIA
the tendency of an
object to resist
changes in motion
Generally speaking,
the more mass an
object has, the more
inertia it has.
INERTIA
the tendency of an
object to resist
changes in velocity
INERTIA
the tendency of an
object to resist
changes in
acceleration
Supertankers illustrate the
principal of inertia; their
engines are cut off a mile
or more before the tanker
enters port and it still
coasts right on in.
For Example…
No motion!
It stays at rest!
When a hand pulls one ball, it’s that
unbalanced force we mentioned…
Voila! Motion!
The toy will stay in motion until….what?
It is acted on by an unbalanced
force.
BALANCED AND
UNBALANCED
FORCES
BALANCED FORCES
(no acceleration)
UNBALANCED FORCES
(motion / velocity / acceleration)
Forces are either balanced, or unbalanced
When forces are
equal, no
acceleration
occurs.
Forces are either balanced, or unbalanced
But when they
are unbalanced,
that’s when
objects
accelerate
Forces are either balanced, or unbalanced
But when they
are unbalanced,
that’s when
objects
accelerate
Forces are either balanced, or unbalanced
But when they
are unbalanced,
that’s when
objects
accelerate
Forces are either balanced, or unbalanced
But when they
are unbalanced,
that’s when
objects
accelerate
NET FORCE
the sum of
all forces acting
on an object
again…it’s a VECTOR!
When an object is
in equilibrium
(acceleration = 0),
Net Force= 0
(also called a
Balanced FORCE!)
Practice Finding the Net
Force
Net Force:
-2 N
__________
Left
__________
Hint: Make sure to have a number and a
direction…
Practice Finding the Net
Force
Net Force:
50 N
__________
Left
__________
Practice Finding the Net
Force
Net Force:
__________
0N
No direction
__________
Practice Finding the Net
Force
Net Force:
15 N
__________
Up
__________
The object accelerates
in the direction of the net
force, the vector sum.
What happens when more than one force acts on something at
the same time?
Review and Practice
• Textbook page 127, numbers 2-9
NEWTON’S
SECOND
LAW
Law
The net force acting on an
object causes an object to
accelerate in the direction
of that force
Law #2: The acceleration of an object depends
on the object’s mass and the strength of the
outside force acting on it.
Acceleration under
nd
Newton’s 2
• An object will only accelerate if
an unbalanced net force is
acting on it!
• The direction of acceleration is
the direction of the net force
Relationships of
nd
Newton’s 2
•If you double the net force, you
double the acceleration
(if mass is held constant)
•If you double the mass, you will
half the acceleration
(if force is constant)
Newton’s Second Law
Applied
(here gravity = 10m/s2)
Air Resistance
Why does the elephant fall faster than the
feather?
The force of gravity on the earth is ALWAYS
9.8 m/s2.
Air Resistance is the MAIN factor! Air
resistance is a type of FRICTION!
The elephant has a greater mass and
therefore has less air resistance.
The feather, due to it smaller mass is more
vulnerable to air resistance!
This is WHY the elephant falls faster!!
Law #2: The acceleration of an object depends
on the object’s mass and the strength of the
outside force acting on it.
If the forces acting on them are equal, who is
going to accelerate more?
Fat
Albert or
Rudy?
Calculating With Newton’s Second
Law.
F
a
m
1. F = 300 N
2. m = 1500 kg
3. a = ?
4. Plug and Chug
What is the acceleration of a
1,500 kg cow when it is pushed
by a force of 300 N?
Whiteboard Practice
What is the acceleration
of a 300 kg walrus being
pulled by a force of
1,000 N?
One step further...
What if we know
acceleration?
A 30,000 kg rocket
2
accelerates at 10 m/s .
How much force is acting
on it?
Force Practice
Problem
If Rebeka pushes a
100kg box so that it
2
accelerates 4m/s , how
much force does she
apply to the box?
Whiteboard Practice
• A 90 kg person is pushed with a force of 100 N.
What is her acceleration?
• A cannon ball is shot out of a cannon with an
acceleration of 10 m/s2. If its mass is 100 kg, how
much force does the cannon fire with?
• What is the mass of a cart if a horse pulls it with a
force of 500 N and accelerates it by 0.5 m/s2?
Forces don’t cause
motion, forces cause
ACCELERATIONS!
Accelerations cause
MOTION!
FRICTION
FRICTION
the force that opposes the
motion between two
surfaces in contact
Types of Friction
• Static Friction—friction between 2
things not moving past each other
• Sliding Friction (Kinetic)—friction
between 2 things that are moving
• Rolling Friction (Kinetic)—friction
between a rolling object and the
object it rolls on
Comparing
Friction
For an equally massed object of
the same substance:
Rolling Friction
is less than
Sliding Friction
which is less than
Static Friction
Static Friction
Friction applied to a
stationary object
Kinetic Friction
Friction
applied to a
moving object
Kinetic friction is
always LESS than
static friction.
WHY: Molecular forces form
on stationary objects.
Friction depends on
the two surfaces
involved.
TERMINAL
VELOCITY
TERMINAL VELOCITY
The point at which a
falling object ceases to
accelerate and falls at a
constant rate. Gravity
reaches an equilibrium!
NEWTON’S
THIRD
LAW
Newton’s
Third Law
For every action,
there is an equal and
opposite reaction
For every “action
force” there is an
equal in size and
opposite in direction
“reaction force”
Newton’s 3rd Law
…is a great one. Very poetic. It goes like
this:
For every action,
there is an equal
and opposite
reaction.
Let’s see an example.
ACTION:
Rocket Gases push
DOWN on air.
REACTION:
Air pushes up on rocket.
Try this one:
• ACTION:
• Wings push DOWN on
air.
REACTION:
Air pushes UP on
wings.
…And another!
• ACTION:
• Feet push back on the floor.
REACTION:
Floor pushes forward on feet.
Identify 6 pairs of actionreaction forces in the
following picture
Action = reaction means the bowler and the ball get
equal and opposite forces when the ball is launched.
But since the bowler’s mass is greater than the ball’s
mass, the bowler accelerates less than the ball.
ma = ma
To get cart B to go the greatest distance, who
should go on which cart?
A
B
Rudy
Fat
Albert
How do Newton’s Laws explain what happens
next?
Newton’s First Law:
Science
English
An object at rest will
stay at rest.
Neither cart will move
until it’s pushed or
pulled.
How do Newton’s Laws explain what happens
next?
Newton’s Second Law:
Science
English
An object’s acceleration
depends on its mass and
the force acting on it.
Fat Albert will not
accelerate as much as
Rudy.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
For every action force
there is an equal
reaction force acting in
the opposite direction.
English
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How do Newton’s Laws explain what happens
next?
Newton’s Third Law:
Science
English
For every action force
there is an equal
reaction force acting in
the opposite direction.
Rudy and Fat Albert will
be pushed with the
same force, in opposite
directions.
How would the following be
examples of Newton’s 3rd
Law?
•A fish swimming through the
water
•A bird flying through the air
•A car moving forward