Download Chapter 12 Forces and Motion

Law of Inertia:
*If an object is motionless, it will stay
motionless unless acted upon by some
force.
*If an object is moving, it will move in a
straight line unless acted upon at an angle
by some force.
*If an object is moving at a constant speed or
velocity, it will continue at the speed unless
acted upon by some force.
The Truck and Ladder
• an object in motion continues in motion with
the same speed and in the same direction unless acted upon by an unbalanced force.
Changing motion
*A force can cause the
motion of an object to change
Balanced and Unbalanced Forces
What is a Force ?
Force is a Push or Pull on an
object.
• Unit of Force is always
measured in Newton.
Balanced forces
• do not cause a change in motion.
• The force exerted by each person is
equal
• but they are pushing in opposite
directions
• Balanced forces are equal and
opposite in direction.
• Ex: 10
10
. newtons
newtons
Balanced forces in action is in a tug of war.
• They are equal in size and opposite in
direction.
• the resulting force is zero and there is
no change in motion.
unbalanced forces
• always cause a change in motion.
• They are not equal and opposite.
• Unbalanced forces are
unequal and opposite to each
other.
• Example:
2 Newton
.
22
8Newton
A net force is found by
subtracting two opposite
forces. 9 N
5N
.
9N-5N=4N
.
What is Friction?
• appears whenever two things rub
against each other
• No matter which direction
something moves in, friction pulls it
the other way
*Move left - pulls right.
*Move up - pulls down.
Sticking together
• Stronger the force pushing the
two surfaces together.
• The micro welds will come in
contact.
• To break these micro welds or
move one surface over other ,a
force must be applied.
Four types of friction
1. Static Friction
2.Sliding Friction
3.Rolling Friction
4.Fluid friction
Static Friction .
Friction between two
surfaces that are not moving
past each other.
Act in the direction opposite
to that of the applied force
Sliding friction
• when two solid surfaces slide
against each other.
• Is less than static friction
• Factors affecting sliding friction
- include weight (normal force)
- the stickiness of the two surfaces
Rolling Friction
Friction between a rolling object
and the surface it rolls on.
is much less than static or sliding
friction
Ex:easier to pull a load in a
wagon rather than dragging it.
Fluid Friction
Opposes the motion of an object
through the fluid .
Ex: Stirring a cake batter
Higher the speeds ,greater the friction
Ex: Bicyclists and speed skaters wearing a
racing suits to reduce the air resistance
Free Fall and the Acceleration of Gravity
• A free-falling object is falling under
the influence of gravity
• any object which is moving and
being acted upon only be the force
of gravity is said to be "in a state of
free fall."
Elephant and Feather - Free Fall
• if the elephant weighs more and
experiences a greater downwards
pull of gravity compared to the
feather, why then does it hit the
ground at the same time as the
feather?
Elephant and Feather - Free Fall
*In the absence of air
resistance, the elephant and
the feather strike the ground at
the same time.
Newton's second law
• The force/mass ratio is the same for each.
• The greater mass of the elephant requires
the greater force just to maintain the same
acceleration as the feather.
Law of Gravitation
Anything that has mass is attracted
by the force of gravity.
The attractive force depends on the
mass of the objects.
Gravitational acceleration is always
going to be 9.8 m/s2
•Horizontal
.,
.Motion
a
.. ...................Vertical
.
Motion
•When an object moves in a
circle, two forces affect its
motion.
•Inertia pulls it outward, trying
to resist changes in motion.
Centripetal force
• attempts to pull the
moving object toward
the center of the circle
that it is moving in.
•If inertia & centripetal
force are equal (balanced),
the object will move in a
circular path.
Inertia tries to
pull off road.
•
Friction
tries to
keep on
If inertia
equals
friction, car
stays on
road.
Newton's Second Law
• The net force acting on an object causes the
object to accelerate in the direction of the net
force
F is the net force in N,
m is the mass of an object in kg and
a is its acceleration in m/s2.
Mass Vs. Weight
Fw = m. ag
Fw or w = force of weight
(newton, N)
M = mass (kilogram, kg)
Ag or g = gravitational
acceleration(9.8m/s2 or m/s/s)
Mass Vs. Weight
Mass
Weight
Fw =mag
Fw or w = force of weight
ag or g =gravitational
acceleration(9.8m/s2 )
Depends on gravity at the place
1
Is always a constant at any
time
2
Is measured in kilograms in SI Is measured in Newtons (Not
unit
in Kilograms as one might
think
3
Is measured using balance
Is measured using Scales
4
Can never be Zero
Can also be Zero
5
Is an intrinsic (the essential
nature) property of a body
and is independent of
external factors
Depends on
1.Mass of object which is attracting it
2.Force with which it is being attracted
(which in turn depends on the
distance between the two
1.What is the weight of an infant whose
mass is 1.76kg?
2.What is the weight of a locomotive with a
mass of 18,000kg?
3.An F-14’s mass is 29,545 kg. What is
its weight?
4.What is the mass of a runner whose
weight is 648 N?
5.A baseball has a mass of 0.5 kg. What
is its weight?
6.A sled has a mass of 28 kg. What is
the weight of the sled?
The Third Law of
Motion
Newton’s Third Law of Motion
• States that for every action,
there is an equal and opposite
reaction.
Action-Reaction Pairs
• All forces come in pairs
• One object exerts a force on a
second object and the second
object exerts a force on the first
that is equal in size and opposite in
direction
Newton's Third Law of Motion
• For every action, there is an equal
and opposite reaction.
Action-Reaction Pairs Examples
Propulsion of Fish
• A - Fish use fins to push water
backwards
R- Water pushes the fish
forwards
Action-Reaction Pairs Examples
Flying motion of birds
• A - Wings of birds pushes air
downwards
R - The air pushes the bird
upwards
Action-Reaction Pairs Examples
Rocket Propulsion
• A - Rocket engine pushes the
gases out
the back of the
rocket
R - The gases push the car
forward
Remember
• This law states that there
are TWO objects BUT…
Only ONE Force
You Try Some
A swimmer is accelerating forward
Action - The swimmer pushes
against the water
Reaction - the water pushes
back on the swimmer and
pushes her forward.
A ball is thrown against a wall
• Action - The ball puts a force on
the wall
• Reaction - the wall puts a force
on the ball so the ball bounces
off.
A person diving off a raft
• Action - The person puts a force
on the raft pushing it
• Reaction - raft puts a force on the
diver pushing her in the opposite
direction.
Last one!!!!
• Action - A person pushes against
a wall
• Reaction - the wall exerts an
equal and opposite force against
the person
The flying motion of birds
The wings of a bird push air
downwards.
the air must also be pushing
the bird upward
The size of the force on the
air equals the size of the
force on the bird
Driving down the road, a firefly strikes
the windshield of a bus
• Each force is the same size
Momentum is the property a
moving object has due to its
mass and velocity.
Momentum =(mass)(velocity)
.
 p = m x v
A large object has a large
momentum because of its
large mass.(example- a ship)
A fast moving object has a
large momentum because of
its large velocity.(ex-a bullet)
Law of conservation of momentum
*If no net force acts on a system ,
then the total momentum does not
change
*The loss of momentum of one = the
gain in momentum of another objectmomentum is conserved.
Gravity and Mass
• Gravitational force of two objects depends
on the mass of the objects and the distance
between the objects.
• Larger the masses exert great gravitational
force
• Gravitational force decreases as distance
increases.
Compare the forces of gravity acting
between each pair of spheres and tell
how you reached your conclusion
1
a
2
a
1
b
2
b
3
a
3
b
Reaction # 3
Jane is tired of pushing her VW van to the gas
station. She gets an idea, “Hey I can go much
faster when I am rollerblading than when I am
walking. Why don’t I wear my roller blades?”
KEY TERMS
1. Friction: Resistance to motion that is
opposite the direction of travel. A force
that opposed the motion of an object.
2. Air resistance: The force that opposes
the movement of an object in air
3. Terminal velocity: Speed at which air
resistance and gravity acting on a falling
object are equal
4. Vacuum: Empty space
Falling Objects
1. All falling objects in the Earth’s
atmosphere are subject to two forces:
~ GRAVITY / AIR RESISTANCE
Falling Objects
2. air resistance
~ the greater the surface area
the greater the air resistance
Gravity
Gravity
FORCE
FORCE
Terminal Velocity
3. When the air
resistance equals
the pull of gravity
the object stops
accelerating and
stay at a constant
Air
velocity.
Resistance
Gravity
Vacuum
1. Vacuum:
~ empty space = no atmosphere
~ only subject to one force:
GRAVITY
EARTH’S ATMOSPHERE
Gravity
Air
Resistance
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Gravity
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