Transcript Newton`s Laws of Motion

NEWTON’S
LAWS OF
MOTION
I. LAW OF INERTIA
II. LAW OF ACCELERATION
III. ACTION-REACTION
FORCES
Any action that can change the state of an object’s motion
A PUSH or PULL that acts on an object
 Causes resting object to:
 Move / Accelerate
 Causes moving object to:
 Accelerate by changing the object’s speed or direction/ can be positive or
negative acceleration (speed up or slow down)
FORCES
Force: action exerted on a body in order to change the body’s state of rest or motion
 Contains:
 Direction
 Magnitude
 Represented by Vectors
4 FUNDAMENTAL FORCES
The 4 fundamental (Basic) forces are: In Order from Strongest to Weakest
Strong nuclear force
Weak nuclear force
Electromagnetic force
Gravity
GRAVITATIONAL AND ELECTROMAGNETIC FORCES
They act over LONG distances
Electromagnetic
Electromagnetic force is about 1/100 the strength of the strong force
Causes FRICTION
Everyday forces such as friction, magnetic, tension, and spring force.
Gravitational
Any two masses are attracted toward each other by gravity
Gravitational force is much weaker than electromagnetic force
Holds e- in rotation around nucleus
Holds planets in rotation around the sun and the causes the moon to orbit the earth
ELECTROMAGNETIC FORCES
1/100 size of a strong force
Everyday forces
Friction
STRONG AND WEAK NUCLEAR FORCES
They act over a SHORT distance
These are negligible over long distances
- Do not act over long distances
Work at the nuclear level (within an atom or between atoms)
Strong Nuclear Force
Holds protons and neutrons together in the nuclei
Strongest of all forces
Weak Nuclear Force
Holds molecules together
2 BASIC TYPES OF FORCES
1. Contact Force
Touch is involved
An object is literally pushed or pulled by contact
2. Field Force
Do not require that objects touch
Attraction or repulsion between two same poles
Ex. The way a magnet will push away from another magnet.
** Both Contact or Field forces cause the MOTION of objects to change**
BALANCED AND UNBALANCED FORCES
**Whenever there is an unbalanced net force acting on an object, the object
accelerates in the direction of the net force**
An object will not accelerate if the net force on the object is zero
REPRESENTING FORCES
Use Vectors (arrows) to represent direction/strength (magnitude) of force
Adding forces
 ----------------> --------> = ------------------------>
Motion to the RIGHT
Subtracting forces
 ------> <-----------------
= <---------
Motion to the LEFT
Equal and opposite forces
---------><--------- = 0
No Motion!!!
BALANCED AND UNBALANCED FORCES
MOTION
Why 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.
FRICTION
Friction is the force that OPPOSES the Motion of two objects that are in contact
- The force of friction always opposes motion
- occurs because the surface of any object is rough
2 TYPES OF FRICTION
1. Static Friction: the force that resists the initiation of sliding motion between two
surfaces that are in contact or at rest
Static Friction makes it possible for us to walk.
**Static = no motion
Example: The resistance you feel when you first start to push your
car on E off to the side of the road.
2. Kinetic Friction: the force that opposes the movement of two surfaces that are in
contact an are moving over each other
**Kinetic = motion
Example: the resistance you feel while you are pushing that same car while it is
rolling .
3. TYPES OF KINETIC FRICTION
1. Sliding Friction: force that opposes the direction of motion of an object as it slides over a
surface.
EX. Pushing a box across the floor
2. Rolling Friction: friction force that acts on rolling objects
Ex. Moving a box on a dolly across the floor.
3. Fluid Friction: opposes the motion of an object through fluids
such as air or water.
Ex. Airplane flying through the air or fish swimming
in the lake.
Slide a book across a table and
watch it slide to a rest position.
The book comes to a rest because
of the presence of a force
- that force being the force of
friction - which brings the book to a
rest position.
FRICTION AND MOTION
**Friction is necessary for many everyday tasks to work correctly
Unwanted friction can be lowered by using lubricants to make the surfaces touching
smoother
ex: non-stick coating on pans; motor oil, air on an air hockey table
Helpful friction can be increased by making the surfaces rougher or adding inertia
ex: golf gloves; paper weight
FORCES SUMMARY
**Force causes Acceleration**
Balanced force = no change in motion/ no acceleration (net force = 0)
Unbalanced force = acceleration (net force >0)
- acceleration occurs in the direction of the unbalanced force
NEWTON’S LAWS OF MOTION
1st Law –
An object at rest will stay at rest, and an object in motion will
stay in motion at constant velocity (at the same speed and in the same direction)
unless acted upon by an unbalanced force.
“ The Law of Inertia”
2nd Law –
Force equals mass times acceleration. F = M x A
“ The Law of Acceleration”
3rd Law –
For every action there is an equal and opposite reaction.
“The Law of Action/Reaction”
NEWTON’S FIRST LAW
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.
AKA: The Law of Inertia
Inertia: the tendency of an object to remain at rest or, if moving, to continue
moving at a constant velocity
**Tendency of an object to resist change in motion **
The greater the mass of the object the more inertia it has.
So a bowling ball has more inertia than a tennis ball.
NEWTON’S FIRST 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.
NEWTON’S FIRST LAW
Unless acted upon by an
unbalanced force, this golf ball
would sit on the tee forever
.
NEWTON’S FIRST LAW
Once airborne, unless acted on by
an unbalanced force (gravity and air
– fluid friction), it would never stop!
NEWTONS’S 1ST LAW AND YOU
NEWTON’S SECOND LAW
NEWTON’S SECOND LAW
The net force/unbalanced force acting on an object equals the
object’s mass x its acceleration
F=MA
A larger mass accelerates less (for equal forces)
FORCES
Measured in NEWTONS (N)
 1 Newton is the force that causes a 1-kilogram mass to accelerate at a
rate of 1 meter per second each second.
 1 N = 1 kg*m/s2
NEWTON’S SECOND LAW
When mass is in kilograms (kg) and acceleration is in m/s/s (m/s 2), the unit of force
is in newtons (N).
One newton is equal to the force required to accelerate one kilogram of mass at one
meter/second/second.
2ND LAW (F = M X A)
How much force is needed to accelerate a 1400 kilogram car 2 meters per
second/per second?
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 m/s2
Solve for the unknown
2800 kg*m/s2 or
2800 N
ACCELERATION
Acceleration depends on force and mass
- Acceleration of an object is directly proportional to the net force on the object and
inversely proportional to the object’s mass
Acceleration = net force/mass
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
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.
• 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
CHECK YOUR UNDERSTANDING
1. What acceleration will result when a 12 N net force applied to a 3
kg object? A 6 kg object?
2. A net force of 16 N causes a mass to accelerate at a rate of 5
m/s2. Determine the mass.
3. How much force is needed to accelerate a 66 kg skier 1
m/sec/sec?
4. What is the force on a 1000 kg elevator that is falling freely at 9.8
m/sec/sec?
CHECK YOUR UNDERSTANDING
1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s 2. Determine the mass.
16 N = 3.2 kg x 5 m/s/s
3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
9800 kg-m/sec/sec or 9800 N
GRAVITY
Weight and Mass
The force on an object due to gravity is called weight.
The amount of matter an object contains is called mass.
Weight is measured in Newtons
Weight is a force
Weight is a can influence shape
LAW OF UNIVERSAL GRAVITATION
Why do objects fall toward Earth?
What keeps the planets in motion in the sky?
**Both questions have the same answer – Isaac Newton
All objects in the universe attract each other through the force of gravity
- Gravitational force increases as one or both masses increase
- Gravitational force decreases as the distance between two masses increases
- G is constant
MATTER AND GRAVITY
There is a gravitational force between ALL objects
Factors Affecting Gravity:
1. Size of the mass
2. Distance between the masses
GRAVITATIONAL FORCE
The force of gravity acts between all objects
Gravitational Force increases as mass increases
Gravitational Force decreases as distance increases
Gravitational Force is week between objects with small masses
FREE FALL
Free fall : Motion of a body when only the force of gravity is acting on the body.
- Free Fall acceleration is directed toward the center of the Earth
**In the absence of air, all objects falling near Earth’s surface accelerate at the same
rate regardless of their mass.
Acceleration depends on both force and mass
Heavy object has greater gravitational force due to its larger mass
Heavy object is more difficult to accelerate due to its larger mass
PROJECTILE MOTION
A curved path followed by an object that is thrown, launched or projected newar the
surface of the Earth.
• Ex: Motion of leaping frogs, arrows shot from bow, a baseball thrown.
• 2 components: When the two components are combined they form a curved
path.
• Horizontal
• Vertical
TERMINAL VELOCITY
Terminal Velocity: constant velocity of a falling object
when
the force of air resistance is equal in magnitude and opposite in direction to the force
of gravity
A falling object stops accelerating when the force of air resistance becomes equal to
the gravitational forces on the object
- Astronauts in orbit are in free fall
- Their distance from the earth increased and gravitational pull has become weak
NEWTON’S THIRD LAW
For every action, there is an equal and opposite reaction.
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.
The size of the force on the
water equals the size of the
force on the fish; the direction of
the force on the water
(backwards) is opposite the
direction of the force on the fish
(forwards).
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.
Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the
bird upwards.
The size of the force on the air equals the size of the
force on the bird; the direction of the force on the
air (downwards) is opposite the direction of the
force on the bird (upwards).
Action-reaction force pairs make it possible for birds
to fly.
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
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.
MOMENTUM
Momentum : A property of all moving objects
Momentum = mass x velocity
For movement along a straight line, momentum is calculated by multiplying an
object’s mass and velocity
Page 415 practice problems
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