Download Forces and Newtons Laws

Newton’s
Laws of
Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction
Forces
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•
A force is a push or a pull
A force can cause
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•
Net force
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the combination of all the forces acting on an object.
changes an object’s state of motion.
Balanced Force
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a stationary object to move
a moving object to stop
an object to accelerate (change speed or direction)
Net force is 0, object at rest
Or constant velocity
Unbalanced
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Net force is not 0, Object moves
Or accelerates
Forces
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What causes and object to change its
velocity?
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When you throw or catch a ball, you exert a
force to change the ball’s velocity.
There are usually many forces that act on
an object at any given time.
The net force is the combination of all of
the forces acting on an object.
An object accelerates in the direction of
the net force.
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It will not accelerate if the net force is zero.
Balanced Forces
Balanced forces – the forces acting on an
object that combine to produce a net force
that is equal to zero.
Unbalanced Forces
Unbalanced forces – the forces acting on an
object that combine to produce a net
nonzero force.
Friction
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Because of friction, a constant force
must be applied to a car on a flat
road just to keep it moving.
In order for a car to reach a certain
speed from rest, the forces on the
car must be unbalanced.
The force pushing the car forward
must be greater than the force of
friction opposing the cars motion.
Friction
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Vehicle tires are
designed to use
friction to increase
grip.
A wet surface
prevents contact.
Frictional Force and
Acceleration
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When a car is accelerating, the
forces are unbalanced. The force
moving the car forward is greater
than the opposing force of friction.
When a car is cruising at constant
speed, the force moving the car
forward is balanced by the force of
friction;
Frictional Force and
Acceleration
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No motion is also a balanced force.
If a car is parked on a hill, it will not
roll because the force of friction
between the brakes and the wheels
balances the forces of gravity.
Frictional Forces
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Frictional force varies depending on the
surfaces in contact.
Frictional forces are relatively great when
both surfaces are rough.
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Ex. New jogging shoes often have rough
rubber soles. Friction between the new shoes
and a carpeted floor will be large enough to
keep you from slipping.
What if the soles of your shoes are smooth?
What if the floor you are walking on has been
waxed?
Air Resistance
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Air resistance is a form of friction
Air resistance opposes the car’s motion.
Air resistance is caused by the interaction
between the surface of a moving object
and the air molecules.
Air Resistance
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The amount of air
resistance on an
object depends on its
size and shape as well
as on the speed with
which it moves.
Aerodynamic –
designed to move
through air with
minimal resistance.
Which is more
aerodynamic?
Gravity
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Gravity – is given as the reason why
the apple falls down from a tree.
Every object exerts a gravitational
force on every other object.
When an apple breaks from its stem,
the apple falls down because the
gravitational force between Earth
and the apple is much greater than
that between the apple and the tree.
Gravity
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Gravity is different from forces such
as friction.
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Gravity acts even when the objects do
not touch.
The force of gravity between two
objects depends on their masses and
the distance between the two
objects.
Gravity
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The gravitational force between two
objects is proportional to the
product of their masses.
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The greater the mass, the larger the
gravitational force it exerts on other
objects.
The gravitational force between most
objects around you is very
small…Earth’s, however, is extremely
massive.
Gravity
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The force of gravity changes as the
distance changes.
Gravitational force is weaker than
other types of forces, even though it
holds the planets, stars, and galaxies
together.
Weight
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The SI unit of weight is
the Newton (N).
Example: a small apple
weighs about 1 N. A
1.0 kg book has a
weight of 1.0 kg x 9.8
m/s2 = 9.8 N
Weight is Different From Mass
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Weight and mass are easy to confuse.
They are proportional to one another, but are not the
same.
Mass is a measure of the amount of matter in an
object.
Weight is the gravitational force an object experiences
due to its mass.
The weight of an object depends on gravity, so a
change in an object’s location will change the
object’s weight.
Weight vs. Mass
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Consider a 66 kg astronaut.
On Earth, this astronaut weighs 66 kg x 9.8 m/s2 =
650 N (about 150 lb).
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1 kilogram = 2.20462262 pounds
But, on the moon’s surface, where g is only 1.6 m/s2,
the astronaut would weigh 66 kg x 1.6m/s2 = 110N
(about 24 lb).
The astronaut’s mass will remain the same on Earth,
the moon, or an orbiting space shuttle.
Mass vs. Weight
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MASS
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How much and what
material an object is
made of (what types of
atoms and how many of
them)
Measured in grams or
kilograms (kg)
Is constant for an object
independent of location
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WEIGHT
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Force of gravity acting
on a mass
Measured in Newtons or
Pounds
Fg=mag
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Depends on what planet
you are on.
Sir Isaac Newton

Sir Isaac Newton
described the
relationship between
motion and force in
three laws that we now
call Newton’s laws of
motion.
Newton’s Laws of Motion
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1st 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.
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2nd Law – Force equals mass times
acceleration.
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3rd Law – For every action there is an
equal and opposite reaction.
Newton’s First Law of Motion
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An object at rest remains at rest and an object
in motion maintains its velocity unless it
experiences an unbalanced force.
Sometimes called law of inertia
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Sliding a book on carpet vs. ice
Riding in a car
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Infant seats
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.
Inertia
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Inertia – the tendency of an object at rest
to remain at rest; or, if moving, to
continue moving with a constant velocity,
until acted upon by an unbalanced force.
All objects have inertia because they
resist changes in motion.
An object with very little mass, such as a
baseball, can be accelerated with a small
force.
It takes a much larger force to accelerate
a car, which has a large mass.
INERTIA
the tendency of an object
to resist any change in its motion
Inertia is a property of matter and does not
depend on the position or location of the object. But it does depend on:
MASS
a quantitative measure of inertia
FORCE
“a push or pull”
Newton’s First Law of Motion
1st Law
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Once airborne,
unless acted on
by an unbalanced
force (like
gravity and
friction), it would
never stop!
1st Law

Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tee forever.
Why then, do we observe every
day objects in motion slowing
down and becoming motionless
seemingly without an outside
force?
Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.
What is this unbalanced force that acts on an object in motion?
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There are four main types of friction:
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Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water resistance
Static friction: initial friction when moving an object
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.
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In the absence of a force of friction, the book
would continue in motion with the same speed
and direction - forever! (Or at least to the end
of the table top.)
Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.
Newton’s Second Law of Motion
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The unbalanced force acting on an object
equals the object’s mass times its acceleration.
Force = mass x acceleration
F = ma
2nd Law
2nd Law
The net force of an object is
equal to the product of its mass
and acceleration, or F=ma.
2nd Law (F = m x a)

How much force is needed to accelerate a 1400
kilogram car 2 meters per second/per second?
Write the formula
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
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2800 kg-meters/second/second or 2800
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N
Newton’s Second Law of Motion
Force is Measured in Newtons
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The SI unit of force is the Newton (N).
One Newton is the force that can give a mass
of 1 kg an acceleration of 1 m/s2
1 N = 1 kg x 1 m/s2
Newton’s Second Law of Motion
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Example:
Zookeepers lift a stretcher that holds a sedated lion.
The total mass of the lion and stretcher is 175 kg,
and the lion’s upward acceleration is 0.657m/s2.
What is the unbalanced force necessary to produce
this acceleration of the lion and the stretcher?
Newton’s Second Law of Motion

Answer:
Given:
mass, m = 175 kg
acceleration, a = 0.657 m/s2
Unknown: force, F = ? N
Force = mass x acceleration
F = ma
F = 175 kg x 0.675 m/s2
F = 115 kg • m/s2 = 115 N
Newton’s Second Law Practice
1.
2.
3.
What is the net force necessary for a 1.6 x 103 kg
automobile to accelerate forward at 2.0 m/s2?
A baseball accelerates downward at 9.8 m/s2. If
the gravitational force acting on the ball is 1.4 N,
what is the baseball’s mass? (Hint: Assume gravity
is the only force acting on the ball.)
A sailboat and its crew have a combined mass of
655 kg. If the sailboat experiences an unbalanced
force of 895 N pushing it forward, what is the
sailboat’s acceleration?
Free Fall
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When the force of gravity is
the only force acting on an
object, the object is said to be
in a free fall.
Free fall acceleration is
directed toward the center of
the Earth.
Because free-fall acceleration
results from gravity, it is often
abbreviated as the letter g.
Near Earth’s surface, g is
approximately 9.8 m/s2.
Weight Equals Mass x Free-Fall
Acceleration
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The force on an object due to gravity is called
weight.
On earth, your weight is simply the amount of
gravitational force exerted on you by Earth.
If you know the free-fall acceleration, g, acting on
a body, you can use F = ma to calculate body
weight.
Weight = mass x free-fall acceleration
w = mg
Check Your Understanding
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1. What acceleration will result when a 12 N net force applied to a 3 kg
object? A 6 kg object?
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2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.
Determine the mass.
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3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
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4. What is the force on a 1000 kg elevator that is falling freely at 9.8
m/sec/sec?
Check Your Understanding
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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
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2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the
mass.
16 N = 3.2 kg x 5 m/s/s
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3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
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4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
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9800 kg-m/sec/sec or 9800 N
Example 1
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A 2.2 kg book is slid across a table. If Fnet =
2.6 N what is the book’s acceleration?
Fnet=ma
2.6 N = (2.2kg) a
a = 1.18 m/s2
Example 2
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If you drop a 20 kg object what is its
acceleration? What is its weight?
acceleration = 9.8 m/s2
Weight = force
Fg=ma
Fg= (20 kg) (9.8 m/s2)
Let’s take a quick break
from Newton’s Laws to talk
about Free Fall and
Projectiles
What is “Free Fall”?
• An object moving under the effects of
the force of gravity ONLY is considered
to be in vertical “free fall”.
• This only works in a vacuum, because
air friction is another force.
• Terminal velocity is the velocity of an
object when the air friction causes an
object to fall at a constant speed.
Free Fall Facts
• For all objects in free fall near the
surface of the earth, the
acceleration due to gravity is a
CONSTANT equal to 9.80665 m/s2
in the negative direction.
Free Fall Facts
• This fact is true:
1. regardless the mass of
the object.
2. only in the absence of
air resistance (in a
vacuum)
Free Fall Facts
If you throw an object
straight up into the air, the
time for an object to rise
from its release point is
EQUAL TO the time for it to
fall back to its release point.
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We now 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
Projectiles
• Airborne objects that move as a result of
their own inertia and gravity.
• They exhibit two dimensional motion.
– Horizontal and vertical motion
• Horizontal and vertical motion are independent of
one another.
– The resultant motion is a combination of
horizontal and vertical motion.
Examples of Projectile Motion
• Launching a Cannon ball
Factors Affecting Projectile Motion
• What two factors would affect projectile
motion?
– Angle
– Initial velocity
Initial Velocity
Angle
Two Types
of
Projectiles
Newton’s Third Law of Motion
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For every action force, there is an equal and
opposite reaction force.
Also called the law of action and reaction.
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Ex.
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Kicking a soccer ball
Letting the air out of a balloon
Newton’s Third Law of Motion
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The action and reaction forces are applied to
different objects.
These forces are equal and opposite, but this is
not a case of balanced forces because two
different objects are involved.
They do not cancel each other out!
Newton’s Third Law of Motion
3rd Law

For every action, there is an
equal and opposite reaction.
3rd Law
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.
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
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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.
3rd Law

Consider the motion of
a car on the way to
school. A car is
equipped with wheels
which spin backwards.
As the wheels spin
backwards, they grip the
road and push the road
backwards.
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.