Transcript Force - Kowenscience.com

Physical Science
Unit:
Forces
FORCE

A PUSH OR A PULL
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A FORCE MAY GIVE ENRGY TO AN OBJECT
AND CAUSE IT TO START MOVING OR
CHANGE ITS MOTION
The force gives energy to an object
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The energy can set the object in motion, stop its
motion or change the speed and direction of its motion
The Nature of Force

By definition, a Force is a push or a pull.
A Push
Or
A Pull
Just like Velocity & Acceleration
Forces have both
magnitude and direction
components
Force
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Force is a vector quantity.
Described by magnitude and direction.
An arrow represents the direction and
strength of a force and the longer the arrow,
the greater the force.
The SI unit for force is a Newton.
Vectors
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Vectors are a method used to visually show forces
A vector is a quantity which has both magnitude (size) and
direction.
 The length of the arrow shows the magnitude of the vector.
 The angle of the arrow shows the vector's direction.
Just like numbers, we can add two or more
vectors together and get a net force called the
resultant
Adding 2 or More Vectors
Fig 1
Fig 2
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Fig 3
Add vectors A and B to get the Resultant C
 A+B=C
Fig 1 - shows the magnitude & direction of the 2 vectors we are adding
Fig 2 – we move the beginning of vector B to the end of Vector A,
making sure to keep the magnitude & direction exactly the same
Fig 3 – Connect the beginning of Vector A to the end of Vector B,
this is your “Resultant” C.
Combining Forces

Often, more than a single force acts on an
object at one time.
 Net Force is the combination of all forces
acting on an object.
 Combined forces that are balanced are
always equal to zero
Balanced Forces

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Balanced Forces are
equal forces acting on
one object in opposite
directions
When equal forces are
exerted in opposite
directions there is no
net force.

Balanced forces acting
on an object do not
change the object's
velocity (keeps velocity
constant)
Unbalanced forces
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Forces that are not opposite and equal
While balanced forces cause no change in
motion, unbalanced forces always cause a
change in motion.
Unbalanced forces

When 2 unbalanced forces are exerted in
opposite directions, the combined force is the
difference between the 2 forces
Balanced & Unbalanced Forces
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With a Balanced force – opposite and equal forces acting on the
same object result in NO motion of the object
Unbalanced forces – two or more forces of unequal strength or
direction acting upon on an object results in the motion of the
object
Friction
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Friction The force that opposes the motion of an
object, It is a force that two surfaces exert on each
other when they rub against each other.
Friction acts in the direction opposite to motion.
and is the force that brings an object to rest
Without friction or other unbalanced forces, an
object will not stop until it hits another object.
Isaac Newton

th
17
century
Said that no force is necessary to keep an object
in motion at a constant speed, and that objects
do not come to rest on their own
Causes of Friction
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Smooth surface has less friction than rough
surface.
Strength of force of friction depends on
types of surfaces involved and how hard the
surfaces push together. (i.e. rubbing hands
together)
4 Types of Friction
1.
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Static Friction - Friction that acts on
objects that are not moving
Example: moving a piece of furniture
across room.
Must use extra force to start friction of
stationary objects.
2. Sliding Friction - Friction where two
solid surfaces slide over each other.
When an object is pulled at a constant velocity
across a level surface, the pulling force
equals the sliding friction.
 The amount of sliding friction present
depends on 2 factors:
1. weight of object moving
2. type of surface object is
sliding across
3.
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Rolling friction - Friction where an
object rolls across a surface.
Rolling friction is less than sliding friction.
4.
Fluid friction - Friction where a solid
object moves through a fluid.

Less than sliding friction.
A substance that changes sliding friction to
fluid friction is called a lubricant, ex parts
of machines bathed in oil.
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Example Problem
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. A 5 kg block is pulled across a table by a
horizontal force of 40 N with a frictional force
of 8 N opposing the motion. Calculate the
acceleration of the object.
Solution:
F=ma
40N-8N=32N of force in a
direction 32N=5(a) a=6.4 m/s2
Types of Forces:
1. Gravitational Force:
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The attraction between any 2 objects that have
mass
As mass increases so does the attraction
As distance increases gravitational force
decreases
2. Electrical forces
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Forces between electrical charges
Objects with different charges pull toward (
attract) each other ex plastic wrap to an object
Objects with the same charge repell
3. Magnetic force
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Forces caused by moving electrical charges ex
common magnet
4. Weak interactions

Forces believed to cause the nuclei of some
atoms to break apart
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A mass can be acted upon by one or more of
these forces at a time
Forces and Motion
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During 1665-1666 Isaac Newton developed his
3 laws of motion.
The 3 laws of motion explain rest, constant
motion, accelerated motion, and how
balanced/unbalanced forces act.
Newton’s 3 Laws of Motion
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Newton’s 1st Law of Motion:
AKA The Law of Inertia

which states an object at rest will remain at
rest, and an object in motion will remain in
motion at a constant velocity until acted on by
another force.
Remember:
The greater the mass of an
object the greater the inertia
Remember:
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Newton called the tendency of objects to remain
in motion or stay at rest: inertia

Inertia: The property of matter to resist any
change in motion ( Latin meaning idle or lazy)
Newton’s 3 Laws of Motion
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Newton’s Second Law of Motion aka F=ma
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Force = mass x acceleration
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Can be written as:
F=ma ; a= F/m ; m= F/a
What is the basic unit for mass? Kilogram
What is the basic unit for acceleration? Meter/sec/sec
Therefore the basic unit for Force is
(kilogram)( meter/sec/sec)
An object with a mass of 1 kg accelerating at 1 m/s/s
has a force of 1 Newton
Remember:
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Acceleration is the change in speed or direction
According to the 2nd law of motion, an
unbalanced force must be present when there is
a change in speed or direction
The acceleration is always in the direction of the
unbalanced force
Newton’s 2nd Law & Weight
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F=ma
Weight is the force of gravity acting on an object’s mass.
Therefore weight is a type of Force
Remember:
1 newton = 0.22 pounds
The formula for weight: Weight = mass x Ag
Since Ag= 9.8 m/s2 then
Weight = mass x 9.8 m/s2
Example Problem Force
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What is the force on an object that goes from 35
m/s to 85 m/s in 20 seconds and has a mass of
148 kg?
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Solution:
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Force = mass x acceleration
370 N
Example Problem Force
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Calculate the force on an object that has a mass
of 12 kg and an acceleration of 4 m/s2 .
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Solution:
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Force = mass x acceleration
F = ma = 12 kg ´ 4 m/s2 = 48 kg ´ m/s2 = 48 N
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Example Problem Force
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What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec2?
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Solution:
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Force = mass x acceleration
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f=9,800 n
Example Problem Force
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What is the acceleration of a 50 kg object
pushed with a force of 500 newtons?
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Solution:
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Force = mass x acceleration
F= ma 500 = 50(a) a= 10 m/s2
Example Problem Force
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The mass of a large car is 1000 kg. How much
force would be required to accelerate the car at a
rate of 3 m/sec2?
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Solution
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Force = mass x acceleration

F= ma
f= 1000 x 3 f= 3000 N
Your weight on
other planets
& 3 different
types of stars
Newton’s 3 Laws of Motion
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Newton’s 3rd Law of Motion:
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For every action there is an equal & opposite reaction.
If an object is not in motion, then all forces acting on it are balanced and the
net force is zero!
Friction – the force that one surface exerts on another when the two rub
against each other.
Sliding friction
Fluid friction
Rolling friction
How Do Rockets Lift Off?
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Rockets •and space shuttles lift into space
using Newton’s third law of motion.
They burn fuel and push the exhaust gases
downward at a high velocity as they lift off.
In turn, the gases push upward on the
rocket.
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A rocket can rise into the air because the gases it
expels with a downward action force exert an equal
but opposite reaction force on the rocket.
As long as this upward force, called thrust, is
greater than the downward pull of gravity, there is
an unbalanced force in the upward direction that
causes a change in the rocket's velocity. As a
result, the rocket accelerates upward into space.
Newton’s 3 laws of motion can
explain all aspects of an objects
motion
Gravity
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Gravity is a force that pulls objects toward
each other.
Newton realized gravity acts everywhere in
universe.
Law of Universal Gravitation states that the
force of gravity acts between all objects in
the universe.
Any two objects in the universe, without
exception, attract each other.
Law of Universal Gravitation
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Newton was the 1st scientist to prove that the
forces pulling an apple to the ground were the
same forces pulling the moon toward the earth
This was the first universal law of force ( applies
to all objects in the universe)
The size of the force depends on 2
factors:
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1. Mass of object
2. Distance between the objects
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The force of gravity increases as objects mass
increases
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Gravitational force decreases as distance
between objects decreases
Free fall
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Free fall – when the only force acting on an object is
gravity.
In free fall, the force of gravity alone causes an object to
accelerate in the downward direction.
All objects in free fall accelerate at the same rate.
Since objects fall at the same speed, their acceleration is the
same.
All objects accelerate at the rate. Here on Earth the rate is:
Ag=9.8 m/s2
Or
Ag=32 ft/s2
air resistance
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Any falling object meets air resistance
As the object falls, the air resistance gradually
becomes equal to the pull of gravity.
When an object reaches the point that it no
longer accelerates, it has reached its terminal
velocity ex T.V. of sky diver is 190 km/hr
Air Resistance
Air resistance keeps
things from falling
equally
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Air resistance is a type of fluid friction.
Air resistance is an upward force exerted on
falling objects.
Objects with greater surface area experience
more air resistance as they fall. (Flat piece of
paper vs. crumpled piece of paper)
Projectile Motion
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A projectile is an object that is thrown.
A ball thrown horizontally falls at the same
rate as ball dropped.
Elastic Forces
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Matter is considered elastic if it returns to its
original shape after is squeezed or stretched.
Compression is an elastic force that
squeezes or pushes matter together.
Tension an elastic force that stretches or
pulls matter.
Mass and Distance
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Force of gravity between object increases
with greater mass and decreases with greater
distance.
The further from Earth it is, the less
gravitational force.
Mass is the measure of the amount of matter
in an object. (Kg)
Gravity & Weight
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Weight is gravitational force exerted on a
person or object at the surface of a planet.
Stepping on scale (weight) shows the
gravitational force Earth exerting on you.
Objects with greater mass have greater
weight.
Weight = Mass x Acceleration due to gravity
Weight – Newtons (N).
Mass – Kilograms (KG).
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Acceleration due to gravity = 9.8 meters per
second per second (m/s²) (at Earth's
surface)
Weight varies with strength of gravitational
force – mass doesn't.
Strength of gravitational force exerted on an
object or person by the moon is 1/6 of the
force exerted by Earth.
Center of Gravity
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The center of gravity of an object is the point at
which all an object’s mass appears to be located
An object is balance if its center of gravity is
supported ( if not it falls)
Force of gravity and weight both decrease as an
object moves away from the Earth’s center of
gravity ( mass stays the same)
Formula used to find weight of
object
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Force = mass of object x constant of 9.8 m/sec2
Example Problem
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An object of mass 30 kg is in free fall in a vacuum where there is
no air resistance. Determine the acceleration of the object.
Solution:
Since it is in free fall and is falling due to the force of
gravity.....the acceleration will always be 9.8 m/s2
Orbital Motion
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An object that orbits another object is in orbital
motion.
This is the result of the combination of the objects
forward inertia and downward pull of gravity on an
object
Ex Satellites in orbit around Earth continuously
fall toward Earth, but because Earth is curved they
travel around it.
The speed with which an object must be thrown in
order to orbit Earth is 7,900 meters per second.
Example Problems
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A bowling ball rolled with a force of 15 N accelerates at
a rate of 3 m/sec2; a second ball rolled with the same
force accelerates 4 m/sec2. What are the masses of the
two balls?
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Solution:
F= ma
15N= (m)3 m=5kg
m=3.75kg
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15N = (m)4
Example Problem
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If a 60 kg person on a 15 kg sled is pushed with
a force of 300 N, what will be person’s
acceleration?
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Solution:
F=ma
a=4 m/s2
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300N=(60+15)(a)
Example Problem
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An object of mass 30 kg is falling in air and
experiences a force due to air resistance of 50
newtons.
Determine the net force acting on the object
and :
f=ma f=30 x 9.8 f=294N Net force is 29450 = 244N
calculate the acceleration of the object.
F= ma
244N=30 (a)
a=8.13
m/s2
Example Problem
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A force of 250 N is applied to an object that
accelerates at a rate of 5 m/sec2. What is the
mass of the object?
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Solution:
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F = ma
250N=(m)5
m= 50kg