Forces in Motion

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Chapter 4

• Forces in Motion

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Force

• Force is a push or a pull
• Example

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• Forces can be balancedwhere the force is equal
  on both sides
• In this example the net force is zerothe object
  doesnt move
• Or they can be unbalancedwhere one side pushes
  or pulls more than the other
• In this example, the object would move in the
  direction of the greater force

• example

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• Examples of force are
• pushair resistance
• pullgravity

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Section 1Gravity and motion

• Galileo theorized that the mass of an object
  does not affect the rate at which it falls

All objects fall with same acceleration
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• The force of gravity is greater between an object
  with large mass than it is between an object with
  less mass
• A greater force has to be applied to the large
  object in order for it to fall
• Less of a force is required to make a small
  object fall
• So the difference in the amount of force is
  cancelled out by the difference in mass

• Gravity and falling objects

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• Acceleration due to gravity

• The rate at which velocity changes (or the speed
  at which an object travels)
• All objects accelerate toward the Earth at a rate
  of 9.8 meters per second per second (or 9.8 m/s2)
• For each second an object is falling, its
  downward velocity increases by 9.8 meters per
  second

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• Velocity of falling objects

• This is written
• ?v g x t
• g is the acceleration due to gravity (9.8m/s2)
• t is the amount of time the object is falling in
  seconds
• Look at the Math focus on page 85.

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• Air resistance slows acceleration

• Gravity and air resistance affect falling objects
• Air resistance is fluid friction that opposes the
  motion of objects through air
• it occurs between the surface of the falling
  object and the air around it
• the amount of air resistance depends on the
  shape, size and velocity of the object

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Which would be affected less by air resistance?
Why?
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• Acceleration stops at terminal velocity

• As speed of falling objects increases, so does
  the air resistance
• The upward force of air resistance continues to
  increase until it equals the downward force of
  gravityat this point the object stops
  accelerating because the net force is zero
• The object then falls at a constant velocity,
  called terminal velocity

Upward air resistance
Net force
object
Downward force of gravity
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• The motion of the elephant and the feather in the
  presence of air resistance is shown.
• But why does the elephant fall faster?
• The elephant and the feather are each being
  pulled downward due to the force of gravity. When
  initially dropped, this force of gravity is
  unbalanced by all other forces and thus, both
  elephant and feather begin to accelerate (i.e.,
  gain speed)
• Once the upward force of air resistance upon an
  object is large enough to balance the downward
  force of gravity, the object is said to have
  reached a terminal velocity.
• the elephant never does reach a terminal velocity
  during its fall the forces never do become
  completely balanced and so there is still an
  acceleration.
• If given enough time, perhaps the elephant would
  finally accelerate to high enough speeds to
  encounter a large enough upward air resistance
  force in order to achieve a terminal velocity.
• On the other hand, the feather quickly reaches a
  terminal velocity.

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• Free fall

• Free fall only occurs when gravity is the only
  force acting on an object
• This can only happen in a place where there is no
  matter (a vacuum) or in space

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• in the absence of air resistance, the elephant
  and the feather strike the ground at the same
  time
• A simple rule to bear in mind is that all objects
  (regardless of their mass) experience the same
  acceleration when in a state of free fall.
• When the only force is gravity, the acceleration
  is the same value for all objects.

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• Orbiting objects are in free fall
• Two motions cause orbiting

• Objects in space are not weightlessthey still
  have mass
• These objects are actually in free fall
• An object is in orbit when
• it is traveling in a circular path around
  another object
• AND
• is also in free fall

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• Many objects are in orbitmoon orbits Earth,
  Earth planets orbit sun

• The role of gravity in orbiting

• All orbiting objects are traveling in nearly
  circular paths
• Circular path is created by unbalanced forces
  acting on the object
• Unbalanced force that causes circular path is
  called centripetal force (toward the center)

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Example The satellite is orbiting the earth The
Earths gravitational force provides a
centripetal force on the satelliteit keeps the
satellite moving in a circular path around the
Earth
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• Projectile Motion and Gravity

• Projectile motionthe curved path an object
  follows when it is thrown or propelled near the
  surface of the Earth
• Has 2 components horizontal and vertical
• Together, those 2 components form a curved path

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• Horizontal Motion
• Vertical Motion

• Horizontal motionmotion parallel to the ground
• Vertical motionmotion perpendicular to the
  ground
• Objects in projectile motion accelerate downward,
  so you have to aim above a target if you want to
  hit it with a thrown object
• Ex pitching a baseball, throwing a dart

motion
Ground
motion
Ground
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Chapter 4 section 2Newtons Laws of Motion
Sir Isaac Newton 1643 - 1727
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• Force and motion are relatedin order for an
  object to be set into motion, there must be a
  force acting upon it
• This is explained in Newtons three laws of
  motion

• An object at rest remains at rest and an object
  in motion remains in motion at constant speed and
  in a straight line unless acted on by an
  unbalanced force.

Newtons First Law of Motion
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• Objects at rest
• Objects in motion

• At rest not moving
• Object will not move unless it is acted on by an
  unbalanced force
• Moving objects will continue to move in the same
  direction and at the same speed unless an
  unbalanced force stops it

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• Friction and the first law

• Friction is an unbalanced force
• Friction is created when two surfaces rub
  together and the irregularities of their surfaces
  get caught on one another
• It will change the motion of an object
• Example as a golf ball rolls across the turf, it
  slows down due to the friction between the ball
  and the grass
• What examples of friction are in the picture to
  the left?

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• Inertia and Mass

• First law is also called the law of inertia
• Inertiathe tendency of all objects to resist any
  change in motion
• Objects at rest will remain at rest until
  something makes it move objects in motion will
  stay in motion until something changes its speed
  or direction
• Example a car cannot stop instantaneouslybrakes
  must be applied and it will slow down

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• Mass is a measure of inertia

• Objects with small mass have less inertia than
  objects with large massthey are less able to
  resist change of motion
• Example
• Bike small mass
• Car large mass
• It is easier to push a bike than it is to push a
  car due to the bikes small mass

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• Newtons Second Law of Motion

• Describes the motion of an object when an
  unbalanced force is acting on it
• The acceleration of an object depends on the mass
  of the object and the amount of force applied

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• Acceleration depends on Mass

• an objects acceleration decreases as its mass
  increases
• an objects acceleration increases as its mass
  decreases
• Example
• when a shopping cart is empty (less mass), it
  is easy to pushor accelerate
• As you add objects (increase the mass), it
  becomes more difficult to push

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• Acceleration depends on Force

• an objects acceleration increases as the force
  applied to the object increases
• the acceleration will decrease if the force
  decreases
• Example the shopping cart will go much faster
  if you give it a hard push than if you pushed it
  gently

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• 2nd law Mathematically

• a F/m or F ma
• aacceleration
• Fforce
• mmass
• you can use this to calculate both acceleration
  and force
• Look at the Math Focus page 95

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• Gravity and 2nd Law

• when the equations are figured, it proves that
  all objects fall to Earth with the same
  acceleration

Watermelon has more mass more inertia (harder
to move) m1.02 kg F 10 N
Pear has less mass less inertia (easier to
move) m0.2 kg F 1 N
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• Newtons Third Law of Motion

• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first object
• Or simply putforces act in pairs called action
  and reaction forces

Body pushing down on chair (ACTION FORCE) Chair
pushing up on body (REACTION FORCE)
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• Do not act on same object
• Action reaction
• Difficult to see

• Force pairs do not act on same objectif they did
  the net force would be zero
• Example action forcemans body weight pushes on
  chair chair exerts force on mans body
• When a force is exerted, there is ALWAYS a
  reaction force
• Reaction forces can be difficult to see
• Example a ball falling exerts a force on earth,
  but the earths mass is so much larger that you
  dont feel or notice the acceleration of the
  earth in reaction to the ball

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• Momentum

• A property of moving objects that depends on the
  objects mass and velocity
• The more momentum an object has, the more
  difficult it is to change its speed or direction
• Momentum is transferred from one object to
  another when 2 objects collide
• Law of conservation of momentum any time two or
  more objects interact, they may exchange momentum
  but the total amount stays the same

Momentum is conserved
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How is the game of bowling an example of the law
of conservation of momentum?
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• Conservation of Momentum Newtons 3rd law

• When one object hits another, the first object is
  exerting the action force and the second is
  exerting the reaction force
• Because the two forces are equal and opposite,
  the momentum is conserved