Unit 2 Forces and Motion

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Unit 2 Forces and Motion
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• Motion
• People walk, ride bikes, drive cars, etc. All the
  time we are dealing with motion
• What must you know in order to describe an
  objects motion?

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• Motion
• To describe motion we speak of
• Position - Where an object is (as measured from
  some reference point)
• Speed an objects rate of change of position (v
  d/t)
• Acceleration an objects rate of change of
  speed (a v/t)

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• Speed and Velocity
• Speed- the rate of change of the position of an
  object.
• Speed(v) Distance (d)
• Time (t)
• Velocity- describes both speed and direction.

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• Speed and Velocity
• Ex1 Find the speed of a car that goes 10 miles
  in 6.6 minutes.
• Ex2 Find the speed and velocity of an airplane
  that flies 805 miles north in 2.3 hours.

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• Acceleration-
• Rate of Change in velocity
• Average Acceleration a
• Final velocity Vf
• Initial velocity Vi
• Time t
• a Vf - Vi or a ?v
• t s t t

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Sample Problem

• A flower pot falls off second story windowsill.
  The flowerpots starts from rest and hits sidewalk
  1.5 seconds later with a velocity of 14.7 m/s.
  Find average acceleration

• What are your givens?

Time 1.5 s Initial velocity 0m/s Final velocity
14.7m/s
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Your Turn

• Ex If you run at a rate of 6.00 km/hour, how
  far will you run in 35 seconds?
• Ex What is the acceleration of a car that goes
  from 1.00m/s to 50.0 m/s in 1.00 minute?
• Ex What is the acceleration of a car that goes
  from 0 mph to 60.0 mph in 6.2 seconds?

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Acceleration requires a change in velocity

• Can you accelerate w/o changing speed?
• Remember maintaining same speed but changing
  direction changes velocity, thus you have
  acceleration!

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Graphing Motion

• You can tell a lot about an objects motion from
  looking at a graph of the motion

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Distance vs. Time Graph

• Speed can be determined from a distance vs. time
  graph.
• Slope of the line is the speed.
• What is slope?

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Velocity vs. Time graph

• Acceleration is equal to zero if velocity is
  constant.
• Be sure to recognize speeding up and slowing down
  on a graph.

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• Slope of velocity vs. time graph is the
  acceleration.
• Acceleration can
• be negative.
• See page 262

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Momentum

• What is momentum (P)?
• The product of the mass and velocity of an
  object.
• P m x v kg m/s
• Momentum has direction, it is the same direction
  as velocity

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• Mass can be thought of as a measure of the amount
  of Inertia of an object.
• Momentum - the amount of mass-motion of an
  object.
• Momentum is used to understand what happens when
  2 objects collide.

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• Find the momentum of the following (watch units)
• A 75 kg football player running at 9.4 m/s
• A 12 lb bowling ball moving at 20 ft/s
• A 2 ton truck moving at 40 mph
• Can a butterfly ever have more momentum than a
  freight train?

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Your Turn

• A person throws a 25g rock at a velocity of
  4.9m/s in an upward direction. What is the
  momentum of the rock?
• Calculate the time in seconds that a runner would
  take to complete a 6.00km run if he ran with an
  average velocity of 3.00 m/s.

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Law of conservation of momentum

• Total momentum in a
• system is conserved.
• If no other forces act upon two marbles when one
  is rolled into another the total momentum would
  remain unchanged.

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The Impulse-Momentum Theorem

• A force applied over a given time is called an
  Impulse.
• When an impulse is applied to a body, it will
  gain or lose the same amount of momentum

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Mechanical Forces

• 1.) What is a Force?
• A push or pull that changes direction, shape, or
  motion.

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• a.) Forces are all around us. Floors and walls
  exert forces on us.
• Friction is the force that opposes the motion
  between two objects that are touching each other.
  ( Walking, driving, writing all require friction.)

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Friction

• If you roll a ball across the floor it eventually
  stops.
• Friction is the force between two objects in
  contact.
• What do you think friction depends on???

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Is there another force acting on a falling object
in Earths atmosphere?

• Air resistance-
• Force exerted by air on a moving object.
• Acts in the opposite direction of the motion of
  the object.

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• Air resistance depends on speed, size, and shape
  of the object.
• Air resistance as speed

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Examples of Forces

• Result in stretching, squeezing, or bending
  matter.
• Friction-Force that opposes the motion between
  two surfaces that are touching each other.
• Depends on types of surfaces and forces pressing
  them together.
• Not dependent on area in contact

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Forces (F)

• What changes the velocity of a body?
• A golf club strikes a ball on a
  tee.
• The ball then strikes a tree and changes
  direction.

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• Does a force always change velocity or
  direction?
• When two forces
• are balanced there is no net force.(tug of war)
• Push on a wall no effect.
• A Net force is required to change the direction
  and or velocity of a body.

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Gravity

• Every object in the universe exerts a force on
  other objects.
• The more mass an object
• has the greater the force of gravity it exerts.

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Gravity and Weight

• Not all forces are exerted by bodies touching one
  another.
• Objects fall because gravity pulls them toward
  Earth.
• Force exerted depends on
• the objects mass.

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• Force of gravity that Earth exerts on an object
  is its weight.
• The unit of weight (or any force) is the
  Newton(N).

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• Example
• Earth has greater mass than moon
• Weight on moon is 1/6 weight on Earth.
• Mass is the same
• Inertia is the same

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• Gravity also depends on the distance between
  objects.
• As distance pull of gravity
• example- rocket and Earth
• double the distance and force is 1/4 X
  distance
• 2X 1/4 3X 1/9

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Newtons Laws

• 1st Law-The Law of inertia
• a.)An object will continue in a state of rest
  or uniform motion unless acted on by an
  unbalanced force.

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• b.) An object that is moving
• at a constant velocity
• continues at that velocity
• unless a net force acts on it.
• Golf ball- at rest until club hits it.
• Bowling ball- continues until it strikes pins and
  back stop.

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• Inertia- the property of a body that resists any
  change in velocity.
• a baseball is pitched at a
• rate of speed that continues
• until the catchers glove or the
• bat contacts the ball)
• The amount of mass an object is a measure of its
  inertia

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• Mass, weight, and Inertia
• Mass - the amount of matter in an object. This
  is a constant.
• Weight can change with the environment. We use
  a balance to mass objects, not a scale.

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• Remember
• Mass is a measure of inertia
• Greater mass means greater difficulty changing
  motion.
• mass amount of matter in an object (kg or g)
• The Physics Classroom

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• Newtons 2nd Law-
• The Law of Acceleration
• When a force is applied to an object it will
  accelerate with an acceleration proportional to
  the amount of the force and inversely
  proportional to the mass. F ma or aF/m

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Units for Force (N)

• Units for acceleration m/s2
• Units for mass kg
• Units for force kgm/s2
• F ma
• 1N1 kgm/s2

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• The acceleration of an object is increased as
  the amount of net force applied from outside the
  object increases.

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• For the same force, a small mass will have a
  greater acceleration than a larger mass.
• F m(a)

a F/m
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Your turn

• An object with a mass of 27.0kg is accelerated at
  a rate of 121m/s2. What was the force applied to
  the object?
• A force of 301.6N is applied to an object and
  results in an accelaration of 59m/s2. What is the
  mass of the object?

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Falling Objects

• Rate of acceleration is a constant value in a
  vacuum.
• 9.8 m/s2
• For a falling object
• If F m(a) then W m(a)
• Weight force of gravity

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• Terminal Velocity
• The largest velocity
• that can be reached
• by a falling object.
• An object has no net force acting on it. It
  keeps falling at a constant velocity.

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• Upward force of air downward force of gravity.
  Object has a constant velocity.
• If there is no air
• resistance
• (i.e. moon or vacuum)
• an object accelerates
• until it hits the ground.

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Weightlessness and Freefall

• What is weightlessness?
• An object is weightless when it is in freefall.
• No force other than
• gravity is acting on
• an object in freefall
• The Physics Classroom.

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• 3rd Law
• The Law of Action Reaction
• 1.) For every applied force, there is an equal
  force in the opposite direction.
• 2.) Forces occur in pairs. One object exerts a
  force on another the 2nd exerts an force on
  the 1st

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• For every force there is an equal and opposite
  force.
• Identifying Action and Reaction Force Pairs

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Newtons 3rd Law

• Why does it hurt when we kick a brick wall?
• How does the 3rd law describe how we walk?.

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The End