Motion and Forces

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Chapter 8

• Motion and Forces

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8.1 - Motion
Discussion slide

• What is motion?
• What are some examples of things in motion?
• What is speed?
• Lets go find out how fast some people are!

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8.1 - Motion
Notes!

• Speed is the distance traveled, divided by the
  time it takes to move that distance
• Speed Distance / Time
• v d / t
• The standard units for speed is meters per
  second, or m/s
• Did our walkers and runners go at a constant or
  variable speed?
• Can you name some things that move at constant
  and variable speeds?

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8.1 - Motion
Graphing?!?

• We can graph time versus distance, to give us
  speed
• We covered 116 feet in how many seconds?
• A jet goes 257m per second
• Eagles fly 51m/s
• Horses run 19m/s
• Our fastest runner ran _______
• And, at rest, were going 0m/s

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8.1 - Motion
Discussion slide!

• Who lives closest to school?
• Can you walk?
• How far is it?
• Where is it?
• What is your walking speed?
• So, how long will it take to get home?
• Lets ask ___________ to walk to your home
• How fast can __________ walk?
• How soon will __________ get there?
• Okay, go!

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8.1 - Motion
Discussion slide!

• What is the difference between speed and
  velocity?
• In our exercise the other day, what was the speed
  of people, as they either walked or ran?
• What was their velocity?
• So, if I said, party at Jims house at 400,
  and everyone walks 2-4 mi/hr, will everyone get
  there in time if Jim lives 2 miles away?

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8.1 - Motion
Back to notes!

• Velocity is the speed and direction of motion
• Velocity changes if either speed or direction
  changes
• Examples
• Speed
• The Broad Street subway trains go 30 mi/hr
• Ninth-graders walk an average of 3 mi/hr
• Velocity
• The Broad Street subway trains take our students
  home at 30 mi/hr toward the south
• Ninth-graders walk an average of 3 mi/hr up the
  hill to Central each morning

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8.1 - Motion
Math exercise! No need to copy.

• During Spring Break, were going to go whitewater
  rafting!
• On Day 1, we should go about 8km down the river
  we will put in at 8 a.m. and stop at 4 p.m., with
  an hour for lunch. What is our average velocity,
  in meters per second?
• Distance 8km time 7 hours
• Distance 8000m time 25200s
• Velocity .32m/s down the river

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8.1 - Motion

• While v d / t, multiplying both sides by t
  yields
• d vt (so we can solve a problem for distance,
  if we know velocity and time)
• And if we know both velocity and distance, we can
  solve for time
• t d / v

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8.1 - Motion
Table exercise!

• Find the
• Velocity of a swimmer going a constant 110m
  toward shore in 72s
• Velocity of a baseball thrown 38m from third to
  first base in 1.7s
• Distance a cyclist would travel in 5 hours at an
  average velocity of 12km/h southwest
• Time a skier would take to finish a 2.6km race at
  an average velocity of 28m/s downhill

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8.1 - Motion
Discussion slide!

• A car and a bus are going the speed limit down
  Olney Avenue, toward Ogontz, when the traffic
  light turns red. Why does it take longer for the
  bus to stop?
• What is mass?
• Have you ever gone so quickly down a hill that
  you lost control (running, on skates or skis)?
  Why did you lose control?

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8.1 - Motion
Notes!!!

• Momentum - for an object moving in a straight
  line, its mass multiplied by its velocity
• momentum mass x velocity
• p mv
• standard units for momentum is kg x m/s
• momentum also has a direction
• So, a car, a bus, and a subway moving north on
  Broad Street at 30 mi/hr would have much
  different momentums Why?

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8.1 - Motion
Table exercise!

• Find the momentum of a
• 75kg dancer moving across the stage at 3m/s
• 150kg dancer moving in the opposite direction
  across the stage at 3m/s
• 12kg dog running toward you at 7m/s
• 45kg student sitting on the subway train, waiting
  for it to leave Olney station
• In our first two problems above, what would
  happen if the dancers collided?

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8.1 - Motion
Back to notes.

• Conservation of momentum The total amount of
  momentum in a system is conserved
• So, if a 75kg dancer, moving left at 3m/s,
  collided with a 150kg dancer, moving right at
  3m/s, what would be the result?
• Objects will either bounce off of each other or
  move in the same direction as the object with the
  greater initial momentum

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8.2 - Acceleration and Force
Discussion slide!

• What is acceleration?
• Might I suggest an example Ninth-graders going
  from sitting in chairs to moving out the door
  when the bell rings?
• Can you have a negative acceleration?

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8.2 - Acceleration and Force
Notes!

• Acceleration is the change in velocity divided by
  the time it took for the change to occur
• Acceleration (final velocity - initial
  velocity) / t a ?v / t
• Standard units are m/s2
• This applies to straight line motion
• Negative acceleration means that the objects
  velocity will decrease

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8.2 - Acceleration and Force
Examples!

• A student sitting on a wall jumps to the ground
  in 2.5s, at a final velocity of 7.5 m/s down
• Students sitting in Room 232 suddenly move out of
  the doorway, in 2s reaching a velocity of 1m/s

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8.2 - Acceleration and Force
Table exercise!

• Find the
• Average acceleration of a southbound subway train
  that slows from 12m/s to 9.6m/s in .8s
• Average acceleration of a skateboarder who goes
  straight from 0m/s to 4m/s in 2.5s
• Time it takes for a car to accelerate from
  24.6m/s to 26.8m/s at an average acceleration of
  2.6m/s2

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8.2 - Acceleration and Force
Demonstration!

• What causes an object to change its velocity or
  to accelerate?
• How did the football go from one part of the room
  to another?
• Objective Everyone at the table needs to blow,
  to make sure that the ball does not move, or,
  better, spins in place!
• Who can explain the action of the ball at each
  table. Why did it move as it did?

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8.2 - Acceleration and Force
Notes!

• Force - the cause of acceleration or change in an
  objects velocity
• If forces act in different directions but are not
  exactly opposite, the combination of forces acts
  like a single force on the object this net force
  will cause the object to accelerate
• How would we need to set up our previous exercise
  to balance the ball on the table?

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8.2 - Acceleration and Force
Notes!

• Balanced forces will not change the motion of an
  object (who can think of some examples of
  balanced forces?)
• Unbalanced forces do not cancel each other
  completely, so the object will move (examples?)
• If we roll a ball across this table, will it keep
  moving at a constant speed? Why or why not?

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8.2 - Acceleration and Force
Discussion slide!

• So, a force must be acting on the ball, causing
  it to slow down this is an unbalanced force
• What do we need to do to keep the ball moving?
• Once we reach the speed we want the ball to go,
  the forces will balance One force in the
  direction of motion, the other force against the
  direction of motion, as we achieve a constant
  speed. The same is true of subways, buses, and
  cars.

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8.2 - Acceleration and Force
Notes!

• Friction is an unbalanced force acting against
  the direction of motion
• There is more frictional force on a rough surface
  than on a smooth surface (for instance, the
  sidewalk outside, as opposed to the hallways
  inside Central)
• Air resistance is a form of friction. For
  example, buses experience more air resistance
  than cars do

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8.2 - Acceleration and Force
Discussion slide!

• If I hold a ball in my hand, as still as
  possible, what is its speed, velocity, momentum,
  and acceleration?
• What force, if any, is acting on it? If any,
  balanced or unbalanced?
• What will happen if I stop holding the ball and
  rotate my hand? Why?

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8.2 - Acceleration and Force
Notes!

• Gravity - the attraction between two objects
  because of their mass
• Every object exerts a gravitational force on
  every other object
• Gravitational force is proportional to mass. In
  other words, the larger an objects mass, the
  greater its gravitational force. So, its not
  gravity that attracts you to your bf/gf, but
  gravity does attract you to Earth!

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8.2 - Acceleration and Force
Notes!

• Distance also affects gravitational force The
  closer two objects are to each other, the greater
  the gravitational force

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8.3 - Newtons Laws of Motion
Discussion slide!

• As we roll a ball across the table, note how far
  it rolls
• Now, observe how far the ball rolls across a
  towel
• What will happen to the coin on the card, if we
  slowly move the card off the top of the glass?
• What if we move the card quickly?
• Can anyone explain these two demonstrations?

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8.3 - Newtons Laws of Motion
Notes!

• Newtons First Law An object at rest remains at
  rest and an object in motion maintains its
  velocity unless it experiences an unbalanced
  force
• Inertia is the tendency of an object to remain at
  rest or in motion with constant velocity
• All objects have inertia because they resist
  changes in motion

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8.3 - Newtons Laws of Motion
Discussion slide!

• In Newtons first law, the net force is zero -
  the object is either at rest or moving at
  constant velocity
• What happens if the net force is not 0?
• Is it easier to push an empty box or a box full
  of books across a table? Why?
• If we gave an empty box and a full box the same
  push, which will move further?

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8.3 - Newtons Laws of Motion
Notes!

• Newtons Second Law - the unbalanced force acting
  on an object equals the objects mass times its
  acceleration
• Force mass x acceleration (F ma)
• The acceleration is always in the direction of
  the net force
• Force is expressed in units called Newtons
• 1N 1kg x 1m/s2

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8.3 - Newtons Laws of Motion
Examples!

• Paramedics lift a stretcher and patient with a
  mass of 100kg, with an upward acceleration of .65
  m/s2. What is the unbalanced force needed to
  produce this acceleration?
• A member of 271 pushes a 88kg 270 student
  sideways off of a frictionless wall with an
  acceleration of .34 m/s2. What is the force
  needed to produce this acceleration?

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8.3 - Newtons Laws of Motion
Table exercise!

• What is the net force needed for a 1.6x103kg car
  to accelerate forward at 2.0 m/s2?
• What is the mass of a baseball accelerating
  downward at 9.8 m/s2, if the gravitational force
  on the baseball is 1.4N? (assume gravity is the
  only force)
• A sailboat and crew have a combined mass of
  655kg. If the boat is moving forward by a force
  of 895N, what is the boats acceleration?

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8.3 - Newtons Laws of Motion
Demonstration!

• Someone from each table come to the front and
  obtain two spherical objects
• Someone else come obtain a balance (we need to
  share!!!!)
• Back at the tables
• Find the mass of each ball
• Make an inference of which ball will hit the
  floor first if both are rolled off the table at
  the same time
• As a table group, test your inference by
  carefully pushing the balls off the table at the
  same time
• Note the results

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8.3 - Newtons Laws of Motion
Notes!

• Free fall - the motion of an object when only the
  force of gravity is acting on it
• The free fall acceleration is directed toward
  Earths center
• Free fall acceleration is abbreviated as g
• Near Earths surface, free fall acceleration is
  constant, at 9.8m/s2 this means that, no matter
  the mass, and in the absence of air resistance,
  all objects dropped from the same height will
  reach the surface at the same time

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8.3 - Newtons Laws of Motion
Discussion!

• Why do all objects have the same free fall
  acceleration?
• Newtons second law shows that acceleration
  depends on both the force on an object and its
  mass. A heavier object experiences a greater
  gravitational force than a lighter object
  however, a heavier object is harder to accelerate
  than a lighter object because it has more mass!
• We will disregard air resistance in our
  discussions, calculations, and quizzes!

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8.3 - Newtons Laws of Motion

• What is weight, as opposed to mass? (for the
  record, our book describes mass as a measure of
  the quantity of matter in an object)
• Weight - the force on an object due to gravity
• Weight mass x free fall acceleration (wmg)
• The standard unit for weight is the Newton
• Compare your mass and weight on Earth to what
  they would be on the moon (g1.6m/s2)
• Mass on Earth? Mass on moon?
• Weight on Earth? Weight on moon?

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8.3 - Newtons Laws of Motion

• Gravitational force affects the shapes of living
  things. What are some examples?
• Strong skeletons are needed for plants and
  animals to live on the earth
• What is terminal velocity?
• Terminal velocity - when the force of air
  resistance equals the gravitational force
  (weight) of the object the object stops
  accelerating and moves at a constant velocity
• Without a parachute, sky divers reach terminal
  velocity of 200 mi/h with the chute, their new
  terminal velocity slows to several km/h

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8.3 - Newtons Laws of Motion
Discussion slide!

• If you were to kick a soccer ball, can you feel
  the force of your foot kicking the ball?
• What if you were a boxer and punched your
  opponent in the nose would you feel that force?
• The ball and the nose both change in motion, but
  are these the only forces present?
• The soccer ball exerts an equal and opposite
  reaction force against your foot

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8.3 - Newtons Laws of Motion
Notes!

• Newtons Third Law - For every action force,
  there is an equal and opposite reaction force
• These forces act in pairs, on different objects
  (the foot and the ball, for example), and they
  occur at the same time
• Another example of this happens with rockets, as
  the burning fuel going out of the bottom of the
  rocket forces the rocket to move in the opposite
  direction