Motion

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Motion
2
Motion

• Motion is the change of an objects position
  over time
• When we say something has moved, we are
  describing motion
• When we are describing motion, we are comparing
  it to a frame of reference

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Motion Activity

• On your graph paper, draw a small picture of
  yourself getting ready to take a walk in the
  center at the top.
• The center is West (Draw a dot)
• You are going to walk 10 squares to the right to
  Marios for a slice of pizza
• How far have you moved from West?
• How far have you traveled?

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Motion Activity

• You forgot your physical science notebook, so you
  have to run home 14 squares in the other
  direction and get it
• How far have you moved from West?
• How far (total squares) have you traveled?

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Motion Activity

• You dont want to be late for Physical Science,
  so you run 4 squares back to West
• How far have you moved from West?
• This is displacement
• How far (total squares) have you traveled?
• This is distance

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Distance vs. Displacement

• Distance the length of the path traveled
  between the initial and final positions of
  something in motion
• Displacement the shortest distance between the
  initial and final positions of something in motion

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Speed

• When we talk about how fast something is
  traveling, we are talking about speed
• Speed is the rate at which an object moves

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Speed

• How do we calculate speed?

Distance
Velocity
Time
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Speed

• How do we calculate speed?

Distance
Speed
Time
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Speed

• Distance is usually measured in m or km
• Time is usually measured in s or hr
• The unit of speed is usually m/s or km/hr
• Speed is used to describe anything that is
  changing position
• Runners, cars
• Ocean currents, winds
• Moon, Earth

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Constant Speed

• Some objects have speeds that stay the same as
  they move
• Constant speed
• Speed does not change
• Plot the distance vs. time for a cheetah
  traveling 30m/s for 5 seconds

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Constant Speed

• A cheetah running at constant speed

Time (s) Distance (m)
0 0
1 30
2 60
3 90
4 120
5 150
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Constant Speed
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Average Speed

• Some objects have speeds that change as they move
• We have to calculate average speed
• Speed changes over time
• Think about driving in a car
• Speed limit changes
• Stop signs and stoplights

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Average Speed
A car in the pick-up line at West
Time (s) Distance (m)
0 0
1 30
2 75
3 90
4 130
5 150
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Average Speed
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Average Speed

• For things moving at a constant speed, the speed
  at any point on the line is the average speed

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Instantaneous Speed

• For things moving at any speed, the speed at a
  given point in time is the instantaneous speed

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Velocity

• Velocity is a speed in a given direction
• If you drive to Raleigh, your average speed could
  be 97km per hr
• (60miles per hr)
• Your velocity would be 97 km per hr east
• You have to include the direction

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Speed and Velocity Practice

• An airplane flew 1000 km in 4 hr. What is the
  airplanes speed?

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Speed and Velocity Practice

• A car travels 50 km west in 30 minutes. What is
  its velocity in km/hr?

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Speed and Velocity Practice

• How long will it take a ball to roll 26m if its
  velocity is 2.0m/s to the right?

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Speed and Velocity Practice

• If a runner moves with a speed of 1.5m/s and
  changes her speed to 2.1m/s over 3s, what is her
  average speed over that 3s?

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Speed and Velocity Practice

• Work on Velocity Problems

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Acceleration

• Think about riding a roller coaster
• You go up the first hill at constant speed
• After the top of the hill, your speed increases
  quickly
• At the bottom of the hill, you slow down and make
  some turns and keep going
• Rapid changes in velocity
• The rate of change in velocity is acceleration

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Acceleration

• Something that is speeding up, slowing down, or
  changing direction is accelerating
• Acceleration Final velocity-original velocity
• Time
• What are the units of acceleration?
• Acceleration tells you how fast velocity is
  changing

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Acceleration

• Sample Problem What is the acceleration if we
  speed up from 10m/s to 30 m/s in 10 seconds?
• A Vf - Vi
• t

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Acceleration

• Sample Problem A roller coaster car speeds up as
  it goes down a hill. As it starts down the hill,
  its speed is 8m/s, and at the bottom of the hill
  3 seconds later, its speed is 28m/s. What is its
  average accleration?
• A Vf - Vi
• t

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Gravity

• Gravity is the acceleration of an object toward
  Earth
• g9.8m/s2

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Gravity

• Example Brandon drops a water balloon out a
  window on Dominic, who is standing below. If it
  takes 10 seconds for the balloon to reach
  Dominics head, what is its speed just before it
  hits Dominic?
• g 9.8m/s2 - put in place of acceleration
• g Vf - Vi What is Vi?
• t

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Momentum

• Think about a football game
• A 100kg fullback runs up the field and collides
  with an 80 kg defensive back. The 100kg fullback
  gets knocked to the ground by the smaller player.
  How does that happen?
• The defensive back has more momentum
• All moving objects have momentum
• The more momentum a moving object has, the harder
  it is to stop.

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Momentum

• If both players had the same velocity, which
  player would have more momentum?
• What are the units for momentum?
• p m x v (p is the abbreviation for momentum)
• Momentum has units of kgm/s

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Momentum

• Momentum depends on the mass of an object and its
  velocity
• MomentumMass x Velocity
• pm x v
• In the football game, how can the smaller player
  knock over the larger player?
• His mass is smaller
• His velocity must be larger

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Momentum

• What are some things that have a large momentum?
• Train has a very large mass
• Car large mass, high speed
• Harder to stop if moving faster
• Stopping distance is directly related to momentum
• Bullet small mass, but high velocity

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Law of Conservation of Momentum

• When two objects collide, one object transfers
  momentum to the other
• The total momentum of a group of objects stays
  the same unless outside forces act on the objects
• No momentum is lost
• Total momentum is the same
• Think about the football players

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Law of Conservation of Momentum

• How do moving objects act?
• Richard_Garriott_Space_Video_Blog_Conservation_of_
  Momentum.avi
• Explains recoil of a gun
• http//www.batesville.k12.in.us/physics/phynet/mec
  hanics/momentum/firing_a_gun.htm

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Mythbusters

• Mythbusters - Lego ball
• Lego Ball