Motion and Momentum

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Motion and Momentum

• Chapter 7

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What is Motion? Section 1

• an object is in motion if it changes position
  relative to a reference point.
• An object moves when its distance from another
  object is changing.
• Whether an object is moving or not depends on
  your point of view (reference point).

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Reference point

• a place or object used for comparison to
  determine if something is in motion.

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• Suppose you are standing on a sidewalk and your
  friend rides past you on her skateboard.
• Which one of you is moving relative to the Earth?
• Are you moving relative to your friend?

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Distance and Displacement

• Distance the total length of the route you will
  travel.
• Odometers measure distance traveled.
• It is 4 miles to drive from my house to school.
• Displacement the distance between the starting
  and ending points and the direction in which you
  traveled.
• As the crow flies.
• It is 1.3 miles from my house to school as the
  crow flies. A straight line distance.

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Speed

• The distance an object
• travels in one unit of time
• is its speed.
• Speed Distance
• Time
• Units of Speed (m/s) or (km/hr)
• Average speed total distance
• total time

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The 3-Step Process

• FOR SHOWING MATH WORK IN SCIENCE
• Step 1 Formula
• Step 2 Substitute (with units)
• Step 3 Solve (with units!!)
• Ex
• Fma
• F20kg x 0.5m/s/s
• F10kgxm/s/s 10N

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Sample Problem

• At what speed did a plane fly if it traveled 1760
  meters in 8 seconds?

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• At what speed did a plane fly if it traveled 1760
  meters in 8 seconds?

• Step 1 Formula
• v d/t
• Step 2 Substitute
• v 1760m / 8s
• Step 3 Solve
• v 220 m/s

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Speed Practice Problems

• A car travels 240 kilometers in 3 hours. What is
  the speed of the car during that time?
• The speed of a cruise ship is 50 km/hr. How far
  will the ship travel in 14 hours?
• A cyclist travels 32 km during the first 2 hours
  of riding, and 13 km during the next hour. What
  is the average speed of the cyclist?

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Average vs Instantaneous Speed

• Average speed
• Speed can change during the time traveled.
• Speeding up and slowing down
• Stopping at traffic lights
• Getting behind slower drivers
• Stopping to use the rest-room or to get a drink
  at the convenient store
• Passing the semi-truck in front of you
• Traffic jam
• Average speed is TOTAL distance traveled during
  the TOTAL time traveled!!

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• Instantaneous Speed
• The speed of an object at one instant of time
• Right NOW!
• Shown on the speedometer!

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Velocity

• speed in a given direction.
• Velocity Distance
• Time
• Note that the velocity equation is the same as
  the speed equation
• A change in v happens when
• 1 its speed changes
• 2 its direction changes
• 3 they both change

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Graphing motion

• Always use a distance vs. time graph (distance on
  the vertical or y-axis and time on the horizontal
  or x-axis).
• Slope tells you the rate of change (speed).
• slope rise
• run

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Acceleration Section 2

• the rate of change in velocity.
• Refers to
• increasing speed (speeding up) positive accel
• decreasing speed (slowing down) negative accel
• or changing direction (turn)
• (a)Final Velocity (vf)-Initial Velocity
  (vi) Time (t)
• a vf vi
• t
• Units for Acceleration km/hr2, km/hr/s or m/s2

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Acceleration Practice Problems

• A roller coaster is moving at 25m/sec at the
  bottom of a hill. Three seconds later it reaches
  the top of the next hill, moving at 10m/sec.
  What is the deceleration of the roller coaster?
• A car is traveling at 60km/hr. It accelerates to
  85km/hr in 5 seconds. What is the acceleration
  of the car?

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Graphing Acceleration

• Velocity vs. Time
• Velocity on y-axis
• Time on x-axis
• The slope of the line gives you the acceleration
  of the object.

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Momentum Section 3

• When a collision occurs, changes in motion of the
  colliding objects depend on their masses and
  their velocities before the collision.

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Mass and Inertia

• Would it be harder to stop a large adult from
  rushing at you or a small child?
• Why?
• Inertia is a property of matter which states that
  objects have the tendency to resist a change in
  its motion!
• Objects with more mass have more inertia!

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Momentum

• This is a measure of how hard it is to stop an
  object.
• Momentum (p) mass (m) x velocity (v)
• pmv
• Mass is measured in kg
• Velocity is measured in m/s
• Units of momentum are kgm/s

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Momentum Practice Problems

• Calculate the momentum of a 14kg bicycle
  traveling north at 2.0m/s
• A 10,000kg train is traveling east at 15m/s.
  What is the momentum of the train?
• What is the momentum of a car with a mass of
  900kg traveling north at 27m/s?

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Final Practice Problems

• One jet plane is flying east at 880 km/h, and
  another plane is traveling north at 880 km/h. Do
  they have the same velocities? The same speeds?
  Explain.
• A swimmer speeds up from 1.1 m/s to 1.3 m/s
  during the last 20 seconds of the workout. What
  is the acceleration during this interval?
• Describe three different ways to change your
  velocity when youre riding a bicycle.
• An object is said to be accelerating if it
  ___________.
• a) speeds up c) changes direction
• b) slows down d) all of these