Linear Motion

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Linear Motion

• Dr. Don Franceschetti
• September 6, 2005

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Guide Questions

• 1. Is there any way to we tell if we are motion?
• 2. Is there a difference between speed and
  velocity?
• 3. Is there a difference between velocity and
  acceleration?
• 4. What is meant by free fall?

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We are moving!

• 107,000 kilometers per hour with respect to the
  sun
• Faster with respect to center of milky way galaxy
  (How do we know--Youll find out.)
• Motion is relative. No meaning to absolute rest.
• Usually we mean relative to the Earths surface.

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Units of Measurement

• The meter
• Originally the distance from the North pole to
  the equator on the meridian passing through
  Paris, France.
• Then the distance between two marks on a Pt-Ir
  rod.
• Then a fixed number of wavelengths of a certain
  wavelength of light
• Now the distance traveled by light in a fixed
  time.

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Units of Measurement

• The second
• Short for the second minute part of an hour
• Originally 1/2600 of 1/24th of a mean solar day.
• Now time for a fixed number of vibrations of a Cs
  atom. (atomic clock)

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Speed

• Speed distance/time
• So units are meters/second (m/s.)
• E. g. 100 km/hr
• Instantaneous speed
• Average speed
• Total Distance Covered
• Time interval

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Velocity

• Velocity is speed and direction, taken together.
  E. g. 100 km/h due north.
• Instantaneous velocity is usually meant.
• Average Velocity is
• Net displacement
• time interval

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Acceleration

• Acceleration
• change in velocity
• time interval
• Measured in meters per second per second
• or m/s2
• Has both magnitude and direction
• Or in mixed units, e. g. 5 km/hrsec
• This is the one you feel

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You have a (nonzero) acceleration if.

• Your speed changes as you move in a straight
  line.
• Your direction changes while your speed does not.
• Your speed and direction are both changing.

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Galileo again

• Found that balls rolling down an inclined plane
  did so wth constant acceleration
• Velocity acceleration x time
• Measured from rest.

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Free fall

• A body responding only to the gravitational pull
  of other bodies is said to be in free-fall.
• This means all other forces are negligible.

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Little g

• Galileo showed that all bodies in free-fall near
  the earths surface accelerate downward at 9.8
  m/s2.
• Call this g. Its best to remember as 10
  meters per second every second.
• Falling from rest vgt.

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Freely Falling bodies

• How far
• D (1/2)gt2 from rest
• Or D v(average) x time, always true

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Study problems

• Calculate the speed and distance covered at 1, 2,
  3, 4, and 5, seconds by.
• A lead brick dropped from rest.
• A rock tossed upward at 20 m/s.
• c) A rock thrown downward at 20 m/s.

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For your further consideration

• Hewitt Ch 3.
• Review Questions 5, 6, 8, 11, 12
• Ex 1, 5, 6, 7, 27, 28, 29