Measuring motion

clocks and meter sticks
Clocks and meter sticks are all that's needed to characterize motion.

Estimate...

Estimate the distance from GC's 8th street entrance (near dining hall) to the RFC by two methods...

  • Take a raw guess at the distance, in miles.
  • Now, take a guess at how long it takes to walk (in minutes), and use the typical speed of walking which is 3 or 4 miles per hour to work out how far it is.

Measuring...in the 21st century


map

Speed

Describing the motion of a moving body...

average speed = distance travelled / time to travel
`s = d / t`

[writing: about speed]

Was their speed always the same at every moment of the trip?

How could you measure your "instantaneous" speed?

A bicycle speedometer (nowadays called a cyclocomputer) uses a magnet attached to a spoke, and a sensor which "feels" the magnet pass.

Velocity

velocity = speed and direction of travel

Jane passes Fred, both of them moving North on 8th Street.

  • Do they have the same speed?
  • Do they have the same velocity?

Jane and Fred pass in opposite directions, both going 15 km / hour.

  • Do they have the same speed?
  • Do they have the same velocity

Jane and Fred are both going North on 8th St at 15 km / hour. But Jane has just crossed College Avenue, and Fred has already passed Plymouth Avenue.

  • Do they have the same speed?
  • Do they have the same velocity

Acceleration

Acceleration: any change in an object's velocity.

Acceleration?

See: HHSTT rocket sled video.

Acceleration?

Orbit of the Hubble Space Telescope around the earth.

hubble orbit

Acceleration?

See: Crash test video.

Acceleration?

A car...

  1. Moving along a straight, level road at 120 km/hr?
  2. Moving along a straight, level road and slowing from 120 km/hr to 70 km/hr?
  3. Rounding a curve at a steady 50 km/hr?
  4. Moving uphill along a straight incline at 50 km/hr?
  5. Rounding a hill top at 50 km/hr?
  6. Acceleration!

    These things all mean "Acceleration" in Physics-speak:

    • speeding up
    • slowing down
    • turning

    What 3 acceleration devices can a car driver control?

    Inertia revisited

    We said...

    A body that is subject to no external influences* will keep moving, if it was moving to begin with, in a straight line with unchanging speed. Or, if it was at rest, such a body will stay at rest.

    But now we can say

    A body subject to no external influences must maintain an unchanging velocity.

    Acceleration while falling

    How do we know objects speed up as they fall?

    acceleration = change in speed (m/s) / time interval (s)

    On the surface of earth all objects are accelerated by the same amount: 9.8 m/s/s

    Suggested Exercises

    Conceptual Exercises in Chapter 3: 8, 11, 15, 16, 19, 27.
    Problems in Chapter 3: 3, 5