2. Point of reference
• Observational point tied to the motion of an
observer.
• In Newtonian physics, all motion must be
defined in terms of a point of reference.
• Relative Motion

3. Point of reference
• Is the speed of the ball different relative to the
pitcher, the truck driver, and the jet pilot? Why or
why not?

4. Point of reference
• Trajectory: The path of a moving body.

5. Distance
• Distance is the measurement from one point
to another. The length of the trajectory
• Distance may refer to a physical length, a
period of time, or an estimation.

6. Displacement
• Displacement ( x) – difference between an object’s final
position and its starting position.
– Does depend on direction.
• Displacement = final position – initial position
• x = xfinal – xinitial
• In order to define displacement, we need directions.
• Examples of directions:
– + and –
– N, S, E, W
– Angles

7. Displacement vs. Distance
• Example of distance:
– The ant walked 3 cm.
• Example of displacement:
– The ant walked 3 cm EAST.
• An object’s distance traveled and its
displacement aren’t always the same!

8. Displacement vs. Distance
• An athlete runs around a track that is 100 meters long
three times, then stops.
– What is the athlete’s distance and displacement?
• Distance = 300 m
• Displacement = 0 m
• Why?

9. Displacement and distance
To go to school from home... your
home
Size = length of
this arrow
your displacement from
school home to school
A displacement has size & direction.

10. Displacement and distance
To go to school from home... your
home
l2 l1
l3
your
school Distance = length of path you
travelled = l1 + l2 + l3
( size of displacement)