September 9, 2009
Physics I Honors
Section 2.1
Displacement and Velocity
Maglev train, magnetic levitation. No friction from track. If no air drag, can get to 4000 mph. Current record is
361 mph (581 kph). Proposed for LV-Anaheim.
Vocab: displacement (net displacement), velocity, speed, instantaneous velocity)
One dimensional motion, must use coordinate system
Must have frame of reference since everything in motion. For train, station is at rest, train is moving.
- Establish coordinate, 0 point (xi)
- Most important part of Physics calculations, determining coordinate system or reference point.
Displacement – the length of the straight line which can be drawn from the initial position of an object to its
final position.
- Displacement not affected by change in starting point (where we put meter stick does not affect net
displacement for object).
- ∆x is change in x direction, ∆y is change in y direction. ∆ delta is symbol for “change” or
“difference”.
- ∆ x= xf – xi
- Displacement is independent of the path, only depends on xf and xi.
(show various scenarios using meter stick and object; include positive and negative displacement)
(Conceptual Challenge on page 41)
- Demonstrate ∆y displacement.
VELOCITY – the total displacement divided by the time interval during which the displacement occurred.
- It has a magnitude AND direction.
- Average velocity (velocity over a time interval): vavg = ∆x/∆y = xf-xi/tf - ti
- This represents the AVERAGE velocity along the route; instantaneous velocity varies along the path
(Bend trip as an example).
- Can it be positive and negative? Remember, displacement can be negative.
Common Misconception – find average velocity: A car travels from city A to city B (100 km). If the first half of
the distance is driven at 50 km/h and the second half is driven at 100 km/h, what is the average velocity?
Misconception is that you can average if equal distances are driven at two velocities speeds. This is only
true if equal TIMES are driven at two different velocities.
100 km/[(50 km ÷ 50 km/h)+(50 km ÷ 100 km/h)] = 67 km/h
Review Sample Problem 2A on pg 44, then do practice on same page
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Practice:
o A doctor travels to the east from city A to city B (75 km) in 1.0 h. What is the doctor’s
average velocity – do not have to convert the time. (75 km/h)
o (Class will do Practice 2A problems – direction of travel must be included with the
magnitude of the velocity).
o 1. 34 min x 60 = 2040 sec
 0.98m/s x 2040 s = 1999.2 m east or 2.0 km east
September 9, 2009
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Physics I Honors
Section 2.1
Displacement and Velocity
2. 12.5 km/hr x 0.25 hr = 3.125 km south or 3.1 km south (sig fig)
3. 9.5 min x 60 = 570 sec
 1.2 m/sec x 570 sec = 684 m to north, or 680 m north
4. 144 km ÷ 48.0 km/hr = 3.00 hr
5. 144 km ÷ 56.0 km/hr = 2.57 hr (but this is NOT the answer to the question)
 3.00 hr – 2.57 hr = 0.43 hr saved.
6. 24/60 = 0.40 hr
 First time: t = x/v = 280 km ÷ 88 km/h = 3.2 hr
 Second time is given: 0.4 hr
 Third time (still same direction): 210 km ÷ 75 km/hr = 2.8 hr
 A. Total time: 3.2 h + 0.4 h + 2.8 h = 6.4 hr
 B. Average velocity: (280 km + 210 km)/6.4 h = 76.56 km/h south or 77 km/hr
south
Velocity must have magnitude and direction, even in hw answers.
Difference between velocity and speed. Velocity depends on net displacement and can be zero if no net
displacement occurs. Speed will still have a value based on overall distance.
Average Speed: distance traveled/time of travel
Cars have speedometers, not velocitometers (Indy 500)
Velocity = slope of the line on a graph of position vs time. (look in book, pg 45).
Conceptual Challenge (p. 45)
1. – displacement zero since starts and finishes at same point.
2. –velocities are not the same since DIRECTION is different.
Review graph on page 46
If you have raw data point to use for your graph, you need the “best fit” line before you can determine the
average velocity.
Instantaneous velocity – the velocity at a specific time. If curved graph – velocity is increasing over time – then
slope of tangent line to graph represents the instantaneous velocity.
Homework: pg. 47; 1, 3, 5 (all), 6 (all)