Dennis Wang
Ms. Crozier
Physics 30 Ns
Physics Notes for September 24, 2010
Distance/Displacement-Time Graphs and What They Mean
+d
constant positive
(or "forward")
velocity
(+v)
+t
+d
constant negative
(or "backwards")
velocity
(-v)
+t
+d
constant velocity of 0 units,
no distance covered
(v = 0)
+t
Quick Reminder: Velocity is equal to *displacement* over time, speed is equal to
*distance* over time. Delta (Δ) means a change in something (such as change in
temperature). Positive displacement results in positive velocity, and negative
displacement results in negative velocity.


 
 d
 d d 2  d 1
v
v

t
t
t 2  t1
Dennis Wang
Ms. Crozier
Physics 30 Ns
Physics Notes for September 24, 2010
Distance/Displacement-Time Graphs and What They Mean Continued
+d
positive ("forward") acceleration of
velocity
(+a)
*if you draw lines tangent to the
curve, the slope, or speed increases
as t increases*
+t
+d
negative ("backwards")
acceleration of velocity (+a)
*if you draw lines tangent to the
curve, the slope, or speed
decreases as t increases*
+t
Dennis Wang
Ms. Crozier
Physics 30 Ns
Physics Notes for September 24, 2010
Assignment: Page 45-47, #1-8
1. A high school athlete runs 1.00 * 10^2 m in 12.20 s. What is the velocity in km/h and
m/s?
Answer: +8.20 m/s, +29.5 km/h
2. A person walks 13 km in 2.0 h. What is the person's average velocity in km/h and m/s?
Answer: +6.5 km/h, +1.8 m/s
3. Using the data in Table 3-1, during what one second long time interval is the car
moving slowest? moving fastest?
Answer: in-between 0.0 and 1.0 s (slowest)
in-between 3.0 and 4.0 s (fastest)
4. Using the data in Table 3-1, find the average velocity of the car in the time interval
between 0.0 and 2.0 s.
Answer: +3 m/s
5. Suppose a car travels at a constant 10 m/s. how far would it move in 1 h? 1 min? in 1
s? in 1 ms? in 1 μs? in 1 ns?
Answer: 600 m in 1 min
36000 m in 1 hr
10 m in 1 s
0.01 m in 1 ms
0.00001 m in 1 μs
0.00000001 m in 1 ns
6. A train leaves the station at the 0.0-m marker travelling with a constant velocity of
36.0 m/s.
a. How many seconds later will the train pass the 1620.0-m marker?
Answer: 45.0 s
b. What is the velocity of the train in km/h?
Answer: +130. km/h
7. At 1:00 p.m., a car, travelling at a constant velocity of 94 km/h toward the west, is 17
km to the west of your school. Where will it be at 3:30 p.m.?
Answer: 260 m [west]
Dennis Wang
Ms. Crozier
Physics 30 Ns
Physics Notes for September 24, 2010
Assignment: Page 45-47, #1-8 Continued
8. Suppose the car in Practice Problem 7 started 17 km east of your school at the same
time, moving in the same direction at the same velocity.
a. Where would it be at 3:30 p.m.?
Answer: 220 m [west]
b. When would it be at your school?
Answer: 0.18 h
Assignment: Page 49, 1.1, 1.3, 1.4
1.1 Describe the position of the period in this sentence using three reference points: one
on the book, the second on your desk, the third fixed in the room.
Answer: The period is stationary relative to all three reference points, provided that
the reference points were not moving relative to the floor.
1.3 Draw a position-time graph of a person who walks one block at moderate speed,
waits a short time for a walk light, walks the next block more slowly, and the final
block very fast. All blocks are of equal length
+d
+t
Dennis Wang
Ms. Crozier
Physics 30 Ns
Physics Notes for September 24, 2010
Assignment: Page 49, 1.1, 1.3, 1.4 Continued
1.4 Suppose that in your town positive house numbers are north of Main Street and
negative numbers are south. You leave your house that is located at -800 at 9:00a.m.
You arrive at Main Street at 9:04 a.m., reach +800 at 9:08 a.m., and stop at +1600 at
9:14 a.m.
a. Draw a position-time graph of your walk.
+1600
+800
Main
Street,
9:00 a.m.
9:04
a.m.
9:08
a.m.
9:14
a.m.
-800
b. Find your average velocity for your entire trip.
Answer: +170 house #s/min
c. Find your average velocities between 9:00 and 9:04, 9:04 and 9:08, 9:08 and 9:14.
Which is largest? smallest?
Answer for first two: +200 house #s/min
Answer for last: +130 house #s/min
First two is largest, last one is smallest
Dennis Wang
Ms. Crozier
Physics 30 Ns
Physics Notes for September 24, 2010
Assignment: Page 58, #1-3, 8, 9
1. If the scale in Figure 3-1 was moved 3 m to the right, would the position of each car
remain the same?
Answer: No
2. If the scale in Figure 3-1 was moved 3 m to the right, would the distance between each
car remain the same?
Answer: Yes
3. A NASA team oversees a space shuttle launch at Cape Kennedy in Florida and then
travels to Edwards Air Force Base in California to supervise the landing. Which group
of people, the astronauts or the NASA team, has the greater displacement?
Answer: they both have the same displacement if the shuttle lands and if the reference
point is Cape Kennedy.
8. Figure 3-21 is a position-time graph of two people running.
a. Describe the position of runner A relative to runner B at the y intercepts.
Answer: Runner A is positioned ahead of runner B.
b. Which runner is faster?
Answer: Runner B is faster than runner A.
c. What occurs at point P and beyond?
Answer: Runner B catches up with runner A. Beyond point P, runner B runs ahead
of runner A.
9. Figure 3-22 is a position-time graph of the motion of two cars on a road.
a. At what time(s) does one car pass the other?
Answer: The two cars pass each other at approximately 1.5 and 8.2 s.
b. Which car is moving faster at 7.0 s?
Answer: Car B is moving faster than car A.
c. At what time(s) do the cars have the same velocity?
Answer: The cars have the same velocity at approximately 3.6 s.
d. Over what time interval is car B speeding up all the time?
Answer: Car B never speeds up.
d. Over what time interval is car B slowing down all the time?
Answer: Car B never slows down during the time interval between 1.3 and 10 s.