PUM Physics II - Kinematics
Lesson 1 Solutions
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1.1 Observe and Find a Pattern
a) As the teacher moves in order to keep the object in the telescope I have to change the
orientation of the telescope. I turn the telescope from right to left.
b) The orientation of the teacher’s telescope doesn’t change during the experiment.
c) Based on your experiences in these two observational experiments:
For me while the teacher was moving I could say that the ball was moving relative to me
since I had to change the orientation of my telescope. For the teacher the ball did not
move since the orientation of his/her telescope did not change. Whether the ball was
moving depended on who the observer was.
In general, if an observer is looking at something through a telescope, and the telescope
moves, then to that person the object moved. If the telescope did not move, then the
object did not move. Motion depends on the observer and whether their telescope moved
or not. For example, a person sitting on the shoulder of the teacher or walking next to the
teacher at the same pace and direction will not see the ball move, because their telescopes
will not move.
If I am sitting on a sidewalk two observers who would see me moving would be the
passenger in a car driving past me or a person walking a dog across the street. In order to
see me they would have to change the orientation of their ‘telescopes’. So relative to
them I would be moving.
Sitting on the same sidewalk a person standing still across the street or my friend sitting
next to me would say I was stationary since to see me they would not have to change the
orientation of their ‘telescopes’. So relative to them I would not be moving.
d) To Chris the car is moving from right to left.
Jodi could be riding in a car right behind the car that Chris is watching. The car might be
getting bigger or smaller depending on whether or not the cars are coming closer together
or farther apart.
PUM Physics II - Kinematics
Lesson 1 Solutions
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1.2 Observe and Find a Pattern
The movement of the front passenger in the blue car
Observer
Describe what she/he sees
The person sitting in the backseat of the blue car.
Does not see the front passenger move.
The pedestrian standing on the sidewalk as the
blue car passes.
Sees the front passenger move.
The driver of the red car moving in the same
direction and passing the blue car.
Sees the front passenger in the blue car as
if he/she is moving backwards in the
opposite direction that the red car is going.
a) The person in the front would not appear to be moving. The pedestrian on the sidewalk
would appear to be moving backwards in the opposite direction that the blue car is going.
The driver of the red car would appear to be moving in the same direction that the blue
car is going, opposite the direction that the pedestrian appeared to be moving.
b) The front passenger and back passenger would appear to be moving together in the blue
car. The red car would be moving in the same direction past the blue car.
c) An object is in motion with respect to an observer, as time progresses, its position is
changing relative to the observer. A telescope test shows that an object is in motion when
you have to change the orientation of the telescope or the object appears to be bigger or
smaller.
d) “Motion is relative” because an object that appears to be moving relative to one observer
may not be moving relative to another observer. It really depends on who you are asking.
1.3 Represent and Reason
(Multiple possibilities)
a) From GW University, travel south for approximately ¼ km until the State Dept. is
located direction in front of you. Them, travel east past the White House. After 1 km,
travel south so that the Washington Monument is west of you. After 1km, travel about ¾
km east. The Smithsonian will be on your left. To arrive by 8:30, you would need to
leave sufficiently early to walk about 3.5 km, perhaps 7:30am.
b) I selected to use landmarks, the compass and the scale on the map to provide directions.
c) The legend helped me to estimate the distances that my friend would have to walk based
on the scale of the map. It also gave me a sense of north, south, east and west.
d) I assumed that the person could locate north and estimate the distances. I also assumed
that the scale and my rough estimates of distance with the scale were accurate enough to
provide guidance. In addition, I assumed that a typical speed for walking was about 3.5
PUM Physics II - Kinematics
Lesson 1 Solutions
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km/ hour, which would mean that my friend would need about an hour to walk from
GWU to the Smithsonian.
e) If I under or over estimated the distance or my friend’s walking speed, his/her arrival
time will change.
1.4 Reason
b) Both are partially correct and neither is really wrong. The problem is that both Meagan
and Beccy are not being specific about their observers when they are describing the
motion of Ryan.
c) If Andrew is the observer then Meagan would be correct in saying that “Ryan must be
moving west.” However to a person standing on the platform at the train station Ryan
appears to be moving East towards New York City, meaning that Beccy is also right.
Homework
1.5 Relate
(Many Responses Possible)
1) Hockey players looking for the right speed and direction while passing pucks need to use
relative motion to make sure that their teammate catches their pass. If one player is
moving faster or slower than the other, it changes how you will hit the puck so that your
teammate will receive it. Absolute motion cannot be considered, only relative motion
between the players.
2) Determining whether you are moving when you’re on a train and there is another train
located out your window. It comes difficult to know who is moving because all you can
detect is the relative motion between the trains. Are we moving backwards or are they
moving forwards?
3) Playing cards in the backseat of a moving vehicle is only possible because we are
concerned with the relative motion of the people in the car. Because both card players are
moving in the same direction at the same speed, there is no relative motion and they can
easily play – even while moving at 65 mph.
1.6 Explain
a) Start at my front door, 4 Smith St., Jerseyville, NJ at 7:15am. Walk south down Smith
Street for .2km, then walk west down Oak street for .4km. Jerseyville HS will be on your
right.
b) When we say that the sun rises and sets, a person on the surface of the earth who is
stationary is the observer. The observer could be on the moon to see that the sun does not
travel around the Earth every 24 hours.
c) We rotate relative to the sun to produce days and nights. However saying that the “Sun
does not move” ignores the ideas of relative motion. Relative to the center of our galaxy
PUM Physics II - Kinematics
Lesson 1 Solutions
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the sun does move around the galactic center. However relative to an imaginary observer
on the sun (ouch) the sun does not move.
1.7 Observe and Explain
a)
1. The observer is stationary on the dock and the 4 sail boats move from right to left
across the picture with the 4th one entering the picture in the last photograph.
2. The sail boats are stationary and the observer moves from left to right on a the dock.
b)
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•
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Student moving – sitting on the bicycle; Student not moving – running next to the
student or somehow be in the student’s backpack.
Street sign moving – Sitting in the police car
Police car moving right – standing on the sidewalk; Police car moving left – running
from the police car towards the student, faster than the police car is moving.
Bicycle moving – sitting on the train; Bicycle not moving – sitting on the bicycle
c) The running student will see the pick-up truck moving backwards because he is moving
in the same direction as the truck but faster than the truck. He would have to rotate his
head backwards to watch the truck.