Kinematics
concepts
3UB PHYSICS
Average Speed twist
A
car travels 1.0 km through a town at 50.0 km/h and then
drives 1.0 km through the country at 100.0 km/h. What is
the average speed of this car?
v
= Δd/Δt
=
total distance/total time
We
need the time for each section of the motion!
Average Speed twist
t1 = d1/v1
= 1.0 km/50.0 km/h
= 0.020h
t2 = d2/v2
= 1.0 km/100.0km/h
= 0.010h
Average Speed twist
v = (1.0 + 1.0)km/(0.020 + 0.010)h
= 67 km/h
Note:
This average speed is NOT the average of 100 km/h
and 50 km/h. Why?
Graphing this question
The
car drove 1.0 km at 50 km/h in 0.20 h and then 1.0
km at 100 km/h for 0.10 h. The average speed we
found was 67 km/h. The two motions are each UM:
UM question 1
The
following graph shows the Northward motion of
a wolf tracking a biology teacher.
UM question 1
A)
What is the wolf’s position at 4.0 s?
b)
What is the wolf’s displacement
between 2.0 s and 8.0 s?
c)
What is the velocity of the wolf?
UM question 1
a)
4.0 m [N]
b)
Δd
=df-di
=
8.0 m [N] – 2.0 m [N]
=
6.0 m [N]
c)
v
= Δd/Δt
=(10m[N]
=
– 0)/(10s – 0)
1.0m/s [N
UM question 2
The
motion of a biology teacher heading North during
migration season. The origin represents a town he
passes through.
UM question 2
a)
What is the total displacement?
b)
What is the total distance traveled?
c)
What is the total displacement from the town?
d)
At what time does the migrating dude reach the
town?
e)
f)
Where does he start?
What is his average velocity?
UM question 2
a)
=
Δd = df-di
60 km – (-40 km) = 100 km [N]
b)
Δd = 100 km
c)
Δd from town = 60 km [N]
d)
Δt = 4 h
e)
He starts 40 km [S] of town.
f)
v = Δd/Δt = (60 km – 0)/(10 h – 4 h)
=
10 km/h [N]
UM Problem 3
An
aimless chemistry teacher wanders as:
UM Problem 3
Notice
that we must label displacement rather
than distance; why?
The
slopes for each line segment, yields the
velocity for that time interval. Find each.
Describe
What
the motion of the above graph.
section shows the highest velocity?
Which
section shows the greatest displacement?
Find Δd for each section.
What
is the total displacement of the object?
Displacement and distance from v-t
graphs.
For
motion comprised of UM and UAM, we will need to
find the total distance and displacement by adding the
areas up.
Displacement and distance from v-t
graphs.
The
total distance is found by adding areas.
The total displacement is found by adding areas above the
x-axis and adding negative areas below the x-axis.
Break up the graph into a series of triangles and rectangles
to find the distance or displacement at certain times. (Useful
info if asked to plot a d-t graph for the same motion!)
In this question, find: distance, displacement, speed and
velocity.
Sketch the distance and displacement versus time graphs
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