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