Mechanics
Topic 2.1 Kinematics _Uniform Motion
Mechanics
The branch of physics involving the motion of
an object and the relationship between that
motion and other physics concepts
Kinematics is a part of mechanics that
studies motion in relationship to time.
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In kinematics, you are interested in the
description of motion
Not concerned with the cause of the motion
(Dynamics)
Quantities in Motion
Any motion involves three concepts
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Displacement
Velocity
Acceleration
These concepts can be used to study
objects in motion and called the
parameters of the motion.
Displacement
Defined as the change in
position of a body when it
moves from an initial
position to a final position
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x  xf  xi
 f stands for final and i stands
for initial
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May be represented as s
or y if vertical
Units are meters (m) in SI.
Vector quantity
Displacement
Displacement is a
vector quantity.
Needs both
magnitude and
direction to be
completely described
Generally denoted
by:
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boldfaced and
an arrow over the
letter
+ or – sign is also
used if motion is
along one direction:
∆s = +70, ∆s =-60
Displacement Versus Distance
The displacement of an object is not
the same as the distance it travels
Example:
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In the adjacent
picture, what is
the displacement?
What is the
approximate
distance traveled?
Kinematic Concepts- Speed
Is the rate at which distance is covered
by an object.
Or the distance covered per unit time
Scalar quantity
Two types of speed:
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Average speed
Instantaneous speed
Average vs Instantaneous Speed
Average Speed is the ratio of the total
distance (d) covered by the object to the
time interval (t) over which the distance
was covered.
Average Speed (v) = total distance (d)
total time (t)
Average speed does not tell us about the
variation in the speed throughout the
journey.
Instantaneous Speed is the speed at a
specific instant of time.
Be Knowledgeable!
Distinguish between Instantaneous and
average speed. (May 2012- 2 marks)
average speed is the speed over a
period of time/distance;
instantaneous speed is the speed at a
particular instant in time/point in space;
Kinematic Concepts- Velocity
Is the rate at which displacement is covered
Velocity is a vector quantity
Average vs instantaneous velocity
The average velocity is the ratio of the total
displacement (∆s) covered by the object to the
time interval (∆t) over which the displacement
was covered.
Average velocity=
= total displacement =
total time
∆𝑠
∆𝑡
Instantaneous Velocity
Is the velocity at any one instant
v = s
t

Where t is tending towards zero
The instantaneous velocity of an object at
point P is its velocity at the instant it
passes P and is the average velocity of
the object calculated over a infinitely
small (but not zero) distance around P.
Speed vs. Velocity
Cars on both paths have the same average velocity
since they had the same displacement in the same
time interval
The car on the blue path will have a greater average
speed since the distance it traveled is larger
Acceleration
When the velocity of an object changes, we
say that it accelerates
Velocity can change by:
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Changing its magnitude, and/or
Changing its direction
Acceleration is the rate at which velocity is
changing or change in velocity per unit time
Acceleration is a vector quantity
SI Unit is ms-²
v vf  vi
a

t
tf  ti
u and vo are also used for vi
More about Acceleration
v vf  vi
a

t
tf  ti
The above equation refers to average acceleration
that takes place over a period of time
Instantaneous acceleration refers to a specific instant
of time when the time interval ∆t tends to zero.
Uniform Motion
Constant velocity
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Constant direction, and
Constant speed
Acceleration is zero (Velocity shown by red
arrows maintaining the same size)
Graphical analysis
What information can we get from the ticker tape and from the
graphs?
Graphical analysis
What information can we get from the ticker tape and from the
graphs?
Graphing Motion –Uniform
Motion in the positive sense
Uniform motion in the positive sense
Acc = 0
Velocity is positive and constant= 12 (unit)
Position-time graph is a straight increasing line whose
slope is equal to the velocity (12)
Area under the velocity-time graph gives the
displacement of the object (Area = 12 * 15 = 180)
Graphing Motion –Uniform
Motion in the negative sense
Uniform motion in the negative sense
Acc = 0
Velocity is negative and constant ( -12). The negative sign
means that the object is moving in the negative sense.
Position-time graph is a straight decreasing line whose slope
is equal to the velocity ( -12)
Area under the velocity-time graph gives the displacement of
the object (-12 * 15 = -180)
Notions about Uniform motion
Graphs
Uniform motion Displacement-time graph:
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the graph is a straight line
Gradiant is the uniform velocity
 Positive gradient: object moving forward
 Negative gradient: object moving backwards
Uniform motion Velocity-time graph:
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the graph is straight horizontal line
Area under graph gives the change in
displacement
 Positive area: positive displacement
 Negative area: negative displacement
Be a Thinker!
1)
The above graph shows four different phases of motion. For each phase:
a. Identify the type of motion and direction
b. Find the velocity
c. Draw the velocity-time graph
2) For the whole journey, determine:
1) the total displacement and total distance.
2) the average velocity and average speed.
Uniform Motion Equation
x = x0 + vt
Or
s = s0 + vt