Calvin Kong’s 2-page Summary
UNIT 2 - Kinematics
1.
Basic Kinematics Quantities
Distance and Speed



Displacement, Velocity and Acceleration
Scalar quantities
Distance is the total length covered between two points.
Speed is the change in distance per unit time.
For objects moving at constant speed, its value it given
by:



Vector quantities
Displacement is distance moved in a specified direction
Velocity is the change in distance in a specified direction
(displacement) per unit time.
velocity 
distancecovered
speed
time taken

For objects moving at non-constant speed, the average
speed is given by:
average speed
total distancecovered
total time taken

SI Unit: m/s or ms-1
2.
Basic Kinematics Graphs
displacement from a fixed point
time taken
Acceleration is the change in velocity per unit time.
For constant acceleration, its value is given by:
acceleration 


change in velocity
time
i.e.
a
v -u
t
SI Unit: m/s2 or ms-2
Acceleration occurs when a body is
(a) Speeding up or
(b) Changing
slowing down
direction
Distance-time Graph
The gradient of distance-time graph gives the speed.
Distance / m

Decreasing
gradient means
decreasing speed
Horizontal line
means:
 gradient = 0
 speed = 0
 object is
stationary
Constant gradient
means constant
speed
Constant speed.
Gentler gradient
means lower speed
Increasing gradient
means increasing
speed (nonuniform speed)
Time / s
Speed-time Graph
The gradient of speed-time graph gives the acceleration.
Area under the speed-time is equivalent to the distance travelled.
Velocity / ms-1


Increasing
gradient means
increasing
acceleration
Constant
negative
gradient means
constant
deceleration
Constant
gradient means
constant
acceleration
Horizontal line
means:
 gradient = 0
 acceleration = 0
 object is at
constant velocity
Gentler
gradient means
lower
acceleration
t1
t2
At the peak:
 gradient = 0
 acceleration
=0
Horizontal line
at speed = 0
means the
object is
stationary
Time / s
Area under the graph between t1 and t2 gives
the distance travelled between t1 and t2.
Kong E K
Kinematics - 1
Physics for Secondary 3 Express
Calvin Kong’s 2-page Summary
3. Acceleration of Freefall


Free fall is defined as the motion of an object under the influence of gravity only. The only force acting on the object is its
own weight.
Air resistance tends to slow down a moving object. It has the following properties.
(a)
(b)
(c)
It always opposes the motion of objects.
It increases with the speed of the object.
It increases with cross-sectional area of the object.
Objects falling without Air Resistance
0 m/s
0 m/s
10 m/s
10 m/s
20 m/s
20 m/s
 In the absence of
air resistance, all
falling objects fall
with the same
constant
acceleration.
 This acceleration
is known as
acceleration of
freefall and is
determined to be
10 m/s2.
30 m/s
30 m/s
vacuum
tube
0 m/s
0 m/s
9.9 m/s
9.8 m/s
16 m/s
18 m/s
16 m/s
16 m/s
25 m/s
tube with
air
The feather has less weight is
subjected to greater air resistance
due to its larger cross sectional area.
As a result, it reaches a lower
terminal velocity is a shorter time.
Despite having different
masses, both coin and feather
will fall with the same
acceleration.
Speed-time graph of a free falling object
Speed / ms
Objects falling with Air Resistance
Speed-time graph of a falling object in air
-1
Speed / ms-1
50
50
40
40
30
30
20
20
10
10
0
 When the feather is released, it accelerates
under the influence of gravity.
 As it speeds up, the air resistance increases.
 This causes the resultant downward force acting
on the feather to decrease. As a result, the
acceleration decreases.
 Eventually, the magnitude of the upward air
resistance equals the magnitude of the
downward weight.
 This leads to the resultant force acting on the
feather to be zero. The acceleration also
becomes zero and the feather reaches it
maximum speed called the terminal velocity.
0
1
2
3
4
1
5
2
3
4
5
6
7
Time / s
Time / s
Kong E K
Kinematics - 2
Physics for Secondary 3 Express