Page 7 - Pra U STPM 2021 Penggal 1 - Physics

Physics Term 1 STPM Chapter 2 Kinematics

Quick Check 1

1. A car starts from rest and accelerates at 2. A car decelerates uniformly from 30 m s to
–1
2.0 m s . 15 m s in a distance of 75 m. Calculate the
–2
–1
(a) Find its speed at (i) 1.0 s, (ii) 2.0 s and further distance travelled before the car comes
(iii) 5.0 s. to a stop.
2 (b) What is the average speed in (i) the first
second, (ii) 2.0 s and (iii) 5.0 s? 3. A car accelerates through three gear changes
(c) Calculate the distance travelled in with the following speed:
–1
(i) 1.0 s, (ii) 2.0 s and (iii) 5.0 s. 20 m s for 2.0 s
40 m s for 2.0 s
–1
60 m s for 6.0 s
–1
What is the overall average speed of the car?
Motion Under Gravity 2009/P1/Q2, 2011/P2/Q1, 2017/P1/Q2

1. When a body is released from rest, it falls towards the earth with an acceleration.

2. In free space where there is no air resistance, all objects irrespective of their mass, fall with the same
acceleration, known as the acceleration of free fall or acceleration due to gravity, g.

3. Close to the earth, the value of g can be as assumed constant. In this book, the value of g is assumed
–2
to be 9.81 m s unless otherwise stated.
4. Since the value of g is constant, equations of motion under uniform acceleration such as:
1
2
v = u + at, s = ut + at and v = u + 2as
2
2
2
can be used for motion under gravity by substituting the acceleration a as +g, or as –g where applicable.
5. When the body is only falling downwards, it may be easier to use a = +g, since the direction of
motion is in the direction of g, that is downwards.
6. When the body is projected vertically and upwards, if it is assumed that the velocity of the body
is positive, it implies that the upwards direction is assumed positive. Since the direction of g is
downwards, hence acceleration a = –g.
7. When the body moves in both directions, upwards then falls downwards or vice versa, it would be
better to assume the upwards direction as positive. Hence, the acceleration a = –g.
Upwards journey Downwards journey
Displacement : + E E Displacement : +
Velocity : + F Velocity : –
Acceleration : a = –g D Acceleration : a = –g
G
C
B H

A Below the level AI
I
Displacement : –
Velocity : –
J Acceleration : a = –g
Free Fall
INFO Figure 2.3

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