Page 6 - Pra U STPM 2021 Penggal 1 - Physics

Physics Term 1 STPM Chapter 2 Kinematics

Example 2

The table below is from the handbook of a car. On a dry road, the car driven by an alert driver will
stop in distances as shown below.

Speed/ Thinking Braking Overall stopping
m s –1 distance/m distance/m distance/m
5.0 3.0 1.9 4.9 2
10.0 6.0 7.5 13.5
15.0 9.0 17.0 26.0
20.0 12.0 30.0 42.0
25.0 15.0 47.0 62.0
30.0 18.0 68.0 86.0
35.0 21.0 92.0 113.00

The thinking distance is the distance travelled by the car during the driver’s reaction time. The
braking distance is the distance travelled by the car before the car is stopped when the brakes are
applied.
(a) Explain why the thinking distance is directly proportional to the speed, whereas the braking
distance is not. State the relationship between the braking distance and the speed.
(b) What is the value of deceleration used in calculating the braking distance?
–1
(c) Calculate the overall stopping distance for a car travelling at 40 m s .
Solution:
(a) During the reaction time, t of the driver, the car travels at a constant speed, u.
Hence, thinking distance, s 1 = ut
s 1 ∝ u
The reaction time t of a driver is constant,
The final speed after the brakes are applied = 0
2
If deceleration = a, using v = u + 2as
2
0 = u – 2as 2
2
Braking distance, s 2 = u 2 ∝ u 2
2a
u 2
(b) Using braking distance, s 2 =
2a
–1
When u = 10 m s , s 2 = 7.5 m
u 2
Deceleration, a =
2s
10 2
=
2 × 7.5
= 6.67 m s –2
(c) When u = 40 m s –1 2
u
Thinking distance, s 1 = 40 × Reaction time Braking distance, s 2 = 2a
40
2
3.0
= 40 × (5.0) = 2 × 6.67
= 24 m = 120 m
Overall distance travelled = (24 + 120) m = 144 m

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02 STPM PHY T1.indd 35 4/9/18 8:19 AM