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Physics Form 4 Chapter 2 Force and Motion I

Important Safety Features of a Car
CONTOH 2.23
1. While striving to improve the performance of
A billiard ball with a mass of 0.25 kg is resting on their cars, car manufacturers also place great
a smooth table. The ball is then given a horizontal importance on the safety features of the cars. A
impulse of 3.0 N s. What is the velocity of the ball good car safety system that can prevent injuries
after the hit? to passengers in the event of an accident is of
Solution utmost importance.
Initial velocity of the ball, u = 0 m s –1 (a) Air bags
Air bags inflate within one hundredth

Velocity of the ball after being hit = v m s –1 second upon impact to cushion passengers
Mass of the ball, m = 0.25 kg from direct impact with the steering wheel,
Impulse = Ft = 3.0 N s dashboard, or windscreen of the car. For
Since Ft = mv – mu safety protection by the air bags, the Chapter
Therefore, 3.0 = 0.25v passengers must also be wearing safety

Hence, v = 3.0 belts at the same time. 2
0.25 (b) Safety belts
= 12 m s –1 The inertia-reel safety belts lock to keep
wearers tightly held to their seats when a
car decelerates rapidly. The passengers are
prevented from being thrown forward due
EXAMPLE 2.24 to their inertia.
(c) Side-impact bar
A 60 kg resident jumps from a burning building.
Before hitting the ground his velocity is 5.0 m s . Strong metal steel bars inside the door
–1
(a) Calculate the impulse when his legs hit the frame provide passengers with extra
ground. protection in the event of a side impact.
(b) What is the impulsive force if he bends his leg (d) Anti-lock braking system (ABS)
and his velocity changes from 5 m s to zero The anti-lock braking system consists
–1
in 0.5 s? of a computerised system that rapidly
(c) What is the impulsive force if he does not bend holds and releases the brake alternately.
–1
his leg and his velocity changes from 5 m s It prevents the wheels from locking when
to zero in 0.1 s? the brake is applied and held by the driver.
(d) What is the advantage of bending his leg? This prevents the car from skidding.
Solution (e) Passenger safety cell
(a) Impulse = Change of momentum The passenger safety cell is a strong rigid
= mv – mu steel cage that will prevent the roof from
= (60 × 0) – (60 × 5) collapsing on the passengers in the event
= –300 N s that the car overturns. The passengers
will be protected from direct impact of
(b) Impulse = Ft = –300 N s
–300 external forces.
Impulsive force, F = 0.5 = –600 N (f) Safety glass
–300 The windscreen glass is specifically
(c) Impulsive fprce, F = 0.1 = –3 000 N designed to fracture into small rounded
(d) Reduces the impulsive force and hence, reduces pieces upon impact instead of shattering
injury. so that the passengers will not be cut by
the pieces that easily.

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