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Concepts Of Physics MCQ Edition [Volume 1]

Exam: Concepts Of Physics MCQ Edition [Volume 1]
Subject: Physics
Chapter: Rest and Motion Kinematics

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Rest and Motion Kinematics

A driver takes 0.20 s to apply the brakes after seeing a need for...

A driver takes

0.20 \text{ s}

to apply the brakes after seeing a need for it. This is termed the reaction time of the driver. If he is driving a car at a speed of

54 \text{ km/h}

and the brakes cause a deceleration of

6.0 \text{ m/s}^2

, determine the distance travelled by the car after he sees the need to put the brakes on, in m.

Options

A $18.75$
B $21.75$
C $3.00$
D $253.80$

Correct answer

B. $21.75$

Step-by-step solution

Initial speed of the car,

u = 54 \text{ km/h} = 54 \times \dfrac{5}{18} \text{ m/s} = 15 \text{ m/s}

Reaction time of the driver,

t = 0.20 \text{ s}

Distance travelled during the reaction time,

s_1 = u \times t = 15 \times 0.20 = 3.0 \text{ m}

Deceleration of the car,

a = -6.0 \text{ m/s}^2

Final velocity,

v = 0

Using the equation of motion,

v^2 - u^2 = 2as_2

0 - (15)^2 = 2 \times (-6.0) \times s_2

s_2 = \dfrac{225}{12} = 18.75 \text{ m}

Total distance travelled by the car,

s = s_1 + s_2 = 3.0 + 18.75 = 21.75 \text{ m}

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