Question

19. A car decreases its speed from 60 km/hour to 40 km/hour in 5 s.
(i) Find the initial speed in m/s.
(ii) Find the final speed in m/s.
(iii) Write the equation to calculate the acceleration. (iv) Find the acceleration of the car.
(v) What is retardation?​

Answer 1

Explanation:

• Initial speed (u) = 60 km/h
• Final speed(v) = 40 km/h
• Time (t) = 5 sec

(i)

$\implies\sf{Initial\: speed=60\:km/h}$

$\implies\sf{Initial\: speed=60\times\dfrac{5}{18}\:m/s}$

$\implies\sf{Initial\: speed= 16.67\:m/s}$

Hence, the initial speed will be 16.67 m/s.

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(ii)

$\implies\sf{Final\: speed=40\:km/h}$

$\implies\sf{Final\: speed=40\times\dfrac{5}{18}\:m/s}$

$\implies\sf{Final\: speed= 11.11\:m/s}$

Hence, the Final speed will be 11.11 m/s.

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(iii)

The equation to calculate the aceleration,

$\longrightarrow\sf{v=u-at}$

[ Reason : Initial velocity is greater than final velocity. So, acceleration will be negative.]

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(iv)

We know that,

${\boxed{\sf{v=u-at}}}$

→ 11.11 = 16.67-a×5

→ 11.11 = 16.67 - 5a

→ 5a = 16.67-11.11

→ 5a = 5.56

→ a = 1.11

Hence, the acceleration will be 1.11 m/s².

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(v)

Retardation :

Retardation means negative acceleration.

Answer 2

Answer:

Given :-

Initial speed (u) = 60 km/h

Final speed(v) = 40 km/h

Time (t) = 5 sec

Solution :-

1]

Initial Speed (m/s) = 60 × 5/18

= 300/18

= 16.67 m/s

2]

Final speed (m/s) = 40 × 5/18

= 200/18

= 11.11 m/s

3]

The equation will be

v = u - at

4]

11.11 = 16.67 - a(5)

11.11 = 16.67 - 5a

5a = 16.67-11.11

5a = 5.56

a = 5.56/5

a = 1.11 m/s²

5]

Sometimes acceleration comes in negative. It is known as retardation.