Question

A body moves with velocity 72km/h and after 5sec it takes brake then find the retardation?

Answer 1

Answer

• Retardation = 4 m/s²

Given

• A body moves with velocity 72 km/h and after 5 sec it takes brake and goes to rest

To Find

• The retardation

Concept Used

• 1 st equation of motion

• v = u + at

• Acceleration and retardation are both same but opposite in direction

Solution

Initial velocity , u = 72 km/h = 72 × ⁵/₁₈ m/s

⇒ u = 20 m/s

Time , t = 5 s

Final velocity , v = 0 m/s

[ ∵ Goes to rest after applying brakes]

Acceleration , a = ? m/s²

Apply 1st equation of motion .

⇒ v = u + at

⇒ 0 = 20 + a(5)

⇒ 5a = - 20

⇒ a = - 4 m/s²

[ - ve sign of acceleration denotes retardation or deceleration ]

More Info

$\boxed{\begin{minipage}{4.5cm} \underbrace{\bf Equations\ of\ motion}\ :\\\\\rm \bigstar \;\; v=u+at \\\\\rm \bigstar \;\; s=ut+\dfrac{1}{2}at^2\\\\\rm \bigstar \;\; v^2-u^2=2as\\\\\rm \bigstar \;\; S_n=u+\dfrac{a}{2}[2n-1] \end{minipage}}$

Answer 2

• Given -

• Body moves with velocity of 72km/h & after 5 sec takes brake.

• To Find -

• The Retardation (Acceleration)

• Solution -

Here,

We have to do some conversions of given intervals,

★ Initial Velocity (u) - 20m/s

$⟹</strong><strong>u</strong><strong> </strong><strong>=</strong><strong>72 \times \frac{5}{18}$

$⟹u = 20m</strong><strong>/</strong><strong>s$

★ Time - 5sec

★ Final Velocity (v) - 0m/s .. (Brakes are applied. No motion)

Placing these values in 1st eqn of motion,

★ v = u + at

Placing values according to given Conditions,

➱ 0 = 20 + a (5)

➱ 5a = 0 - 20

➱ 5a = - 20

➱ a = -20/5

➱ a = -4 m/s²

⛬ Retardation of body is -4 m/s²