Question

If a body is moving with a velocity 40m/s and after 4 sec it stops. What will be its deceleration? 

a) -20
b) 20
c) 10
d) -10
​

Answer 1

★ Cᴏɴᴄᴇᴘᴛ :

According to the question, the body is moving with an initial velocity of 40 m/sec and after 4 sec it stops. Hence, the body is at rest so it's final velocity will be 0 m/sec. After which we need to find it's retardation.

For getting the retardation or negative acceleration we need to apply the first equation of motion. In which we will get acceleration's value and as we have considered retardation as negative acceleration so the opposite of acceleration will be taken

★ Sᴏʟᴜᴛɪᴏɴ :

❍ First Equation : v = u + at

Given Data :

Initial Velocity (u) = 40 m/sec

Final Velocity (v) = 0 m/sec

Time (t) = 4 sec

__________________________________

• v = u + at

• 0 = 40 + a(4)

• -40 = 4a

• a = -40/4

• a = -10

As the acceleration is -10 m/s²

So, Deacceleration = -(-10) = 10 m/s²

Henceforth, the correct answer is option C i.e. 10

Answer 2

$\large \dag$ Question :-

A car travelling at 67 km/h slows down to 33 km/h in 15 seconds find the retardation.

$\large \dag$ Answer :-

$\red\dashrightarrow\underline{\underline{\sf  \green{Deacceleration   \:  is \:  0.63\: m/s^2 }} }$

$\large \dag$ Step by step Explanation :-

Here we have :

Initial Velocity = u = 40 m/s

Final Velocity = v  = 0 m/s

Time = t = 4 s

Let Acceleration be = a m/s²

❒ We Have 1st Equation of Motion as :

$\large \bf \red\bigstar \: \: \orange{ \underbrace{ \underline{\blue{v = u + at}}}}$

⏩ Applying 1st Equation of Motion ;

$:\longmapsto \rm 0 = 40 + a \times 4$

$:\longmapsto \rm 4a+40 = 0$

$:\longmapsto \rm 4a =  - 40$

$:\longmapsto \rm a =  \cancel\frac{ - 40}{ \: 4}$

$\purple{ \large :\longmapsto  \underline {\boxed{{\bf a =  - 10} }}}$

Hence,

$\large\underline{\pink{\underline{\frak{\pmb{\text Acceleration=-10\: m/s^2 }}}}}$

Here negative sign denotes Deacceleration

Hence,

$\large\underline{\pink{\underline{\frak{\pmb{Deacceleration = 10 \: m/s^2 }}}}}$

Therefore,

$\large\green\dashrightarrow\blue{\underline{{\underline{\frak{\pmb{Option \:C\:is \: Correct }}}}}}$