Question

The velocity of a particle at an instant is $10\mathrm{m}/\mathrm{s}$. After $5sec$, the velocity of the particle is $20\mathrm{m}/\mathrm{s}$. The velocity at $3seconds$ before from the instant when the velocity of a particle is $10\mathrm{m}/\mathrm{s}$.

• A. $8\mathrm{m}/\mathrm{s}$
• B. $4\mathrm{m}/\mathrm{s}$
• C. $6\mathrm{m}/\mathrm{s}$
• D. $7\mathrm{m}/\mathrm{s}$

Answer 1

The correct option is B

$4\mathrm{m}/\mathrm{s}$

Step 1:- Given data

The initial velocity of the particle $u=10m/s$

The final velocity of the particle $v=20\mathrm{m}/\mathrm{s}$

Time $t=5\sec$

Find the acceleration $a=?$

Step 2:- Acceleration

We know that, acceleration $\left(a\right)=\frac{v-u}{t}$
$a=\frac{20-10}{5}=2{\mathrm{ms}}^{-2}$

The velocity at $3seconds$ before
Using the formula $v_1=u_1+at$,

Substitute the value in the above equation we get,

$10=u_1+2\times 3$ or $u_1=4\mathrm{m}/\sec$.

Thus, the velocity $3seconds$ before was $4m/s$.

Hence, option B is the correct answer.