Question

At time t = 0 a particle starts travelling from a height $7\hat z cm$ in a plane keeping $z$ coordinate constant. At any instant time it's position along the $x$ and $y$ directions are defined as $3t$ and $5t^3$ respectively. At $t = 1s$ acceleration of the particle will be

• A. $-30 \hat {y}$
• B. $30\hat {y}$
• C. $3 \hat {x}+ 15\hat {y}$
• D. $3\hat{x}^+15\hat{y}+7\hat{z}$

Answer 1

Answer: (B)

$30\hat {y}$

Answer 2

$\vec r=3t\hat {i}+5t^3\hat{j}+7\hat{k}$

$\frac{d^2\vec r}{dt^2}=30t\hat{j}$ At $t=1$

⇒ $\frac{d^2\vec r}{dt^2}$= $30\hat{j}$