First slide

Second law

Question

A body with mass 5kg is acted upon by a force $f = (- 3 \hat{i} + 4 \hat{j}) N$ . If its initial velocity at t=0, $v = (6 \hat{i} - 12 \hat{j}) {ms}^{- 1}$ , the time at which it will just have a velocity along the y axis is

Moderate

A

15 sec
B

10 sec
C

2 sec
D

Never

Solution

When the body has velocity along y-axis its velocity along the X-axis becomes zero then V_x = 0
$\begin{aligned} V_{x} & = u_{x} + a_{x} t \\ = u_{x} + \frac{F_{x}}{m} t \\ 0 & = 6 \hat{i} - \frac{3 \hat{i}}{5} t \\ \Rightarrow & 3 t = 30 \\ \Rightarrow & t = 10 \sec \end{aligned}$

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