First slide

Second law

Question

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

Moderate

A

12 s
B

14 s
C

10 s
D

15 s

Solution

$\vec{a} = \frac{F}{m} = \frac{- 3}{5} \hat{i} + \frac{4}{5} \hat{j} m / s^{2}$
Velocity is along x-axis when y-component of velocity becomes zero.
$v_{y} = u_{y} + a_{y} t 0 = (- 12) + (4 / 5) t 12 = 4 / 5 t ∴ t = \frac{12 \times 5}{4} = 15 s$

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