Ncert Exercises & Solutions

A body with a mass of \(5\) kg is acted upon by a force \(\vec{F}=\left ( -3\hat{i} +4\hat{j}\right )\) N. If its initial velocity at \(t=0\) is \(\vec{v}=\left ( 6\hat{i} -12\hat{j}\right )\) m/s, the time at which it will just have a velocity along the Y-axis is:
1. never
2. \(10\) s
3. \(2\) s
4. \(15\) s

(b)

Hint: The final velocity along the x-axis is equal to zero.

Step 1: Find the acceleration of the particle.

Given, mass = 5 kg

\begin{array}{l} Acting force = \vec{F} = (- 3 \hat{i} + 4 \hat{j}) N \\ Initial velocity at t = 0, \vec{u} = (6 \hat{i} - 12 \hat{j}) m / s \\ Acceleration, \vec{a} = \frac{\vec{F}}{m} = (- \frac{3 \hat{i}}{5} + \frac{4 \hat{j}}{5}) m / s^{2} \end{array}

Step 2: Put the final velocity along the x-axis equal to zero.

As final velocity is along Y-axis only, its x-component must be zero.

From v = u + at. As v_x = 0,

\Rightarrow 0 = 6 \hat{i} - \frac{3 \hat{i}}{5} t

\Rightarrow t = \frac{5 \times 6}{3} = 10 s

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