Question

A body of mass 5 kg starts from the origin with an initial velocity $\overrightarrow{u} = (30\widehat{i} + 40\widehat{j})ms^{- 1}$. If constant force $( - 6\widehat{i} - 5\widehat{j})N$acts on the body, the time in which the y-component of the velocity becomes zero is :

• A. 5 s
• B. 20 s
• C. 40 s
• D. 80 s

Answer 1

Answer: (C)

40 s

Answer 2

Here , $m = 5kg$

$\overset{\rightarrow}{u} = (30\overset{\land}{i} + 40\overset{\land}{j})ms^{- 1}and\overset{\rightarrow}{F} = ( - 6\overset{\land}{i} - 5\overset{\land}{j})N$

$\therefore u_{y} = 40ms^{- 1}andF_{y} = - 5N$

$a_{y} = \frac{F_{y}}{m} = \frac{- 5}{5} = - 1ms^{- 2}$

$v_{y} = 0$

As $v_{y} = u_{y} + a_{y}t$ $$\therefore 0 = 40 - 1 \times t$$

Or $t = 40s$