Question

A body of mass 0.4 kg starting at an origin at t=0 with a speed of $10m{s}^{-1}$ in the positive x-axis direction is subjected to a constant F=8 N towards negative x-axis. The position of the body after 25 s is:

• A. $-6000m$
• B. $-8000m$
• C. $+4000m$
• D. $+7000m$

Answer 1

The correct option is A $-6000m$

Here, Mass of the particle, $m=0.4kg$
$F=-8N$ (minus sign for direction of force)
$\therefore$ Acceleration, $a=\frac{F}{m}=\frac{-8N}{0.4kg}=-20m{s}^{-2}$
The position of the body at any time t is given by
$x=x_0+ut+\frac{1}{2}a{t}^2$
The position of the body at t=0 is 0, therefore $x_0=0$
$\therefore x=ut+\frac{1}{2}a{t}^2$
Position of the body at t=25 s
Here, $u=10m{s}^{-1},a=-20m{s}^{-2},t=25s$
$\therefore x=10\times 25+\frac{1}{2}(-20)(25{)}^2$ $=250-6250=-6000m$