Question

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

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

Answer 1

Answer: (A)

Here,

Mass of the particle, $m=0.4kg$

$F=-8N$ (minus sign for direction of force)

$\therefore$ Acceleration, $a=a=\frac{F}{m}$

$=\frac{-8N}{0.4kg}$

$=-20m{s}^{-2}$

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 $t=25s$

Here, $u=10m{s}^{-1},a=-20m{s}^{-2},t=25s$

$x=10\times 25+\frac{1}{2}(-20)(25{)}^2$

$=250-6250$

$=-6000m$