Question

A body of mass $0.4$kg moving with a constant speed of $10$m/s to the north is subject to a constant force of $8$N direction toward the south for $30s$. Take the instant the force is applied to be $t=0$, the position of the body at the time to be $x=0$ , and predict its position at $t=-5s,25s,100s$.

Answer 1

Given, $Mass=0.4kg,force=8N,acceleration=20m/s^2$

Now, Assume the direction from south to north is positively at the instant the force act as,

$u=10m/s,a=-20m/s^2.X_0=0$

$X=X_0+ut+0.5a{t}^2$

a) Before $t=0s$ when the force was not acting,

$0=x+ut\Rightarrow 0=x+10\times 5\Rightarrow x=-50m$

So, particle was $50m$ south of $x=0,t=-5s$

b) $t=25s\Rightarrow X=10\times 25-0.520\times {25}^2=-6000m$

So, particle was $-6000m$ south of $x=0,t=25s$

c)$t=100s\Rightarrow X=10\times 100-0.5\times 20\times {100}^2=-99000m$

So, particle was $99000m$ south of $x=0,t=100s$