Question

A truck starts from rest and accelerates uniformly at $2.0m{s}^{-2}$. At $t=10s$, a stone is dropped by a person standing on the top of the truck ($6m$ high from the ground). What are the (a) velocity, and (b) acceleration of the stone at $t=11s$ ? (Neglect air resistance.)

Answer 1

(a) Initial velocity of the truck, u = 0
Acceleration, a = 2 m/s${}^2$
Time, t = 10 s
As per the first equation of motion, final velocity is given as:
v = u + at
= 0 + 2 x 10 = 20 m/s
The final velocity of the truck and hence, of the stone is 20 m/s.
At t = 11 s, the horizontal component (vx) of velocity, in the absence of air resistance, remains unchanged, i.e.,
vx = 20 m/s
The vertical component (vy) of velocity of the stone is given by the first equation of motion as:
vy = u + ayt
Where, t = 11 - 10 = 1 s and ay = g = 10 m/s${}^2$
vy = 0 + 10 x 1 = 10 m/s
The resultant velocity (v) of the stone is given as:

v = (vx${}^2$ + vy${}^2$)${}^{1/2}$
= (20${}^2$ + 10${}^2$) ${}^{1/2}$
= 22.36 m/s
Let be the angle made by the resultant velocity with the horizontal component of velocity, vx be $\theta$

$tan\theta =vy/vx$

$\theta =ta{n}^{-1}\left(vy/vx\right)$

= tan${}^{-1}$ (10 / 20)

= 26.570
(b) When the stone is dropped from the truck, the horizontal force acting on it becomes zero. However, the stone continues to move under the influence of gravity. Hence, the acceleration of the stone is 10 m/s${}^2$ and it acts vertically downward.