NCERT Solutions
Class 11 Physics
Motion in a Plane

Q. 3.18
A particle starts from the origin at t = 0 s with a velocity of 10.0 ĵ m/s and moves in the x-y plane with a constant acceleration of (8.0 î + 2.0 ĵ) m s-2.
(a) At what time is the x- coordinates of the particle 16 m? What is the y- coordinate of the particle at that time?
(b) What is the speed of the particle at the time?
Given:-
Velocity of the particle v = 10.0m/s î
Acceleration of the particle a = (8.0 î + 2.0 ĵ) m s-2.
a = d (v)/dt = (8.0 î + 2.0 ĵ) m s-2
Also,
But, a = (dv/dt) = (8.0 î + 2.0 ĵ)
dv = (8.0 î + 2.0 ĵ)
Integrating both sides:
v(t) = (8.0 î + 2.0 ĵ + u)
Where,
Velocity vector of the particle at t = 0 = u
Velocity vector of the particle at time t = v
But, v = (dr/dt)
dr= v dt = (8.0 î + 2.0 ĵ + u)dt
Integrating the equations with the conditions: at t = 0; r = 0 and at t = t; r = r
But,
r = ut + (1/2)8.0t2 î + (1/2) x 2.0t2 ĵ
= ut + 4.0 t2 î + t2 ĵ
= (10.0 ĵ) t + 4.0 t2 î + t2 ĵ
(x î + y ĵ) = (4.0 t2 î + (10t+ t2) ĵ
Since the motion of the particle is confined to the x-y plane, on equating the coefficients of î and ĵ, we get:
x = 4t2
t =(x/4)1/2
And y = 10t + t2
When x = 16 m:
t= (16/4)1/2 =2s
Therefore, y = 10 × 2 + (2)2 = 24 m
b) Velocity of the particle is given by:
v (t) = (8.0 î + 2.0t ĵ + u)
At t=2s
v (t) = 8.0 x2 î + 2.0 x2 ĵ + 10 ĵ
= (16 î + 14 ĵ)
Therefore, speed of the particle;
|v| = √ (16)2 + (14)2
=√ (256 + 196) = √ (452)
=21.26m/s