Question:

Three particles A B & C start from the origin at the same time; A with U velocity 'a' along x - axis, B with a velocity 'b' along y-axis and C with velocity 'c' in XY plane along the line x = y. The magnitude of 'c' so that the three always remain collinear is :

Updated On: Nov 13, 2023
  • $\frac{a+y}{2}$
  • $\sqrt{ab}$
  • $\frac{ab}{a+b}$
  • $\frac{\sqrt{2}ab}{a+b}$
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The Correct Option is D

Solution and Explanation

comparing the slopes $\frac{b - \frac{c}{\sqrt{2}}}{0 - \frac{c}{\sqrt{2}}} = \frac{b - 0}{0 - a}$ $\frac{b - \frac{c}{\sqrt{2}}}{ \frac{c}{\sqrt{2}}} = \frac{b }{a} = ab - \frac{ac}{\sqrt{2}} = \frac{bc}{\sqrt{2}}$ $\therefore \, ab = \frac{c (a + b)}{\sqrt{2}}$ $c = \frac{\sqrt{2} ab}{a + b}$
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Concepts Used:

Motion in a Plane

It is a vector quantity. A vector quantity is a quantity having both magnitude and direction. Speed is a scalar quantity and it is a quantity having a magnitude only. Motion in a plane is also known as motion in two dimensions. 

Equations of Plane Motion

The equations of motion in a straight line are:

v=u+at

s=ut+½ at2

v2-u2=2as

Where,

  • v = final velocity of the particle
  • u = initial velocity of the particle
  • s = displacement of the particle
  • a = acceleration of the particle
  • t = the time interval in which the particle is in consideration