Question

A horse runs along a circle with a speed of $20$ km/hr. A lantern is at the centre of the circle. A fence is along the tangent to the circle at the point at which the horse starts. The speed with which the shadow of the horse move along the fence at the moment when it covers $1/8$ of the circle in km/hr is

• A. $20$
• B. $60$
• C. $30$
• D. $40$

Answer 1

Answer: (D)

$40$

Angle when 1/8 circle covered = $\theta =\frac{\pi }{4}$
speed = $v=r\frac{d\theta }{dt}=20$
Point of shadow = $rtan\theta$
Speed of shadow = $rse{c}^2\theta \frac{d\theta }{dt}$
= $vse{c}^2\theta$
= $20\times se{c}^2\pi /4$
$=40km/hr$