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# Solution - A Fighter Plane Flying Horizontally at an Altitude of 1.5 Km with Speed 720 Km/H Passes Directly Overhead an Anti-aircraft Gun. at What Angle from the Vertical Should the Gun Be Fired for the Shell with Muzzle Speed 600 M S–1 To Hit the Plane? at What Minimum Altitude Should the Pilot Fly the Plane to Avoid Being Hit - CBSE (Science) Class 11 - Physics

ConceptRelative Velocity in Two Dimensions

#### Question

A fighter plane flying horizontally at an altitude of 1.5 km with speed 720 km/h passes directly overhead an anti-aircraft gun. At what angle from the vertical should the gun be fired for the shell with muzzle speed 600 m s–1 to hit the plane? At what minimum altitude should the pilot fly the plane to avoid being hit? (Take = 10 m s2).

#### Solution 1

Height of the fighter plane = 1.5 km = 1500 m

Speed of the fighter plane, = 720 km/h = 200 m/s

Let θ be the angle with the vertical so that the shell hits the plane. The situation is shown in the given figure.

Muzzle velocity of the gun, u = 600 m/s

Time taken by the shell to hit the plane = t

Horizontal distance travelled by the shell = uxt

Distance travelled by the plane = vt

The shell hits the plane. Hence, these two distances must be equal.

uxt = vt

u sin theta  = v

sin theta = v/u

= 200/600 = 1/3 = 0.33

theta = sin^(-1)(0.33)

= 19.5^@

In order to avoid being hit by the shell, the pilot must fly the plane at an altitude (H) higher than the maximum height achieved by the shell.

:.H = (u^2sin^2(90-theta))/"2g"

=((600)^2cos^2theta)/(2g)

= (360000xxcos^2 19.5)/(2xx10)

 =18000xx(0.943)^2

=16006.482 m

=16 km

#### Solution 2

Velocity of plane, vp=720 x 5/180 ms-1=200 ms-1

Velocity of shell = 600 ms^(-1)

sin theta = 200/600 =    1/3

or theta = sin^(-1)(1/3)  = 19.47^@

This angle is with the verticle.

Let h be the required minimum height

Using equation

v^2 - u^2 = 2as we get

(0)^2-(600 costheta)^2 = -2xx10 xxh

or h = (600xx600(1-sin^2theta))/20

=30xx600(1-1/9) = 8/9xx30xx600m = 16 km

Is there an error in this question or solution?

#### APPEARS IN

NCERT Physics Textbook for Class 11 Part 1 (with solutions)
Chapter 4: Motion in a Plane
Q: 30 | Page no. 88

#### Reference Material

Solution for question: A Fighter Plane Flying Horizontally at an Altitude of 1.5 Km with Speed 720 Km/H Passes Directly Overhead an Anti-aircraft Gun. at What Angle from the Vertical Should the Gun Be Fired for the Shell with Muzzle Speed 600 M S–1 To Hit the Plane? at What Minimum Altitude Should the Pilot Fly the Plane to Avoid Being Hit concept: Relative Velocity in Two Dimensions. For the courses CBSE (Science), CBSE (Arts), CBSE (Commerce)
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