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Q.A projectile is fired with velocity

(v) making an angle (θ) with horizontal:
(a) Show that its trajectory is parabola.
(b) Obtain expression for its horizontal range.
Himachal HpboseHPBOSE Himachal Pradesh Class 11 Board Exam 2021Subjective· 3mImportance★★★★★
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Eliminating time from the horizontal and vertical equations of motion of a projectile gives y in terms of x as y = x tan(theta) - (g/(2 v^2 cos^2(theta))) x^2 -- the equation of a parabola; setting y = 0 (landing point) gives the range R = v^2 sin(2 theta)/g.

A projectile is launched with initial speed v at angle theta to the horizontal. Taking the point of projection as the origin, with x horizontal and y vertical (upward positive):

Horizontal motion (no acceleration): x = (v cos theta) t, so t = x/(v cos theta)

Vertical motion (acceleration = -g): y = (v sin theta) t - (1/2) g t^2

  1. Trajectory is a parabola: Substituting t = x/(v cos theta) into the vertical equation: y = (v sin theta)[x/(v cos theta)] - (1/2)g[x/(v cos theta)]^2 y = x tan(theta) - [g/(2 v^2 cos^2(theta))] x^2 This equation is of the form y = Ax - Bx^2, where A = tan(theta) and B = g/(2 v^2 cos^2(theta)) are constants (for a given launch speed v and angle theta). An equation of the form y = Ax - Bx^2 represents a parabola. Hence the trajectory of a projectile is parabolic.
  2. Horizontal range: The range R is the horizontal distance covered when the projectile returns to the same height (y = 0) from which it was launched (other than the starting point x = 0). …

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