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Motion in A Plane question

2023 · 31 Jan · Shift 2 · Q66
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  5. /2023 · 31 Jan · Shift 2 · Q66

Motion in A Plane question

2023 · 31 Jan · Shift 2 · Q66

JEE MainPhysicsMotion in A PlaneNumerical+4 / −1
Two bodies are projected from ground with same speeds 40 ms−140 \mathrm{~ms}^{-1}40 ms−1 at two different angles with respect to horizontal. The bodies were found to have same range. If one of the body was projected at an angle of 60∘60^{\circ}60∘, with horizontal then sum of the maximum heights, attained by the two projectiles, is m\mathrm{m}m. (Given g=10 ms−2\mathrm{g}=10 \mathrm{~ms}^{-2}g=10 ms−2 )
Numerical answer
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Correct answer: 80

  1. For a projectile with speed uuu and angle of projection θ\thetaθ, the horizontal range is R=u2sin⁡2θg.R=\frac{u^2\sin 2\theta}{g}.R=gu2sin2θ​.

  2. Since both bodies have the same speed and the same range, sin⁡2θ1=sin⁡2θ2.\sin 2\theta_1 = \sin 2\theta_2.sin2θ1​=sin2θ2​. One angle is given as θ1=60∘.\theta_1=60^\circ.θ1​=60∘. So, sin⁡2θ1=sin⁡120∘=sin⁡60∘.\sin 2\theta_1 = \sin 120^\circ = \sin 60^\circ.sin2θ1​=sin120∘=sin60∘. For projectiles with the same speed and same range, the two projection angles are complementary: θ2=90∘−θ1=30∘.\theta_2 = 90^\circ-\theta_1 = 30^\circ.θ2​=90∘−θ1​=30∘.

  3. Maximum height of a projectile is H=u2sin⁡2θ2g.H=\frac{u^2\sin^2\theta}{2g}.H=2gu2sin2θ​. Given u=40 m/su=40\ \text{m/s}u=40 m/s and g=10 m/s2g=10\ \text{m/s}^2g=10 m/s2, H=\frac{40^2\sin^2\theta}{2\cdot 10}= rac{1600\sin^2\theta}{20}=80\sin^2\theta.

  4. For θ=60∘\theta=60^\circθ=60∘, H1=80sin⁡260∘=80⋅34=60 m.H_1=80\sin^2 60^\circ=80\cdot \frac{3}{4}=60\ \text{m}.H1​=80sin260∘=80⋅43​=60 m.

  5. For θ=30∘\theta=30^\circθ=30∘, H2=80sin⁡230∘=80⋅14=20 m.H_2=80\sin^2 30^\circ=80\cdot \frac{1}{4}=20\ \text{m}.H2​=80sin230∘=80⋅41​=20 m.

  6. Therefore, the sum of maximum heights is H1+H2=60+20=80 m.H_1+H_2=60+20=80\ \text{m}.H1​+H2​=60+20=80 m.

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