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Motion in A Plane question

2021 · 1 Sep · Shift 2 · Q51
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  5. /2021 · 1 Sep · Shift 2 · Q51

Motion in A Plane question

2021 · 1 Sep · Shift 2 · Q51

JEE MainPhysicsMotion in A PlaneMCQ+4 / −1
The ranges and heights for two projectiles projected with the same initial velocity at angles 42 ∘^\circ∘ and 48 ∘^\circ∘ with the horizontal are R1, R2 and H1, H2 respectively. Choose the correct option :
  1. A
    R1 > R2 and H1 = H2
  2. B
    R1 = R2 and H1 < H2
  3. C
    R1 < R2 and H1 < H2
  4. D
    R1 = R2 and H1 = H2
View written solutionFree

Correct answer: B

  1. For a projectile projected with speed uuu at angle θ\thetaθ:

    R=u2sin⁡2θgR = \frac{u^2 \sin 2\theta}{g}R=gu2sin2θ​ H=u2sin⁡2θ2gH = \frac{u^2 \sin^2 \theta}{2g}H=2gu2sin2θ​

  2. Compare the ranges for θ1=42∘\theta_1 = 42^\circθ1​=42∘ and θ2=48∘\theta_2 = 48^\circθ2​=48∘.

    R1=u2sin⁡(2⋅42∘)g=u2sin⁡84∘gR_1 = \frac{u^2 \sin(2\cdot 42^\circ)}{g} = \frac{u^2 \sin 84^\circ}{g}R1​=gu2sin(2⋅42∘)​=gu2sin84∘​ R2=u2sin⁡(2⋅48∘)g=u2sin⁡96∘gR_2 = \frac{u^2 \sin(2\cdot 48^\circ)}{g} = \frac{u^2 \sin 96^\circ}{g}R2​=gu2sin(2⋅48∘)​=gu2sin96∘​

    Now,

    sin⁡96∘=sin⁡(180∘−96∘)=sin⁡84∘\sin 96^\circ = \sin(180^\circ - 96^\circ) = \sin 84^\circsin96∘=sin(180∘−96∘)=sin84∘

    Hence,

    R1=R2R_1 = R_2R1​=R2​

    This also follows from the fact that 42∘+48∘=90∘42^\circ + 48^\circ = 90^\circ42∘+48∘=90∘, so complementary angles give equal range for the same speed.

  3. Compare the maximum heights.

    H1=u2sin⁡242∘2gH_1 = \frac{u^2 \sin^2 42^\circ}{2g}H1​=2gu2sin242∘​ H2=u2sin⁡248∘2gH_2 = \frac{u^2 \sin^2 48^\circ}{2g}H2​=2gu2sin248∘​

    Since 48∘>42∘48^\circ > 42^\circ48∘>42∘ and sin⁡θ\sin \thetasinθ increases in this interval,

    sin⁡48∘>sin⁡42∘\sin 48^\circ > \sin 42^\circsin48∘>sin42∘

    Therefore,

    sin⁡248∘>sin⁡242∘\sin^2 48^\circ > \sin^2 42^\circsin248∘>sin242∘

    so,

    H2>H1H_2 > H_1H2​>H1​

    Hence,

    H1<H2H_1 < H_2H1​<H2​

  4. Final conclusion:

    R1=R2andH1<H2R_1 = R_2 \quad \text{and} \quad H_1 < H_2R1​=R2​andH1​<H2​

Therefore, the correct option is B.

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