MCQEasyJEE 2023Projectile Motion

JEE Physics 2023 Question with Solution

The maximum vertical height to which a man can throw a ball is 136m136 \, \text{m}. The maximum horizontal distance up to which he can throw the same ball is:

  • A

    192m192 \, \text{m}

  • B

    136m136 \, \text{m}

  • C

    272m272 \, \text{m}

  • D

    68m68 \, \text{m}

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Maximum vertical height is Hmax=136mH_{\text{max}} = 136 \, \text{m}.

Find: The maximum horizontal range for the same ball.

For a vertical throw, the maximum height is

Hmax=v22gH_{\text{max}} = \frac{v^2}{2g}

So,

v2=2gHmaxv^2 = 2gH_{\text{max}}

The maximum horizontal range for the same speed is

Rmax=v2gR_{\text{max}} = \frac{v^2}{g}

Substituting v2=2gHmaxv^2 = 2gH_{\text{max}},

Rmax=2gHmaxg=2HmaxR_{\text{max}} = \frac{2gH_{\text{max}}}{g} = 2H_{\text{max}}

Therefore,

Rmax=2×136=272mR_{\text{max}} = 2 \times 136 = 272 \, \text{m}

Therefore, the maximum horizontal distance is 272m272 \, \text{m} and the correct option is C.

The solution labels the correct option as D, but its working gives 272m272 \, \text{m}, which matches option C.

Common mistakes

  • Using the formula for range at an arbitrary angle instead of the maximum range. Maximum range occurs at 4545^\circ, so for the same speed one must use Rmax=v2gR_{\text{max}} = \frac{v^2}{g}.

  • Forgetting that the given 136m136 \, \text{m} is the maximum vertical height for a vertical throw, so it must be related through Hmax=v22gH_{\text{max}} = \frac{v^2}{2g}. Treating it directly as range gives the wrong result.

  • Missing the relation Rmax=2HmaxR_{\text{max}} = 2H_{\text{max}} for the same projection speed. Once HmaxH_{\text{max}} is known, the range is not equal to it; it is twice that value.

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