MCQEasyJEE 2023Velocity & Acceleration

JEE Physics 2023 Question with Solution

A person travels xx distance with velocity v1v_1 and then xx distance with velocity v2v_2 in the same direction. The average velocity of the person is vv, then the relation between vv, v1v_1, and v2v_2 will be:

  • A

    v=v1+v2v = v_1 + v_2

  • B

    1v=1v1+1v2\frac{1}{v} = \frac{1}{v_1} + \frac{1}{v_2}

  • C

    2v=1v1+1v2\frac{2}{v} = \frac{1}{v_1} + \frac{1}{v_2}

  • D

    v=v1+v22v = \frac{v_1 + v_2}{2}

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: A person travels equal distances xx and xx with velocities v1v_1 and v2v_2 respectively in the same direction.

Find: The relation between average velocity vv, v1v_1, and v2v_2.

For the first part of the journey, time taken is

t1=xv1t_1 = \frac{x}{v_1}

For the second part of the journey, time taken is

t2=xv2t_2 = \frac{x}{v_2}

Total displacement is

2x2x

and total time is

t1+t2=xv1+xv2=x(1v1+1v2)t_1 + t_2 = \frac{x}{v_1} + \frac{x}{v_2} = x\left(\frac{1}{v_1} + \frac{1}{v_2}\right)

Average velocity is

v=Total displacementTotal time=2xx(1v1+1v2)=2(1v1+1v2)v = \frac{\text{Total displacement}}{\text{Total time}} = \frac{2x}{x\left(\frac{1}{v_1} + \frac{1}{v_2}\right)} = \frac{2}{\left(\frac{1}{v_1} + \frac{1}{v_2}\right)}

Therefore,

2v=1v1+1v2\frac{2}{v} = \frac{1}{v_1} + \frac{1}{v_2}

So, the correct option is C. The solution labels the option as D, but the derived relation clearly matches option C in the given options.

Common mistakes

  • Using arithmetic mean v1+v22\frac{v_1+v_2}{2} for equal distances. That is wrong because arithmetic mean applies directly when time intervals are equal, not distances. For equal distances, use total displacement divided by total time.

  • Adding velocities directly as v=v1+v2v=v_1+v_2. This is wrong because average velocity is not the sum of segment velocities. First compute time for each segment, then divide total displacement by total time.

  • Forgetting that the motion is in the same direction and using distance treatment without time calculation. Even though displacement equals total distance here, the correct method still requires summing the times of both parts separately.

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