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JEE Main 2020 (6 Sep, Shift 2) · Relative Velocity in Two Dimensions

Relative Velocity in Two Dimensions question from JEE Main 2020 (6 Sep, Shift 2)

Q17JEE Main 2020 · 6 SEP · SHIFT 2Medium

Relative Velocity in Two Dimensions

When a car is at rest, its driver sees rain drops falling on it vertically. When driving the car with speed v, he sees that rain drops are coming at an angle 60° from the horizontal. On further increasing the speed of the car to (1 + β\beta)v, this angle changes to 45o. The value of β\beta is close to:

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Answer: (b)

  1. Rain falls vertically with speed uu; relative to the car it has horizontal component equal to car speed.
  2. At 60∘60^\circ from horizontal: tan⁡60∘=uv⇒u=3v\tan 60^\circ = \frac{u}{v} \Rightarrow u = \sqrt{3}v
  3. At 45∘45^\circ: u=(1+β)v⇒1+β=3u = (1+\beta)v \Rightarrow 1 + \beta = \sqrt{3}
  4. β=0.732\beta = 0.732, so the answer is (b) 0.73.
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