A particle is projected from a horizontal surface with a velocity of 20 m/s at an angle of 60° with the horizontal. When it hits the horizontal surface, its velocity makes an angle of 30° with the horizontal. Apart from gravity, a horizontal force acts on the particle during the motion. The speed of the particle when it hits the ground is

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G1 Physics · Accepted Answer

Vertical motion is unaffected by the horizontal force. Since the particle returns to the same level, its impact vertical component is vᵧ = -u sin60° = -10√3 m/s. At impact the velocity is 30° to the horizontal, so |vᵧ|/|vₓ| = tan30° = 1/√3, giving |vₓ| = 30 m/s. Therefore impact speed = √(30² + (10√3)²) = 20√3 m/s. Hence option B.