Relative Velocity Problem: Finding Train Speed from Raindrop Angle

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SUMMARY

The discussion focuses on solving the relative velocity problem involving a train and falling raindrops. A stationary observer notes that raindrops fall vertically at 5.0 m/s, while the train moves at an unknown speed, causing the raindrops to appear at a 25° angle. The correct speed of the train is determined to be 2.3 m/s, contrasting with the initial calculation of 2.16 m/s. The solution involves understanding the components of velocity and applying trigonometric principles to resolve the vectors accurately.

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  • Understanding of relative velocity concepts
  • Familiarity with trigonometric functions (sine, cosine)
  • Ability to draw and interpret right triangles in physics
  • Knowledge of vector addition and subtraction
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  • Study vector decomposition in physics
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  • Explore trigonometric applications in physics problems
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Homework Statement


A person looking out the window of a stationary train notices that rain drops are falling vertically down at a speed of 5.0 m/s relative to the ground. When the train moves at a constant velocity, the raindrops make an angle of 25° when they move past the window. How fast is the train moving?

Homework Equations


##\vec{v_{AC}} = \vec{v_{AB}} + \vec{v_{BC}}##

The Attempt at a Solution


T = train, R = rain, G = ground
##\vec{v_{TG}} = \vec{v_{TR}} + \vec{v_{RG}}##
##\vec{v_{TG}} = \vec{v_{RG}} - \vec{v_{RT}}##
##v_x = 0~m/s-[(5~m/s)\sin25^{\circ}] = 2.1131~m/s##
##v_y = (-5~m/s) -[(5~m/s)\cos25^{\circ}] = -0.468~m/s##
##\vec{v_{TG}} = \sqrt{(2.1131~m/s)^2 + (-0.468~m/s)^2} = 2.16~m/s##

However, according to the solution manual, the correct answer is 2.3 m/s. Where am I going wrong?
 
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Try drawing a right triangle.

The vertical length is the trains speed.
The horizontal length is the rain drop speed of 5.0.

Which angle are you looking from?
What trig will help you solve it in one quick calculation?
 
RJLiberator said:
Try drawing a right triangle.

The vertical length is the trains speed.
The horizontal length is the rain drop speed of 5.0.

Which angle are you looking from?
What trig will help you solve it in one quick calculation?
What do you mean "the vertical length is the train's speed"? Do you mean horizontal?
 

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