Relative Velocity Problem: Finding Train Speed from Raindrop Angle

In summary, a person on a stationary train observes raindrops 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 pass the window. To determine the speed of the train, the equation ##\vec{v_{TG}} = \vec{v_{TR}} + \vec{v_{RG}}## is used, where T represents the train, R represents the rain, and G represents the ground. By drawing a right triangle and using trigonometry, it can be determined that the train is moving at a speed of 2.3 m/s.
  • #1
Mr Davis 97
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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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  • #2
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?
 
  • #3
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?
 

What is relative velocity problem?

Relative velocity problem is a type of problem in which the motion of two objects is considered in relation to each other. It involves calculating the velocity of one object with respect to another, taking into account their direction and speed.

How do you solve a relative velocity problem?

To solve a relative velocity problem, you need to first identify the motion of the two objects in relation to each other. Then, use the relative velocity formula to calculate the velocity of one object with respect to the other. Finally, take into account the direction of motion to determine the final velocity.

What is the relative velocity formula?

The relative velocity formula is VAB = VA - VB, where VAB is the relative velocity of object A with respect to object B, VA is the velocity of object A, and VB is the velocity of object B.

What are some common examples of relative velocity problems?

Some common examples of relative velocity problems include a boat moving in a river, a car overtaking another car on a highway, and a plane flying in the wind. These examples involve calculating the velocity of one object with respect to another in a given scenario.

Why is it important to consider relative velocity in certain situations?

In certain situations, such as when two objects are moving at different speeds in different directions, it is important to consider relative velocity in order to accurately determine the motion of the objects. This is especially important in fields such as physics and engineering, where precise calculations are necessary.

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