A simple physics problem [frame of reference]

AI Thread Summary
The discussion revolves around a physics problem involving a heavy box sliding down an inclined plane and a particle projected inside it. The key points include finding the distance from the projection point to where the particle lands, with the solution given as u^2sin2α / g cosƟ. Additionally, the speed of the box relative to the ground when the particle is projected is derived as ucos(Ɵ + α) / cosƟ. A participant questions whether the assumption that α + θ equals 90 degrees is valid, suggesting the problem's complexity. The thread emphasizes the need for clarity in physics problems, particularly when they resemble homework questions.
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A large heavy box is sliding without friction down a smooth plane of inclination Ɵ. From a point P on the bottom of the box, a particle is projected inside the box. The initial speed of the particle with respect to the box is 'u' and the direction of projection makes an angle α with the bottom as shown in the figure.
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a : Find the distance along the bottom of the box between the point of projection P and the point Q where the particle lands [assume that the particle does not hit any other surface of the box. Neglect the air resistance]

b: If the horizontal displacement of the particle as seen by an observer on the ground is zero, find the speed of the box with respect to the ground at the instant when the particle was projected.
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http://imageshack.us/photo/my-images/834/uploadv.jpg/
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Answers :
a : u^2sin2α / g cosƟ
b : ucos(Ɵ + α ) / cosƟ

I could not reach answer b, is alpha + theta = 90 degree a valid assumption in this case?
this is not a homework question, I just saw this over the internet
 
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You will probably get better responses if you post this in the homework section. Even though it is not homework it is certainly homework-style.
 

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