A roller coaster starts from rest at its highest point and then descends on its (frictionless) track. Its speed is 26 m/s when it reaches ground level. What was its speed when its height was half that of its starting point?

The velocity is  

Explanation:

From the question we are told that

   The speed of the roller coaster at ground level  is

Generally we can define the roller coaster speed at ground level using the an equation of motion as

 u is  zero given that the roller coaster started from rest

      So

So  

=>      

Now the displacement half way is mathematically represented as

So

Where   is the velocity at the half way point

=>  

=>  

snell's law

refractve index = sin incidence / sin refraction