material Science..

29.2 Ans: Roof is made up of rough material and Ski base is made up of very smooth material. So frictional force between roof and snow is greater and that between Ski and snow is negligible.
As frictional force between roof and snow is higher, it prevents the movement of snow on roof of greater slope like 24 degree. Though vertical component of snow weight acts downward, but it is lesser than the frictional force when roof angle upto 24 degree.
Vertical component of snow weight = Weight of snow x sine of angle of slope.
Sine of angle increases with the increase of value of the angle. So, when the value of angle greater than 24 degree, vertical component of snow weight overcomes the frictional force. And snow slides off.
In other hand, due to smooth bottom of Ski, frictional resistance against movement of Ski is very less. So even...