the drag force on the smooth ball like a golf ball compared

the drag force on the smooth ball (like a golf ball) compared to that on the rough ball, was ______________ at speeds below around 125 mph; and ___________ at speeds above around 125 mph, respectively.

A) smaller, greater
B) smaller, smaller
C) greater, greater
D) greater, smaller

Solution

See for a real fluids flow over a solid boundary with boundary layer zone for a fluid particle to move forward has to do work against friction at an expense of kinetic energy .The loss in energy will be recovered from adjacent layer .If the work increases a stage will come at a point where fluid layer may not be able to stick to the boundary .This is called point of separation.Separation is undesirable because behind separation negative pressure exist.

Inorder to prevent seperatiom turbulence should increase

We know that golf ball is example of turbulent....for this velocity will high...total drag will be reduced as boundary layer over surface of a cylinder separate further down...

As speed should low so that Reynolds number increases ....boundary layer increases...

Option

Smaller and greater