Continuation of Problem 8. Now assume that Eq. 6-14 gives...

Continuation of Problem 8. Now assume that Eq. 6-14 gives the magnitude of the air drag force on the typical 20 kg stone, which presents to the wind a vertical cross-sectional area of 0.040 m2 and has a drag coefficient C of 0.80. Take the air density to be1.21 kg/m3 , and the coefficient of kinetic friction to be 0.80. (a) In kilometers per hour, what wind speed V along the ground is needed to maintain the stone’s motion once it has started moving? Because winds along the ground are retarded by the ground, the wind speeds reported for storms are often measured at a height of 10 m. Assume wind speeds are 2.00 times those along the ground. (b) For your answer to (a), what wind speed would be reported for the storm? (c) Is that value reasonable for a highspeed wind in a storm?

Continuation of Problem 8. Now assume that Eq. 6-14 gives
the magnitude of the air drag force on the typical 20 kg stone,
which presents to the wind a vertical cross-sectional area of 0.040
m2 and has a drag coefficient C of 0.80. Take the air density to be1.21 kg/m3
, and the coefficient of kinetic friction to be 0.80. (a) In
kilometers per hour, what wind speed V along the ground is
needed to maintain the stone’s motion once it has started moving? Because winds along the ground are retarded by the ground,
the wind speeds reported for storms are often measured at a
height of 10 m. Assume wind speeds are 2.00 times those along
the ground. (b) For your answer to (a), what wind speed would be
reported for the storm? (c) Is that value reasonable for a highspeed wind in a storm?

Key Concepts

Wind Speed Profile and Measurement Height

Wind speed can vary with height due to factors like surface roughness and obstacles. The wind speed measured at a standard height, such as 10 meters during storms, may be higher than the wind speed near ground level. This difference is important when translating measured wind speeds to the effective speed at which forces like drag act on objects close to the ground.

Kinetic Friction

Kinetic friction is the resistive force that acts between moving surfaces in contact. It is proportional to the normal force and is quantified using the coefficient of kinetic friction. In dynamics problems, kinetic friction opposes motion and must be overcome by any applied force for maintenance of constant velocity, often requiring a force balance between driving forces and frictional losses.

Air Drag Force

This is the resistive force exerted on an object due to its motion relative to a fluid, such as air. It is determined by factors like air density, the object's cross?sectional area, the drag coefficient, and the square of the relative speed. In many physics problems, this force is modeled by an equation where the drag force increases quadratically with speed, making it a crucial factor in analyzing motion in fluids.

Drag Coefficient

The drag coefficient is a dimensionless number that characterizes the resistance of an object in a fluid. It encapsulates the effects of the object's shape, surface roughness, and flow conditions. A lower drag coefficient indicates a more streamlined object, while a higher value represents more resistance to motion. It appears in the air drag force formula to scale the influence of the object's geometry on the overall drag.

Transcript

Please give Ace some feedback