Deck 15: Human Movement in a Fluid Medium

Question 1

A cyclist riding against a 12 km/hr headwind has a velocity of 28 km/hr with respect to the wind. What is the cyclist's absolute velocity? (16 km/hr)

Answer 1

Remember that the relative velocity of a body with respect to a fluid is equal to the vector subtraction of the absolute velocity of the fluid from the absolute velocity of the body:
The absolute velocity of the cyclist is 16 km/hr

Question 2

A boy is swimming with an absolute speed of 1.5 m/s in a river where the speed of the current s 0.5 m/s. What is the velocity of the swimmer with respect to the current when the boy swims directly upstream? Directly downstream?

Answer 2

If the boy swims with the current, his speed will be equal to the sum of the speed of his swimming plus the speed of the current pushing him downriver:
The boy will move downriver at 2 m/s.
The opposite occurs when the boy is swimming against the current, as his motion is being opposed by the speed of the current:
Swimming against the current, the boy will move with a velocity of 1.0 m/s.

Question 3

A swimmer crossing a river proceeds at an absolute speed of 2.5 m/s on a course oriented at a 45° angle to the 1 m/s current. Given that the absolute velocity of the swimmer is equal to the vector sum of the velocity of the current and the velocity of the swimmer with respect to the current, what is the magnitude and direction of the velocity of the swimmer with respect to the current? (3.3 m/s at an angle of 32.6 ° to the current)

Answer 3

NOT ANSWER

Question 4

A cyclist is riding at a speed of 14 km/hr into a 16 km/hr headwind. What is the wind velocity relative to the cyclist? What is the cyclist's velocity with respect to the wind?

Question 5

What maximum average density can a body possess if it is to float in freshwater? Seawater?

Question 6

A skier traveling at 5 m/s has a speed of 5.7 m/s relative to a headwind. What is the absolute wind speed?

Question 7

A scuba diver carries camera equipment in a cylindrical container that is 45 cm long, 20 cm in diameter, and 22 N in weight. For optimum maneuverability of the container under water, how much should its contents weigh? (120.36 N)

Question 8

A 700 N man has a body volume of 0.08 m 3. If submerged in fresh water, will he float? Given his body volume, how much could he weight and still float?

Question 9

A 50 kg person with a body volume of 0.055 m 3 floats in a motionless position. How much body volume is above the surface if the water is fresh? If the water is salt? (a. 0.005 m 3 , b. 0.0063 m 3 )

Question 10

A racing shell has a volume of 0.38 m 3. When floating in fresh water, how many 700 N people can it support?

Question 11

A 670 N swimmer oriented horizontally in fresh water has a body volume of 0.07 m 3 and has his center of volume located 3 cm superior to his center of gravity. (a) How much torque does the swimmer's weight generate? (b) How much torque does the buoyant force acting on the swimmer generate? (c) What can the swimmer do to counteract the torque and maintain a horizontal position? (a. 0, b. 20.1 N-m, c. rotate the body so that the CG and CV are vertically aligned.)

Question 12

How much body volume must a 60 kg person have to float in fresh water?

Question 13

Based on your knowledge of the action of fluid forces, speculate as to why a properly thrown boomerang returns to the thrower.

Question 14

Explain the implications for flotation due to the difference between the specific weight of freshwater and the specific weight of seawater.

Question 15

Explain the aerodynamic benefits of drafting on a bicycle or in an automobile.

Question 16

What strategy can people use to improve their chances of floating in water? Explain your answer.

Question 17

What is the practical effect of streamlining? How does streamlining alter the fluid forces acting on a moving body?

Question 18

What types of individuals may have a difficult time floating in water? Explain your answer.

Question 19

Explain why a curveball curves. Include a discussion of the aerodynamic role of the seams on the ball.

Question 20

A beach ball weighing 1 N and with a volume of 0.03 m 3 is held submerged in seawater. How much force must be exerted vertically downward to hold the ball completely submerged? To hold the ball one-half submerged?