Question 1

Suppose your instructor staggers into class after a week - long science lab equipment convention. She proceeds to explain Magnus lift in the following way. She has you consider a ball thrown to the left and spinning clockwise, having the wind velocity profile shown below. This is a "fastball" so it will rise, as correctly indicated. Please explain why the force F_m is inconsistent with the wind velocity profile.

Accepted Answer

The answer of Suppose your instructor staggers into class after...

Question 2

In some motion pictures, airplane passengers are sucked out of the plane when a door is opened and the airplane is in flight. Comment on whether such an effect is scientifically realistic or not.

Accepted Answer

The answer of In some motion pictures, airplane passengers are...

Question 3

Consider a baseball thrown to the left, spinning clockwise and experiencing laminar flow as shown in the picture below. Based upon the wind speed experienced at the top and bottom of the ball, comment on whether or not this would account for the ball's obtaining a lift (the vertical component of the purple arrow) from spinning. Also, mention whether the ball shown is a good candidate for the Magnus force or for wake deflection.

Accepted Answer

The answer of Consider a baseball thrown to the left,...

Question 4

You are out for a bicycle ride on a calm, windless day. You are heading northward on a level road and are experiencing a pressure drag force that pushes you toward the south. This air resistance explains why you have to keep pedaling to maintain your constant speed. If someone were to examine the air that you have left behind you after you have passed through it, they would find that the air's velocity is

Accepted Answer

The air left behind a moving object like a bicycle is not stationary or moving uniformly in one direction. Instead, it tends to swirl or create vortices due to the object displacing it as it moves through. This swirling motion is a result of the complex interactions between the air molecules and the object, leading to a mix of speeds and directions in the air's movement, hence "zero on average but that it is swirling rapidly in all directions."

Question 5

When a Frisbee is flying horizontally through the air, the speed of the air flowing over the Frisbee's top is

Accepted Answer

According to Bernoulli's principle, when the speed of a fluid (in this case, air) increases, its pressure decreases. Therefore, if the speed of the air flowing over the top of the Frisbee is higher, the pressure on top of the Frisbee will be less than the pressure beneath it, which helps to generate lift.

Question 6

Aerodynamic forces consist of

Accepted Answer

Aerodynamic forces primarily consist of drag and lift, which are the forces that act against an object moving through the air, with drag opposing the direction of motion and lift acting perpendicular to the direction of motion.

Question 7

As the Reynolds number decreases,

Accepted Answer

The answer of As the Reynolds number decreases,
...

Question 8

Surface roughness help golf and tennis balls travel with less drag, but yet objects that travel under water (submarines by human design and animals by natural design) generally have streamlined, smooth surfaces. Please explain such a difference.

Accepted Answer

The answer of Surface roughness help golf and tennis balls...

Question 9

The top surface of a calm, smoothly flowing stream is always at atmospheric pressure. As water in this stream suddenly speeds up, the water's top surface

Accepted Answer

According to Bernoulli's principle, as the speed of a fluid increases, its pressure decreases. Therefore, if water in a stream suddenly speeds up, the pressure at the top surface decreases, causing the water level to drop slightly below the normal stream level.

Question 10

Consider a baseball thrown to the left, spinning clockwise and experiencing laminar flow as shown in the picture below. Based upon the wind speed pattern shown, comment on whether or not this would account for the ball's obtaining drag. Also, mention whether the ball shown is a good candidate for the Magnus force or for wake deflection.

Accepted Answer

The answer of Consider a baseball thrown to the left,...

Question 11

Suppose your instructor staggers into class after a week - long physics joke convention. She proceeds to explain drag in the following way. She has you consider a ball thrown to the left, having the wind velocity profile shown below. She goes on to say that the air is compressed in front of the ball and rarefied behind it, so there is air pressure pushing backwards, which is exactly drag. Is the picture shown consistent with her explanation?

Accepted Answer

The answer of Suppose your instructor staggers into class after...

Question 12

A fan can be used to circulate air around a room. The pressure at the inlet side of the fan is

Accepted Answer

The answer of A fan can be used to circulate...

Question 13

Which of the following would cause a household fan to stall?

Accepted Answer

A fan can stall if the blades are too sharply angled for the speed, causing inefficient airflow, or if it rotates too slowly, failing to move air effectively.

Question 14

You are holding two identical-looking balloons, one filled with air and one filled with water. You drop these two balloons from a very tall bridge and notice that the water-filled balloon hits the ground first because its terminal velocity is larger. The terminal velocity of the water balloon is larger than that of air balloon because

Accepted Answer

The larger force of gravity on the water balloon, due to its greater mass, requires a larger drag force to balance it out. This larger drag force is achieved at a higher speed, resulting in a larger terminal velocity for the water balloon compared to the air-filled balloon.

Question 15

Suppose you are a teacher and, to your excitement, the local weather person comes for a classroom visit! She is talking about tornadoes and says that there is air moving very rapidly upwards through the funnel but air standing basically still outside the funnel. Such motion would cause

Accepted Answer

Rapid upward movement of air within the tornado funnel leads to lower pressure inside the funnel compared to the surrounding air. This pressure difference is a key factor in the formation and maintenance of the tornado.

Question 16

When a fish hovers over the bottom of a lake, what forces act on it, and what is the net force?

Accepted Answer

When a fish hovers over the bottom of a lake, both lift and weight forces act on it. The lift force acts upwards, while the weight (due to gravity) acts downwards. For the fish to hover without moving up or down, these forces must be equal and opposite, resulting in a net force of zero.

Question 17

You are on an airplane trip to the islands for vacation. Your first flight is on a propeller driven airplane. The air flowing toward the spinning propeller blades doesn't slow down before it encounters those blades. Your second flight is on a turbofan-driven jet airplane. The air flowing toward the spinning fan blades slows down significantly before it encounters those blades. The air's pressure just before it encounters the turbofan's fan blades is

Accepted Answer

The air slows down before it encounters the turbofan's blades, which increases its pressure due to the Bernoulli's principle. This makes the air's pressure just before it encounters the turbofan's blades much higher than the air's pressure just before it encounters the propeller's blades, where the air does not slow down significantly.

Question 18

In old western movies, after having fired their guns people would blow across the end of their guns and smoke would come out the barrel. Please explain.

Accepted Answer

The answer of In old western movies, after having fired...

Question 19

Please consider the funnel of a tornado which holds its size (radius) and therefore its rotation rate. Now imagine that the funnel is made up of many tiny particles moving around in circles. You are examining one of those particles, which represents a tiny portion of the funnel cloud. You may ignore gravity for this problem.
-Please identify the direction of the net force acting on one of the particles.

Accepted Answer

The answer of Please consider the funnel of a tornado...

Question 20

Please consider the funnel of a tornado which holds its size (radius) and therefore its rotation rate. Now imagine that the funnel is made up of many tiny particles moving around in circles. You are examining one of those particles, which represents a tiny portion of the funnel cloud. You may ignore gravity for this problem.
-Why would it make sense if we said that air is moving very rapidly up through the center of the funnel and almost stationary outside the funnel for it to remain in uniform circular motion?

Accepted Answer