Reading Quiz 22 Key

Due 3:30 pm, Thursday, November 12th

Physics 105, Fall Term, 2009


Reading: Chapter 9.8

Did you complete the reading assignment?

Yes
No

KEY CONCEPTS:

1) The Venturi Effect is a specialized from of Bernoulli's Principle.

2) Lift is a force that is a result of pressure differences from fluid flow and Newton's laws.

The Venturi Effect states that the pressure and velocity of a fluid are inversely related to each other. Since this concept was covered with Bernoulli's Principle in the last reading quiz we will not look any more into the Venturi Effect during this reading quiz, but feel free to go back to reading quiz 21 or the key for reading quiz 21 if you are struggling to understand this concept.

Lift is a force related to Bernoulli's Principle and the Venturi Effect. Lift is caused by a difference in pressure in a fluid on different sides of an object. This difference in pressure applies a force on an object in a direction perpendicular to the direction of fluid flow. This principle has been adapted to technologies like wings on a plane; where the object moving through a relatively stationary fluid acts the same as a stationary object in a fluid that is flowing.

Go to this nifty NASA applet: http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html. There are a ton of options that you can change on this simulation, but we will only look at a few of them. Make sure to leave the flow setting on ideal flow. You can change the units from 'English Units' to 'Metric' if you'd like, but I prefer keeping it on 'English' units because I have a better understanding of pounds that Newtons. Directly above the animation part of the applet you have the option to change the view and display. Change the view to 'Side 3-D' and the display to 'Moving'. Notice on the right hand side there is a plot of the pressure of the airfoil for the pressure under the wing (yellow) and above the wing (fuchsia). Above this plot shows the magnitude of the lift force. The three settings you will change on the airfoil will be the angle of inclination, the camber, and the thickness. The angle of inclination is the angle that the front end of the wing is with respect to the horizontal, the camber deals with the shape of the concavity of the wing, and the thickness is the percentage of how thick the wing is compared to how wide the wing is. Don't worry too much about the units for camber or thickness; focus on the effects they have on airflow and lift.

Play with the 3 airfoil settings mentioned above while watching the pressure graph, lift force, and airflow animation.

Under what conditions will the lift force be in a downward direction rather than an upward direction?

When the lift force is positive, which part of the wing has a greater pressure?

Top of wing
Bottom of wing
Pressure is the same at all parts

When the lift force is positive, which part of the wing has a faster airflow?

Top of wing
Bottom of wing
Airflow is the same at all parts

As the angle of inclination is increases (for positive lift), what happens to the lift force?

Lift force increases
Lift force decreases
Lift force does not change

As the camber increases (for positive lift), what happens to the lift force?

Lift force increases
Lift force decreases
Lift force does not change

As the thickness increases (for positive lift), what happens to the lift force?

Lift force increases
Lift force decreases
Lift force does not change

Which of the three factors influences lift force the least?

Angle of inclination
Camber
Thickness

It would be really cool if humans could fly without the aid of technology. Lets see if Usain Bolt, the current World Record holder in the 100 m and 200 m dashes can fly. Change the airfoil shape (found just under the animation part of the applet) to 'Ellipse' to make is shaped more like an arm. Also set the camber to about -10% and the thickness to about 25% so that we are getting the closest representation to an actual arm that we can. Also change the angle of inclination to 20% which would mean that as he runs he would need to hold his arms straight out to the side with his palms raised up to a 20 angle above the horizontal. Change the model under 'Student Mode' to 'Stall Model' to give a more accurate model of how air will actually act around the arms. Now change the input type on the right side of the screen (in blue) to 'Flight Test'. Bolt is estimated to have max out at about 27.2 mph at his top speed and weighs 190 pounds. If he can match this top speed while holding his arms straight out at a 20 angle of inclination could he fly on an average Earth day at sea level?

Yes
No

Provo's altitude is about 4500 feet above sea level. If I weigh 175 pounds, how fast would I need to run on an average Earth day to fly?


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