Take the challenge exam!

Transcription

1 Take the challenge exam! Today is the last day to take it! Read the book Focus on new concepts Answer the questions at the end of each chapter Vocabulary test #1 deadline: Friday 25 Sept. First exam deadline: 11am Mon Sept 28 (begins Sep 26) Late deadline: Sept. 29 (for a $5 fee!!) Chapters 1-91 Closed book, closed notes, etc Testing center (Grant building) Busier in the afternoons on deadline dates. Last test handed out 1 hour before closing.

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3 Moving in an Elevator Forces are: balanced (A) before starting, (E) after stopping, and (C) while gliding in uniform motion between floors. unbalanced upward when you start to move (B) unbalanced downward as you stop (D). A change in the contact force creates the imbalance in forces. E D C B A

4 Going into Orbit When in orbit, the shuttle no longer needs to fire its rockets. It glides easily along in free fall under the sole influence of gravity.

5 Quiz: The Moon s s motion around the Earth can be understood in terms of: a) One force directed towards the Earth b) One force directed away from the Earth c) One force in the direction of the Moon s s orbit d) Two forces which balance e) Two forces which do not balance

6 Centripetal Acceleration Sideways forces cause objects to turn toward the direction of the force. The stronger the force, the tighter the turn. Revisited

7 Quiz: I have read all the chapters through a) Chapter 4 b) Chapter 5 c) Chapter 6 d) Chapter 7 e) Chapter 8

8 Pressure = Force/Area F = 100 lbs A = 1 in lbs 100 lbs F = 100 lbs A = 100 in 2 P = 100lbs/in 2 P = 1lb/in 2 The forces are equal, but the pressures are different.

9 Quiz: If the area on which the force acts is cut in half (and the force remains the same), the pressure a) is cut in half. b) is cut to one fourth. c) is doubled. d) increases by 4. 4 e) stays the same.

10 Pressure Although the ballerina s s weight doesn t t change, she exerts a greater pressure on the floor when on point because her weight is distributed over a smaller area.

11 A few definitions A fluid is anything that flows: air, water, oil, gel, glass, etc. It could be what we think of as liquid or gas. A bounded fluid is any fluid completely contained inside a closed container; e.g. air in a balloon, hydraulic fluid in a pipe, the air inside a spray can, toothpaste in a tube, the air inside a tire, etc.

12 Pascal s s Law Pressure applied to any part of a bounded fluid is transmitted to every other part of the fluid in exactly the same way. The pressure acts at right angles to any surface in contact with the fluid. Inside pressure Outside Pressure Inside pressure Less Pressure Greater Pressure

13 Pressure in Bounded Fluids Pressure distributing uniformly through toothpaste makes the different stripes emerge from the tube in the same proportions as they are inside.

14 Hydraulic Systems The pressure on the right side is the same as the pressure on the left and everywhere in the pipe. Imagine that the area of the left piston is ten times greater than the area of the right piston, the total force on the left piston is correspondingly ten times greater than the force on the left piston. (What( is the tradeoff?)

15 Unbounded fluids A fluid that has at least one open side. Examples: Oceans, lakes, rivers Clouds, the atmosphere A glass of water, an open can of soda A star

16 Some rules for pressure in unbounded fluids Pressure depends only on depth. Pressure increases as depth increases. Pressure is the same for all points (think of a single drop of water) at the same depth regardless of local effects.

17 Some rules for pressure in unbounded fluids At any point (think of a single drop of water), at some depth, pressure is the same in all directions. The pressure in a fluid is always perpendicular to a submerged object.

18 Explain this:

19 Another example: air pressure The atmosphere is an unbounded fluid. Air pressure at sea level is 14.7 lbs per square inch which is a lot! But at the top of Mount Everest the air pressure is only 30% of this value!

20 Quiz: Which pressure law (for unbounded fluids) is stated incorrectly? A. Pressure is greater at deeper depths. B. Pressure is the same for all points at the same depth regardless of local effects. C. Pressure at a given point is larger in the direction of deeper depths than towards shallower depths. D. Pressure in a fluid is always perpendicular to the submerged surface.

21 The Buoyant Force and Archimedes Principle The pressure in unbounded fluids increases with depth. The pressure on objects submerged in a fluid will vary along the height of the submerged object. There will be a net upward force (on submerged objects) called the buoyant force.

22 Archimedes Principle The buoyant force on an object equals the weight of the displaced fluid

23 Quiz: How much water does it take to A. A few cups B. The whole ocean C. A volume of water equal to the volume of the ship D. A weight of water equal to the weight of the ship float an aircraft carrier?

24 Floating and Sinking A heavy lead ball and a light beach ball,, both of equal volume, are submerged. They displace the same amount of water so the buoyant force on each is the same. Both go in the direction of net force. The heavier lead ball sinks while the lighter beach ball floats.

25 Floating and Sinking A solid and a hollow iron ball, both of equal weight, are submerged. Because they displace different amounts of water, the buoyant force on each is different. Both go in the direction of net force. The larger hollow ball floats while the smaller solid ball sinks.

26 Quiz: A wooden sphere is submerged in water. What will happen? A. It will accelerate upwards, because the gravitational force on it is small. B. It will accelerate upwards, because the buoyant force pointing up is greater than the gravitational force pointing down. C. It will remain at rest, because all forces are balanced D. It will sink, because the gravitational force on it is large. E. It will sink, because the gravitational force pointing down is greater than the buoyant force pointing up.

27 Floating and Sinking Solid objects float if they are less dense than water. Freshwater ice is 90% the density of freshwater and floats 90% submerged. Freshwater ice is just 70% the density of heavier saltwater and floats 70% submerged. Remember: Density = Mass/Volume

28 Quiz: Which ball weighs the least? A A B C B C

29 Quiz: Which ball is the most dense? A B C A B C

30 Quiz: Which ball has balanced forces? A. All A B C B. A only C. A and B only D. B and C only Note: The red ball is sinking at constant speed.

31 Buoyancy in the Atmosphere Solid objects in water will float or sink but not hover because water keeps the same density with depth. A hot-air balloon in air will hover because the buoyant force decreases with height as air density decreases. But helium balloons will go up until they burst.

32 Buoyancy in the Earth Earth s s crust floats on the outer layer of the mantle. Continental crust is thicker and has a greater total weight than ocean crust. As a result, continents float deeper in the mantle than ocean basins.

33 Wind currents Convection comes when hot air rises and cool air rushes in to take its place. It rises because it is less dense, not It rises because it is because it is hot.

34 Quiz: How will the different balls move lead water cork

35 Quiz: How will the different balls move buoyant force lead water cork weight Demo