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Forces in Fluids Chapter 13

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Chap 13 Pretest If two forces act in opposite directions on an object, how do you determine the net force on the object? Subtract the smaller force from the larger force.

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Chap 13 Pretest What effect do balanced forces have on the motion of an object? The forces cause no change in the object’s motion.

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Chap 13 Pretest What effect do unbalanced forces have on the motion of an object? The object’s speed increases in the direction of the greater force.

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Chap 13 Pretest Which units could you use to describe the area of a chalkboard? A. meters (m) B. square meters (m2) C. cubic meters (m3) D. all of the above

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**Chap 13 Pretest Which of the following defines weight?**

A. the amount of matter in an object B. the same as mass C. the force due to gravity acting on an object D. all of the above

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Chap 13 Pretest A tennis ball has a volume of 125 cm3 and a mass of 57 g. What is the ball’s average density? (Hint: density = mass/volume) Density = 57g / 125cm3 = 0.46 g/cm3

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Chap 13 Pretest The distance between a home and school is 6.35 km. What is this distance in meters? 6.35 km * 1000 m / 1km = 6350 m

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Chap 13 Pretest Which of Newton’s laws describes action-reaction forces? Newton’s third law

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Pressure of an Object What happens when the same force is applied to two areas of different size? Grab a heavy book at its spine using one hand. Make sure to wrap your entire hand around the book. Hold the book several centimeters above your desktop for 20 seconds. Next, repeat the process using only your thumb and forefinger. Again hold the book for 20 seconds. What difference do you notice in the two methods of holding the book? Does the weight of the book change? Does the area over which you apply the force need to hold the book change?

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13.1- Pressure Pressure is the result of a force distributed over an area. Pressure = Force / Area Pa = N / m2 1 kPa = 1000 Pa If area decreases, pressure increases (inversely proprotional) If force increases, pressure increases (directly proportional) The SI unit for pressure is the pascal (Pa). The pascal is named for French scientist Blaise Pascal.

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**13.1 – Pressure Example Problem pressure = force / area**

A box has a weight of 2500 N resting on the ground. If the side of the box touching the ground is 1 m wide x 5 m long, what pressure does the box exert on the ground? pressure = force / area P = 2500 N / (1m x 5 m) = 2500 N / 5 m2 = 500 Pa = kPa

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13.1 – Pressure in Fluids A fluid is a substance that assumes the shape of its container. Both liquids and gases are fluids. Examples: water, oil, gasoline, air, helium Water pressure increases as depth increases. The pressure in a fluid at any given depth is constant, and it is exerted equally in all directions.

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13.1 – Pressure in Fluids Fluid pressure is determined by the type of fluid and its depth. The amount of fluid (volume or weight) does not affect pressure. The shape and the area of its bottom surface do not affect fluid pressure.

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**13.1 – Air Pressure and the Atmosphere**

The Earth’s atmosphere exerts a pressure of 101 kPa at sea level. The atmosphere is exerting more than 1000 N of force on the top of your head! Air pressure increases with the depth of the atmosphere, or with the altitude above sea level. Air pressure decreases as the altitude increases. Example: flying in a plane, riding in an elevator, driving up the mountains.

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**13.1 - Assessment What must you know to calculate pressure?**

Force and area What is the relationship between the depth of water and the pressure it exerts? Water pressure increases as depth increases. How is pressure distributed at a given level in a fluid? At a given depth, pressure is constant and exerted equally in all directions.

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Assessment How does the pressure exerted by the atmosphere change as altitude increases? Atmospheric pressure decreases as altitude above sea level increases. Why don’t you feel the pressure exerted by the atmosphere? Because the pressure inside your body balances the pressure outside.

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Assessment Some deep-sea fish have been known to explode as they are brought to the ocean’s surface. How do pressure changes cause this to happen? Deep-sea fish have high internal body pressures in order to counteract the extreme water pressure that exists where they live. When the fish is suddenly brought to the surface, where the pressure is much less, its internal pressure can cause it to rupture or explode.

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Assessment A 500 N student stands on one foot. A 750 N student stands on two feet. If both students wear the same size shoe, which exerts the greater pressure. The 500 N student exerts greater pressure than the 750 N student does

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Assessment A circus performer on a pair of stilts exerts a pressure of 32 kPa on the ground. If the performer stands on one stilt, what pressure does the stilt exert on the ground? The pressure exerted when standing on one stilt is twice the pressure exerted when on two stilts. Pressure = 2 x 32 kPa = 64 kPa

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Assessment A book with a weight of 12 N rests on its back cover. If the back cover measures 21 cm by 28 cm, how much pressure does the book exert? Pressure = force / area Convert centimeters to meters (100 cm = 1m) Pressure = 12 N / (0.21m x 0.28m) =12 N / m2 = 204 Pa

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**Using Pressure Differences to Move Objects**

What do you think will happen when high-speed air is blown between two strips of paper? Hold two strips of paper facing one another, about 10 centimeters apart. Blow gently between the strips of paper and observe what happens. Now blow forcefully between the strips of paper.

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**Using Pressure Differences to Move Objects**

What happens when gentle, low-speed air is blown between the strips of paper? What happens when high-speed air is blown between the strips of paper? Based on your observations, try to infer how the speed of air affects the air pressure acting on the inside and outside faces of the strips of paper.

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**13.2 – Transmitting Pressure in a Fluid**

According to Pascal’s principle, a change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid. Hydraulics is the science of applying Pascal’s principle.

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13.2 – Hydraulic Systems A hydraulic system is a device that uses pressurized fluid acting on pistons of different sizes to change a force. In a hydraulic lift system, an increased output force is produced because a constant fluid pressure is exerted on the larger area of the output piston.

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**13.2 – Bernoulli’s Principle**

According to Bernoulli’s principle, as the speed of fluid increases, the pressure within the fluid decreases. Examples: Wings and Lift Spray Bottles

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**13.2 – Bernoulli’s Principle**

Wings and Lift Air traveling over the top of a wing moves faster than the air passing underneath, which creates a low-pressure area above the wing. The pressure difference between the top of the wing and the bottom of the wing creates an upward force known as lift. Spray Bottles The fast moving water creates a low-pressure area at the top of the tube. The pressure differnce between the solution chamber and the tube forces the solution up the tube.

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**13.2 - Assessment Describe Pascal’s principle in your own words.**

A change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid. How is a hydraulic system able to increase force? The increased output force is produced because the fluid pressure is exerted on the larger area of the output piston.

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**13.2 - Assessment How are fluid speed and fluid pressure related?**

As the speed of the fluid increases, the pressure within the fluid decreases. How does an airplane wing produce lift? Lift of a wing is largely explained by Bernoulli’s principle. Air traveling faster over the top of the wing produces and area of reduced pressure. The resulting pressure difference between the top and bottom of the wing produces lift.

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Assessment Water is nearly incompressible. Air is easily compressed. Predict what will happen when an air-filled balloon is pulled to the bottom of a swimming pool. The water pressure increases with increasing depth in the pool, causing the air-filled balloon to be compressed.

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13.2 – Assessment The area of an output piston is 25 times greater than the area of the input piston. If the input force is 40 newtons, what is the output force? area of output piston / area of input piston = 25 output force = input force x 25 output force = 40 N x 25 = 1000 N

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Assessment Two table tennis balls are suspended from strings so they hang at the same height with a small space between them. When a hair dryer is used to blow air between the two balls, the balls come together and touch. Explain why this occurs. The moving air between the balls has a lower pressure than the surrounding, nonmoving air. The pressure difference forces the balls together.

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Assessment When cars pass one another in opposite directions on the highway, they tend to be forced together. Use Bernoulli’s principle to explain why this happens. Each moving car pulls a layer of air with it as it moves. When the cars pass, these two areas of moving, lower-pressure air meet, producing a pressure difference, which forces the cars toward each other.

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Buoyancy

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