13.3 Buoyancy. Buoyant Force

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1 The forces from pressure acting on the bottom of this golf ball are greater than those on the top. This produces a net force called the buoyant force that acts upward on the ball.

2 Buoyant Force What is the effect of buoyancy on the apparent weight of an object? Buoyancy is the ability of a fluid to exert an upward force on an object placed in it. Buoyancy results in the apparent loss of weight of an object in a fluid.

3 Buoyant Force Every object in a fluid experiences buoyancy. Water pressure increases with depth. Forces pushing up on the bottom of the object are greater than the forces from pressure pushing down on the top. This upward force, which acts in the opposite direction of gravity, is called the buoyant force.

4 Archeimedes Principle According to Archimedes principle, the buoyant force on an object is equal to the weight of the fluid displaced by the object. A submerged object pushes aside, or displaces, a volume of fluid equal to its own volume. A floating object displaces a volume equal to the volume of the part of the object that is submerged.

5 Density and Buoyancy How can you determine if an object will float or sink in a fluid? If an object is less dense than the fluid it is in, it will float. If the object is more dense than the fluid it is in, it will sink. When the buoyant force is equal to the weight, an object floats or is suspended. When the buoyant force is less than the weight, the object sinks.

6 Density and Buoyancy Two forces act on every object in a fluid weight and the buoyant force. The force of gravity, equal to the object s weight, acts downward on the object. The buoyant force, equal to the weight of the volume of displaced fluid, acts upward on the object.

7 Density and Buoyancy Suspended An object that has the same density as the fluid it is submerged in will be suspended (it will float at any level) in the fluid. The buoyant force acting on the suspended object exactly equals the object s weight. Submarines and some fish are able to suspend themselves in water partly by adjusting their density.

8 Density and Buoyancy Sinking When a ship s weight becomes greater than the buoyant force acting on it, the ship will sink. As a sinking ship takes on water, the ship displaces less water, and the buoyant force decreases.

9 Density and Buoyancy Floating A solid piece of steel sinks in water. A heavy steel ship floats because of the shape of its hull. The hull is shaped so that it displaces a large volume of water, creating a large buoyant force.

10 Density and Buoyancy The weight and buoyant force determine if an object sinks or floats. Weight Buoyant force

11 Density and Buoyancy The weight and buoyant force determine if an object sinks or floats. Weight Weight Buoyant force Buoyant force

12 Density and Buoyancy The weight and buoyant force determine if an object sinks or floats. Weight Weight Weight Buoyant force Buoyant force Buoyant force

13 Density and Buoyancy Objects also float more easily in dense fluids. The denser the fluid is, the greater is the weight displaced. The greater displaced weight results in a greater buoyant force. This is why it is easier for a person to float in very salty water. The dense salty water produces a larger buoyant force when displaced by the person's body.

14 Density and Buoyancy The exposed green and red stripes on the ship s hull indicate that the ship is riding high in the water. If the cargo ship were completely loaded, it would need to displace more water in order to float.

15 Assessment Questions 1. According to Archimedes principle, the buoyant force acting on an object is equal to a. the weight of the object. b. the pressure exerted by the fluid at the point of contact with the object. c. the pressure exerted at the bottom of the fluid. d. the weight of the fluid displaced by the object.

16 Assessment Questions 1. According to Archimedes principle, the buoyant force acting on an object is equal to a. the weight of the object. b. the pressure exerted by the fluid at the point of contact with the object. c. the pressure exerted at the bottom of the fluid. d. the weight of the fluid displaced by the object. ANS: D

17 Assessment Questions 2. How much fluid is displaced by an object that is floating, partially submerged, in a fluid? a. a volume of fluid equal to the volume of the object b. a volume of fluid equal to the volume of the submerged part of the object c. a mass of fluid equal to the mass of the object d. a mass of fluid equal to the mass of the submerged part of the object

18 Assessment Questions 2. How much fluid is displaced by an object that is floating, partially submerged, in a fluid? a. a volume of fluid equal to the volume of the object b. a volume of fluid equal to the volume of the submerged part of the object c. a mass of fluid equal to the mass of the object d. a mass of fluid equal to the mass of the submerged part of the object ANS: B

19 Assessment Questions 3. An unknown substance is suspended in water. What can be concluded about the substance s density? a. The substance s density is greater than water s density. b. The substance s density is less than water s density. c. The substance s density is equal to water s density. d. Nothing can be concluded about the substance s density.

20 Assessment Questions 3. An unknown substance is suspended in water. What can be concluded about the substance s density? a. The substance s density is greater than water s density. b. The substance s density is less than water s density. c. The substance s density is equal to water s density. d. Nothing can be concluded about the substance s density. ANS: C

21 Assessment Questions 4. A 10,000-N ship is floating in the Great Salt Lake, whose salty water is denser than pure water. What is the buoyant force acting on the ship? a N b N c. 10,000 N d. 12,000 N

22 Assessment Questions 4. A 10,000-N ship is floating in the Great Salt Lake, whose salty water is denser than pure water. What is the buoyant force acting on the ship? a N b N c. 10,000 N d. 12,000 N ANS: C