1. Buoyancy and Density

2. Predict whether the following objects will float or sink in water: • an apple • an orange • a penny • a nail True or False • The shape of an object helps determine if it will float • Steel will not float • Floating depends on weight

3. Imagine that you pushed ice in a glass down with a straw, what makes it come straight back to the surface? • Buoyant force is the upward force that keeps an object immersed in or floating on a liquid. • Buoyant force is caused by differences in fluid pressure.

4. Read the paragraph ‘Buoyant force and fluid pressure” on page 412 Why is the pressure at the bottom of an object in a fluid greater than the pressure at the top of the object?

5. Calculating Buoyant Force • Archimedes’ principle = the buoyant force on an object in a fluid is an upward force equal to the weight of the volume of fluid that the object displaces.

6. Weight vs. the Buoyant Force • An object in a fluid will sink if the object’s weight is greater than the buoyant force (the weight of the fluid that the object displaces). • An object floats only when the buoyant force on the object is equal to the object’s weight. • Examine Figure 3 on page 414 • What happens if the buoyant force is greater than the weight of the object?

7. Density is mass per unit volume. • Any object that is denser than the surrounding fluid will sink. • An object that is less dense than the surrounding fluid will float. • To determine the density of an object, you need to know the object’s mass and volume. • What are the three ways to determine volume?

8. Changing Density • The overall density of an object can be changed by changing the object’s shape, mass, or volume. • Most bony fishes have an organ called a swim bladder that allows them to adjust their overall density. • An inflated swim bladder increases the fish’s volume, which decreases the fish’s overall density so the fish does not sink.

10. Summary • Examine Figure 5 on page 417 • Explain why the boat made out steel floats while the block of steel sinks

11. Why can a duck float on water but not on air? • Would you feel more pressure 5 m underwater in a pool, or 2 m underwater in a lake? Explain your answer. • In a closed soda bottle, the force on the cap due to the carbonation of the soda is 16 N. If the area of the bottle cap is 8 cm2, what is the pressure on the cap?

12. Why does water exert more pressure than air does? a. because water is cooler than air c. because water is less dense than air b. because water is a liquid d. because water is denser than air • What is the equation for pressure? a. pressure = area/force c. pressure = mass/volume • pressure = force/area d. pressure = volume/mass • Fluid pressure is exerted evenly in all directions, which explains why a. some objects float and some sink. b. birds and airplanes can fly. c. bubbles are round. d. objects seem to weigh more in air than in water.

13. If a fluid flows from area A to area B, then area B must be an area of lower a. temperature. c. volume. b. mass. d. pressure. • How does a fluid flow? a. from low pressure areas to high pressure areas b. from high pressure areas to low pressure areas c. from low density areas to high density areas • from liquids to solids • What is the SI unit for pressure? a. pound c. pascal • meter d. Calorie • What is a fish’s swim bladder? • an organ that helps it swim quickly • an organ that allows it to adjust its density • an organ that makes it fly