Chapter 11 Forces in Fluids
Chapter 11 Level A
Pressure
Use Target Reading Skills
Sample questions and answers:
Q. Why does pressure change with elevation and depth?
A. The weight of the fluid above any point changes depending on elevation and depth.
Q. Why does pressure increase as depth below water increases?
A. The amount of water exerting pressure increases.
1. Snowshoes distribute a person’s weight over the large area of the snowshoes, resulting in less downward pressure on the snow compared to regular shoes. With less downward pressure, the person doesn’t sink into the snow.
2. false
3. the force exerted on a surface divided by the total area over which the force is exerted
4. d
5. a, c
6. false
7. fluid
8. a, b, d
9. In fluids, particles are constantly moving in all directions. They are constantly colliding with each other and with any surface they meet.
10. All the forces exerted by the individual particles in a fluid add together to make up the pressure exerted by the fluid.
11. area
12. atmospheric pressure
13. The weight of air, or the force of gravity on air’s mass, produces air pressure.
14. true
15. false
16. At the higher elevation, there is less air pushing down from above and therefore less weight to support than at a lower elevation.
17. true
18. barometer
Floating and Sinking
Use Target Reading Skills
Sample causes:
Weight is greater than buoyant force.
The object’s density is greater than the density of the fluid in which it is placed.
The object is compressed and becomes denser than the fluid.
1. density
2. Density = Mass ÷ Volume
3. 1.0 g/cm3
4. Left to right: b, a, c
5. false
6. more
7. Helium is less dense than air, and objects rise in air if their densities are less than air’s density. A balloon filled with air does not rise because the density of the air inside the balloon is not less than the outside air’s density.
8. By pumping water out, the submarine’s overall mass decreases. Since its volume remains the same, its density decreases when its mass decreases.
9. The air reduces the ship’s overall density, and so allows it to float on the water’s surface.
10. volume
11. buoyant
12. a, b, d
13. It displaces a volume of fluid equal to the object’s own volume.
14. The buoyant force on an object is equal to the weight of the fluid displaced by the object.
15. false
16. The object will remain suspended at a constant level within the fluid.
Pascal’s Principle
Use Target Reading Skills
Possible steps in the sequence:
Step 1: The driver pushes the brake pedal.
Step 2: The brake pedal pushes on a small piston.
Step 3: The piston exerts pressure on the brake fluid.
Step 4: The increased pressure is transmitted through the fluid in the brake lines to larger pistons in the wheels of the car.
Grade 8 Physical Science ANSWER KEY
Step 5: The larger pistons push on the brake pads.
Step 6: The brake pads rub against the brake disc.
Step 7: Friction slows down the motion of the wheels.
1. The pressure in the water increases everywhere in the bottle.
2. When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.
3. Both pistons will experience the same force.
4. The larger piston will experience a force twenty times larger.
5. The increase in pressure on a small surface area is transmitted to another part of a confined fluid, which pushes on a larger surface area.
6. true
Bernoulli’s Principle
Use Target Reading Skills
Possible steps in the sequence:
Step 1: The fire burns the wood and produces smoke.
Step 2: Smoke begins to rise in the chimney because warm air rises.
Step 3: Wind blowing across the top of the chimney lowers the air pressure there.
Step 4: The air pressure at the bottom of the chimney is higher than at the top.
Step 5: The higher air pressure at the bottom of the chimney pushes the smoke upward.
Step 6: The smoke exits the chimney and blows away.
1. false
2. As the speed of a moving fluid increases, the pressure exerted by the fluid decreases.
3. true
4. The moving air above the tissue paper exerts less pressure than the air below the paper.
The greater pressure below the paper pushes it upward.
5. true
6. upward
7. downward
8.
9. pressure
10. Lift is an upward force on a wing, created by a difference in pressure.
11. Both are curved on top.
12. Wind blowing across the top of a chimney lowers the air pressure there. The higher pressure at the bottom then pushes air and smoke up the chimney.
13. Squeezing the bulb causes air to move quickly past the top of the tube. The moving air lowers the air pressure at the top of the tube.
14. false
Chapter 11 Forces in Fluids
Chapter 11 Level B
Pressure
1. a. pascal b. force
2. b, c
3. a. fluid b. particles
4. true
5. balanced
6. C
7. A
8. b
Floating and Sinking
1. c
2. a. Sinks b. Floats
3. b, a
4. buoyant force
5. a, b
6. a. Sinks b. Floats
7. b
8. false
Pascal’s Principle
1. a. fluid b. Pascal’s principle
2. a. Force b. Pressure c. Fluid
3. a, c
4. truePhysical Science ANSWER KEY
Bernoulli’s Principle
1. false
2. c
3. low
4. a
5. a. Faster b. Slower