Ch. 6

The four rules that describe how pressure distributes through a fluid at rest.

1. Pressure depends on depth. Greater depths have greater pressure. 2. Pressure is the same for all points at the same depth. 3. Pressure at a given depth is independent of direction. 4. Pressure is always perpendicular to the surface of a submerged object.

The base of a dam is reinforced in case the pressure increases in the future. Which of the following would cause such an increase?

A change in climatic patterns that would increase the lake's depth over the next several years.

Which exerts the largest amount of force on the ground?

An identical swimming pool full to the exact same level with a 50-lb. block sitting on the bottom.

3. Pressure at a given depth is independent of direction.

At any given point, the pressure measured from any direction is the same.

Identify the following as an unbounded or bounded fluid.

Can of shaving cream=bounded. Bowl of water=unbounded Fluid in a hydraulic system=bounded Mountain lake=unbounded Venus' atmosphere=unbounded

Lava lamps provide an example of _____ currents.

Convection

How do convection currents form?

Convection currents begin with a fluid that has different temperatures within it. If a fluid has different temperatures then it also has different densities. The less dense sections will tend to rise and the more dense sections will tend to sink. A source heats the dense sections while they sink and makes them less dense. Meanwhile, the less dense sections rise and tend to cool and become more dense. Once the rising fluid has become more dense than its surrounding, it begins to sink. Likewise, once the fluid that sinks becomes less dense than its surroundings, it begins to rise. The fluids follow this pattern of rising and cooling and of sinking and heating, creating convection currents.

In grade school, you are told that density determines whether an object will float or sink. In this course, you are told to compare buoyant and gravitational forces. Explain how the two are equivalent.

Density is just the amount of mass in a certain volume. If an object has less mass than the same volume of fluid, then when it is submerged in fluid, it will displace a mass of fluid greater than its own mass. The buoyant force will be bigger than the gravitational force, and it will rise to the surface. Similarly, if it is more dense than the fluid, it weighs more than the volume of fluid it displaces and will sink.

Pressure in an unbounded fluid increases with ____.

Depth

A man standing in a swimming pool weights less because water partially shields him from the full force of gravity.

FALSE

If an object is floating, the buoyant force on it is larger than its weight.

FALSE

Objects that float in air, such as helium balloons, are weightless.

FALSE

Explain why understanding the motion of a body on or inside a fluid almost always involves balancing two or more forces.

Forces in fluids change with speed and depth. Even when the forces start off unbalanced, they change as the object accelerates until they end up balanced.

4. Pressure is always perpendicular to the surface of a submerged object.

If there is ever a net sideways force in fluid, the fluid will flow until it balances out. Once balanced out, the fluid only exerts a force perpendicular to the surface of a submerged object.

2. Pressure is the same for all points at the same depth.

Pressure does not depend on surface area or volume. All points at the same depth will have the same amount of pressure on them.

1. Pressure depends on depth. Greater depths have greater pressure.

Pressure equals the weight of the column of fluid directly above it. As depth increases, the weight of the column also increases, creating greater pressure.

Scuba divers can get the bends if they change pressure too rapidly. Would it be more dangerous to surface rapidly from a depth of 40 ft. in the ocean, or a depth of 40 ft. in a narrow pool formed in an old quarry?

Pressure for a fluid depends on the weight of the column of fluid above that point. If you are at the same depth, the weight of the column of water above you remains the same regardless of whether you are in the ocean ,in a quarry, or in a narrow irrigation pipe.

How do contact forces and pressure give rise to the buoyant force?

Pressure pushes on the bottom of a submerged object more than on the top of a submerged object. The net upward contact force is buoyancy.

Explain why pressure is a useful way of talking about forces exerted by fluids.

Pressure=Force/Area. The pressure is the same everywhere at the same depth in a fluid, while the total force depends on the volume of an object placed into the fluid. Everyone and everything on the surface of Earth has the same air pressure, but the force on each object from the air is unique.

If an object is completely submerged, the buoyant force is equal to the weight of ____. If it is floating, the buoyant force is equal to ____.

The displaced liquid. The weight of the floating object.

In convection currents, warmer air rises and cooler air descends. Which of the following is correct?

The warmer air has a lower density than the cooler air which it displaces.

Air is considered a fluid.

True

Frictional forces increase when an object moves faster through a fluid.

True

If you apply a specific force over a larger area, the pressure decreases.

True

The buoyant force on an object submerged in a fluid can be changed by

changing the volume of the object without changing its weight AND changing the density of the fluid. (NOT by changing the weight of the object without changing its volume.)

If a block of wood floats with half its volume submerged,

the weight of the block is the same as that of the displaced water.