PHYS 9A Lecture

What are the three basic quantitative measurements?

Length, mass, and time.

What are the SI units for length, mass, and time?

Meter (m), kilogram (kg), and second (s).

What do the prefixes micro-, milli-, centi-, and kilo- mean?

Micro = 10⁻⁶; milli = 10⁻³; centi = 10⁻²; kilo = 10³.

What do the prefixes nano-, mega-, giga-, and tera- mean?

Nano = 10⁻⁹; mega = 10⁶; giga = 10⁹; tera = 10¹².

What is a conversion factor?

A ratio equal to 1 that changes units without changing the physical value.

Convert 4.50 g/cm³ to kg/m³.

4.50 × 10³ kg/m³.

How many significant figures are in 500 m, 0.005 m, and 0.0050 m?

500 m has 1; 0.005 m has 1; 0.0050 m has 2.

How can scientific notation clearly show significant figures?

500 m written as 5 × 10² m has 1 significant figure, while 5.0 × 10² m has 2.

How many significant figures do exact definitions have?

Infinite significant figures; for example, 2.54 cm = 1 in exactly.

What are the four basic states of matter?

Solid, liquid, gas, and plasma.

How do solids, liquids, and gases differ in shape and volume?

Solids have definite shape and volume; liquids have definite volume but not shape; gases have neither.

What is a fluid?

A substance that flows and conforms to the boundaries of its container; liquids and gases are fluids.

Why are gases compressible?

Gas molecules are far apart, allowing their spacing and the gas volume to decrease.

Why are liquids approximately incompressible?

Liquid molecules are already close together, leaving little empty space to remove.

What is the equation for density?

ρ = m/V.

What is the SI unit of density?

Kilograms per cubic meter (kg/m³).

What is the density of water at 4°C?

Approximately 1,000 kg/m³; water is most dense near 4°C.

How do the densities of most liquids compare with air?

Most liquids are approximately 1,000 times denser than air.

What is the equation for pressure?

P = F⊥/A.

What does F⊥ represent in the pressure equation?

The component of force perpendicular to the surface.

What is the SI unit of pressure?

The pascal: 1 Pa = 1 N/m².

Is pressure a scalar or vector?

Pressure is a scalar; it has magnitude but no direction.

What are the main atmospheric-pressure conversions?

1 atm = 1.013 × 10⁵ Pa = 101.3 kPa = 760 torr = 14.7 psi.

What is absolute pressure?

The total pressure measured relative to a perfect vacuum.

What is gauge pressure?

Pressure measured relative to atmospheric pressure.

What equation relates absolute and gauge pressure?

Pabsolute = Pgauge + Patm.

What absolute pressure corresponds to zero gauge pressure?

1 atm, or approximately 101.3 kPa.

A gauge reads 235 kPa. What is the absolute pressure?

235 kPa + 101.3 kPa = 336.3 kPa.

What does a negative gauge pressure mean?

The pressure is lower than atmospheric pressure.

What causes a container to collapse when its internal pressure is low?

The greater external atmospheric pressure creates a net inward force.

What factors determine pressure in a static fluid?

Surface pressure, fluid density, gravitational acceleration, and vertical depth.

Does hydrostatic pressure depend on container shape?

No. It depends on vertical depth, fluid density, and surface pressure.

What is the hydrostatic pressure-change equation?

ΔP = −ρgΔy.

Why is there a negative sign in ΔP = −ρgΔy?

Pressure decreases as vertical position increases and increases as vertical position decreases.

What equation is used when moving downward by a depth h?

P₂ = P₁ + ρgh.

What is the gauge pressure at depth h below an open liquid surface?

Pgauge = ρgh.

What is the absolute pressure at depth h below an open liquid surface?

Pabsolute = Patm + ρgh.

What is true of points at the same depth in the same connected static fluid?

They have the same pressure, regardless of container shape.

What is the pressure 0.60 m below an open water surface?

P = Patm + ρgh ≈ 1.072 × 10⁵ Pa, or 1.06 atm.

How does fluid density affect pressure change with depth?

A denser fluid produces a greater pressure change over the same depth.

What is the ideal-gas law?

PV = nRT.

What does each symbol in PV = nRT represent?

P = pressure; V = volume; n = moles; R = gas constant; T = absolute temperature.

What value of R is used in the lecture, and what temperature scale is required?

R = 8.31 J/(mol·K), and temperature must be in kelvin.

At constant temperature and fixed amount of gas, how are pressure and volume related?

They are inversely proportional: P₁V₁ = P₂V₂.

A diving bell becomes three-fourths full of water. What is the pressure of its trapped air?

The air occupies one-fourth its original volume, so its pressure increases from 1 atm to 4 atm.

How is the maximum mass supported by an ideal suction cup calculated?

Use ΔPA = mg, so mmax = ΔPA/g; for a 50.0 cm diameter cup with a vacuum inside, mmax ≈ 2.03 × 10³ kg.