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Mass Weight And Density

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课程笔记

Mass & Weight

  • Mass is the quantity of matter in an object; it resists changes in motion (inertia).
  • Mass is a scalar quantity measured in kilograms (kg).
  • Weight is the gravitational force on an object; it is a vector measured in newtons (N).
  • Weight depends on the gravitational field strength (g): W = m × g.
  • On Earth, g ≈ 9.8 N/kg; on the Moon, g ≈ 1.6 N/kg – weight changes but mass stays the same.

Mass and Weight on Earth and the Moon

Mass and Weight on Earth and the Moon

Gravitational Field Strength

  • Gravitational field strength (g) is the force per unit mass: g = W / m.
  • It is measured in N/kg and is equivalent to the acceleration of free fall (9.8 m/s² on Earth).
  • The weight of an object is given by W = m × g.

Gravitational Field Strength

Gravitational Field Strength

Density

  • Density (ρ) is the mass per unit volume: ρ = m / V.
  • Units: kg/m³ or g/cm³ (1 g/cm³ = 1000 kg/m³).
  • Low-density materials (e.g., gases) have particles spread out; high-density materials (e.g., solids) have particles closely packed.
  • Volume of regular solids: cube = l³, cuboid = l × w × h, cylinder = πr²h, sphere = (4/3)πr³.

Volume Formulas for Common Shapes

Volume Formulas for Common Shapes

Measuring Density of Regular Solids

  • Measure mass using a digital balance.
  • Measure dimensions with a ruler, Vernier calipers, or micrometer.
  • Calculate volume from dimensions, then density = mass / volume.
  • Repeat measurements and take averages to reduce random error.

Equipment for Measuring Regular Solids

Equipment for Measuring Regular Solids

Measuring Density of Irregular Solids

  • Measure mass using a digital balance.
  • Use a displacement can (Eureka can) filled with water just below the spout.
  • Submerge the object; collect displaced water in a measuring cylinder.
  • Volume of displaced water = volume of object; then calculate density.

Eureka Can Setup

Eureka Can Setup

Measuring Density of Liquids

  • Measure mass of empty measuring cylinder, then add known volume of liquid.
  • Mass of liquid = (mass of cylinder + liquid) – mass of empty cylinder.
  • Density = mass of liquid / volume of liquid.
  • Zero the balance before each measurement to avoid systematic error.

Measuring the Density of a Liquid

Measuring the Density of a Liquid

Floating & Sinking

  • An object floats if its density is less than the fluid's density; sinks if greater.
  • Upthrust is an upward force on a submerged object, opposite to weight.
  • Floating occurs when upthrust = weight; sinking when upthrust < weight.
  • Upthrust depends on fluid density and volume of fluid displaced.

Upthrust and Weight on a Boat

Upthrust and Weight on a Boat

幻灯片

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练习题

免费预览——61 题中的 8 题。注册即可查看全部。
  1. 1.Which of the following is a measure of the quantity of matter in an object?

    Easy
    • AWeight
    • BMass
    • CDensity
    • DVolume
  2. 2.What is the unit of weight?

    Easy
    • Akilogram
    • Bnewton
    • Ckg/m³
    • Dm/s²
  3. 3.Mass is a vector quantity.

    Easy

    True or false?

  4. 4.State the equation that links weight, mass and gravitational field strength.

    Easy
  5. 5.An object has a mass of 4.5 kg. What is its mass in grams?

    Easy
    • A4950
    • B5400
    • C4050
    • D4500
  6. 6.A paving slab has a mass of 73 kg and dimensions 0.04 m × 0.5 m × 0.85 m. Calculate its density in kg/m³ (round to two significant figures).

    Hard
    • A4730
    • B4300
    • C5160
    • D3870
  7. 7.Which of the following correctly describes the relationship between density and floating?

    Easy
    • AAn object floats if its density is greater than the density of the fluid.
    • BAn object floats if its density is less than the density of the fluid.
    • CAn object floats if its density equals the density of the fluid.
    • DAn object always floats regardless of density.
  8. 8.Name the force that pushes upwards on an object submerged in a fluid.

    Easy

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