3A5 Weight, Mass, and Density

Question 1

Define:

Mass

Answer 1

The amount of matter in an object.

Mass is constant regardless of location.

Question 2

What are the common units of mass?

Answer 2

Some common units used are:

• Grams (g)
• Kilograms (kg)
• Pounds (lbs)

Mass can be measured in various units depending on the system used.

Question 3

Define:

Weight

Answer 3

The downward force acting upon an object due to gravity.

Weight varies based on the strength of gravity at different locations. The equation is represented by W=mg, where m is equal to mass and g is equal to the force of gravity.

Question 4

What is the difference between mass and weight?

Answer 4

• Mass is the amount of matter in an object.
• Weight is the gravitational force acting on that mass.

Mass is measured in kilograms (kg) and weight in newtons (N).

Question 5

What misconception do people often have about weight and mass?

Answer 5

People often use “weight” and “mass” interchangeably, though they are distinct concepts.

Weight depends on gravity, while mass is intrinsic to the object.

Question 6

True or false:

Mass remains constant, but weight changes depending on gravity.

Answer 6

True

Weight depends on the local gravitational acceleration (e.g., it’s lower on the Moon).

Question 7

Fill in the blank:

Weight is a _____ quantity, while mass is a scalar quantity.

Answer 7

vector

Weight has both magnitude and direction, but mass only has magnitude.

Question 8

Why is weight considered a force?

Answer 8

Because it is the result of gravitational acceleration acting on mass.

Weight follows F=mg,
where “F” is force (i.e., weight), “m” is mass and “g” is gravitational acceleration.

Question 9

What is the SI unit of weight?

Answer 9

Newton (N)

Weight is a force.

Question 10

Calculate the weight of a 10 kg object on Earth.

Answer 10

Weight = 10kg × 9.8m/s² = 98N

For this equation, use g = 9.8 m/s² to represent the acceleration due to gravity.

Question 11

True or false:

An object’s mass changes if it’s taken to the Moon.

Answer 11

False

Mass remains constant regardless of location; only weight changes.

Question 12

How can weight vary even if mass remains constant?

Answer 12

Weight changes due to variations in gravitational acceleration, such as on different planets.

g depends on the mass and radius of the celestial body.

Question 13

How does weight vary on the Moon compared to Earth?

Answer 13

Weight on the Moon is approximately 1/6 of the weight on Earth due to weaker gravity.

The Moon’s gravitational acceleration is about 1.63m/s²

Question 14

What happens to a person’s weight in outer space?

Answer 14

It becomes negligible because they are in free fall, experiencing microgravity, even though their mass remains unchanged.

Weight depends on gravitational force, which is much weaker in outer space.

Question 15

True or false:

Weightlessness means the absence of mass.

Answer 15

False

Weightlessness occurs when there is no force supporting the object, but mass remains constant.

Question 16

Why do astronauts experience weightlessness in space?

Answer 16

Because they are in free-fall along with their spacecraft, constantly falling towards Earth but never hitting it.

Weightlessness occurs when no normal force counters gravity, as in orbiting spacecraft.

Question 17

Does weightlessness mean zero gravity?

Answer 17

No - gravity still acts, but there’s no support force to resist it.

Gravity is still acting on the object, but its effects are not felt due to free-fall conditions.

Question 18

How does the weight of an object vary at the poles and the equator?

Answer 18

Weight is slightly higher at the poles due to stronger gravitational acceleration.

Earth’s shape causes gravity to be weaker at the equator.

Question 19

Why do objects weigh less at higher altitudes?

Answer 19

Gravitational acceleration decreases as the distance from Earth’s center increases.

The effect is negligible for most everyday altitudes but noticeable on mountains.

Question 20

Fill in the blank:

On Jupiter, a person’s weight would ______ compared to Earth

Answer 20

increase

Jupiter’s stronger gravity increases the weight of the same mass.

Question 21

Give an example of how weight is used in designing safety equipment.

Answer 21

Parachutes are designed to counteract weight with air resistance for safe descent.

Understanding weight helps optimize safety designs and material strength. Weight affects helmets, airbags, and parachutes to reduce impact forces.

Question 22

What happens to an object’s weight when submerged in water?

Answer 22

The apparent weight decreases due to the buoyant force of the displaced water.

Weight measured underwater is less than actual weight in air.

Question 23

What device is used to measure mass rather than weight?

Answer 23

A balance or beam scale.

Balances compare masses directly, independent of gravitational variations.

Question 24

How does a scale measure weight?

Answer 24

A scale measures the gravitational force acting on an object and converts it into a weight reading.

Digital and mechanical scales use different mechanisms but rely on W=mg.

Question 25

How do **spring scales** work to measure weight?

Answer 25

They measure the **extension of a spring** proportional to the gravitational force acting on the object. ## Footnote Hooke’s Law is applied: F=kx.

Question 26

# Define: Density

Answer 26

Measure of how much mass is contained in a given volume, calculated as mass divided by volume (**𝜌=M/v**) ## Footnote Density helps compare how compact different substances are.

Question 27

# Define: Volume

Answer 27

Amount of three-dimensional **space occupied** by an object or substance, measured in units like cubic meters (m³), liters (L), or milliliters (mL). ## Footnote Volume can be calculated differently depending on the object's shape, e.g., using geometry for regular shapes or displacement methods for irregular ones.

Question 28

What are the common **units** of **density**?

Answer 28

Kilograms per cubic meter (**kg/m³**) in the SI system and grams per cubic centimeter (**g/cm³**) in the CGS system. ## Footnote For liquids, density is often expressed in **g/mL** or **kg/L**.

Question 29

# True or false: Objects with the same **mass** always have the same **density**.

Answer 29

False ## Footnote Density depends on both mass and volume.

Question 30

Calculate the **density** of an object with a mass of 200g and a volume of 50cm³.

Answer 30

Density = 200g/50cm³ = **4g/cm³** ## Footnote Ensure units are consistent when calculating density.

Question 31

What happens to the density of an object if its **volume decreases** but mass remains constant?

Answer 31

Density increases. ## Footnote Density is inversely proportional to volume.

Question 32

# Fill in the blank: **Less dense** objects \_\_\_\_\_\_ in a denser fluid.

Answer 32

float ## Footnote **Buoyancy** depends on relative densities of the object and the fluid.

Question 33

# Define: Buoyant force

Answer 33

The **upward force** exerted by a fluid on a submerged or floating object. ## Footnote The magnitude of buoyant force equals the weight of the fluid displaced by the object.

Question 34

How does the **buoyant force** depend on the **density** of the fluid?

Answer 34

Higher fluid density results in a **greater buoyant force**. ## Footnote Buoyant force is expressed as Fb=𝜌𝑉g, where 𝜌 is the fluid density, V is displaced volume, and g is gravity.

Question 35

# True or false: The **buoyant force** always acts **upward**.

Answer 35

True ## Footnote Buoyant force counteracts gravity and acts perpendicular to the fluid’s surface.

Question 36

What happens to the **buoyant force** when an object is **fully submerged** compared to partially submerged?

Answer 36

The buoyant force is **greater** when the object is fully submerged. ## Footnote Fully submerged objects displace a larger volume of fluid.

Question 37

How does Archimedes' principle apply to **ship design**?

Answer 37

Ships are designed to **displace water** equal to their **weight**, ensuring they float. ## Footnote The upward **buoyant force** must balance the ship’s weight to stay afloat.

Question 38

How does the principle of **buoyancy** apply to **submarines**?

Answer 38

Submarines adjust their buoyancy by **controlling the amount of water** in ballast tanks to sink or rise. ## Footnote This is achieved by changing the overall density of the submarine.

Question 39

Why do **icebergs float** on water?

Answer 39

Ice is **less dense** than water, so it displaces enough water to equal its weight before fully submerging. ## Footnote Approximately 90% of an iceberg is submerged.

Question 40

How does **Archimedes’ principle** explain the working of a **hydrometer**?

Answer 40

A hydrometer **floats** at different levels depending on the liquid’s **density**. ## Footnote The depth of immersion changes with the liquid’s buoyant force.

Question 41

Why do **life jackets** help people stay **afloat**?

Answer 41

They increase the wearer’s overall volume, decreasing density and **increasing buoyancy**. ## Footnote The displaced water’s buoyant force supports the person’s weight.

Question 42

How is **Archimedes’ principle** used in **metal purity** testing?

Answer 42

By measuring the **volume** of water displaced, the density of the metal can be calculated to check purity. ## Footnote This method is often used to test gold and other precious metals.

Question 43

# True or false: A **steel ship** floats because it has a lower density than water.

Answer 43

True ## Footnote The overall density of the ship (including air inside) is less than water.

Question 44

Why do **helium balloons** rise in air?

Answer 44

Because helium has a **lower density** than air. ## Footnote Buoyant force exceeds the balloon’s weight.

Question 45

# True or false: A denser object will always **sink** in water.

Answer 45

False ## Footnote If the object’s shape increases its volume enough, it can still float (e.g., ships).

Question 46

**Compare the density** of a liquid and a solid with the same mass but different volumes.

Answer 46

The **solid has higher density** due to its smaller volume. ## Footnote Density increases as volume decreases for constant mass.

Question 47

Why is **density** an important property in **material selection**?

Answer 47

It determines the material’s suitability for applications like **buoyancy, construction, or strength**. ## Footnote High-density materials are often used for strength, while low-density materials are used for lightness.

Question 48

Why does a **hot air balloon** rise?

Answer 48

The heated air inside the balloon is **less dense** than the cooler surrounding air, creating buoyancy. ## Footnote The buoyant force exceeds the weight of the balloon.

Question 49

What happens if an object’s **density** equals the density of the fluid it’s in?

Answer 49

The object remains **suspended** at the same level in the fluid. ## Footnote The forces of gravity and buoyancy balance each other.

Question 50

# True or false: **Weight** is only dependent on the object's **density** and not its volume.

Answer 50

False ## Footnote Weight depends on both the object's density and volume, as Weight = Density × Volume × g.

Question 51

# Fill in the blank: Objects with higher \_\_\_\_\_\_ generally **weigh** more for the same volume.

Answer 51

density ## Footnote Weight is directly proportional to density for a given volume, as Weight=Density×Volume×g.

Question 52

How does the **density** of a material affect the design of weighing **scales**?

Answer 52

Scales must account for density to ensure the material's **weight corresponds to its mass** accurately, especially in fluids or uneven terrains. ## Footnote **Buoyant forces** acting on less dense materials can affect readings, requiring calibration.

Question 53

Why do **denser objects sink** while less dense objects float in a liquid?

Answer 53

Denser objects sink because they **cannot displace** enough liquid to counteract their weight. ## Footnote The buoyant force depends on the liquid's density and the displaced volume of the object.

Question 54

# Fill in the blank: A \_\_\_\_\_\_ is commonly used to **measure the density** of liquids by floating in them.

Answer 54

hydrometer ## Footnote A hydrometer determines liquid density based on the buoyant force exerted on it.

Question 55

# True or false: A **pycnometer** can measure the density of both solids and liquids.

Answer 55

True ## Footnote A pycnometer calculates density by comparing the mass of a known volume of the material with that of a reference liquid.

Question 56

How does a **digital density meter** measure density?

Answer 56

It uses the **oscillation principle** to calculate density with high precision. ## Footnote The device measures the change in oscillation frequency caused by the sample's mass in a vibrating tube.