ScienceCBSE Class 8 · Exploring Forces

Weight, Mass, Spring Balance, Floating and Sinking: Class 8 Science

Q: Why is the weight on a spring balance marked in both N and g?

The balance has one scale for weight in newton and another for the corresponding mass in gram, marked on the assumption that it is used on the Earth.

Q: Can we weigh an object to find its mass?

For practical purposes yes, because weight is almost the same everywhere on the Earth, but science keeps the two ideas separate.

Q: Why does the mug feel lighter in water?

Water exerts an upward force, the upthrust, on it.

Q: What decides whether an object sinks?

If its weight is greater than the upthrust it sinks.

Updated 17 October 2026 · Fact-checked

Weight is the force with which the Earth pulls an object; it is measured in newton with a spring balance. Mass is the amount of matter, measured in gram or kilogram, and does not change from place to place, whereas weight does. A liquid pushes an object upwards with an upthrust equal to the weight of liquid displaced; an object floats when this balances its weight and sinks if its weight is greater.

The mass stays 1 kg everywhere, but the weight, the pull shown by the red arrow and the spring stretch, changes from planet to planet.

Illustration drawn for QuestionBuddy.

Understand Weight, Mass, Spring Balance and Floating and Sinking

Weight. The force with which the Earth pulls an object towards itself is the weight of the object. It is a force, so its SI unit is the newton (N). When different objects (a pencil box, a tiffin box, a small stone) hang from the same spring, the stretch is different, so the Earth pulls different objects with different forces; weights differ.

Spring balance. A spring fixed at one end with a hook at the other; the stretch shows the weight on a scale in newton, and a second scale shows the corresponding mass in gram (marked assuming use on the Earth). Look at the balance carefully before using it.

Range: the maximum weight it can measure; the balance in Fig. 5.13 reads 0 to 10 N. Never hang an object heavier than this or the balance may be damaged.
Smallest reading: weight difference between two big marks divided by the number of small divisions between them. In the chapter 1 N is split into 5 divisions, so one division is 1 ÷ 5 = 0.2 N.
Hang the object from the hook, let it settle and read the scale.

Mass versus weight.

Mass: amount of matter; measured in g or kg; same everywhere.
Weight: gravitational force by the Earth (or other planet); measured in N; changes slightly from place to place on the Earth and a lot between planets.

The chapter's table for a 1 kg object gives the weight as 10 N on the Earth, 1.6 N on the Moon, 3.8 N on Mars, 9 N on Venus and 25.4 N on Jupiter. The mass stays 1 kg in each case. In daily life people say 'the weight of the wheat bag is 10 kg'; in science that is the mass, and the correct terms and units should be used. A beam balance finds mass by comparison with a known mass.

Floating and sinking. Gravity pulls every object downward, yet some objects float. Push an empty closed bottle into water: you feel an upward push and the bottle bounces up. All liquids apply such an upward force, called upthrust or buoyant force. On an object in a liquid, gravity acts downward and the buoyant force upward. If gravity is greater than the buoyant force the object sinks; if they are equal it floats. The buoyant force depends, among other things, on the density of the liquid (density is in a later chapter).

Archimedes' principle. When an object is fully or partly immersed in a liquid, it experiences an upward force equal to the weight of the liquid it displaces. If the weight of liquid displaced is less than the weight of the object, the object sinks; if it is equal, the object floats. Pumice is a light, porous volcanic rock whose air pockets make it less dense than water, so it floats.

The earlier topic gives types of forces including gravity.

Weight, Mass, Spring Balance and Floating and Sinking: formulas and key facts

Weight: Force of Earth's pull on an object | SI unit: newton (N); Measured with a spring balance.
Smallest reading of a spring balance: (Value between two big marks) ÷ (Number of small divisions between them); Chapter example: 1 N ÷ 5 = 0.2 N. Range example: 10 N.
Weight of 1 kg in the chapter table: $E a r t h 10 N ∣ M oo n 1.6 N ∣ M a r s 3.8 N ∣ V e n u s 9 N ∣ J u p i t er 25.4 N$; Mass is 1 kg everywhere.
Floating and sinking rule: Weight greater than upthrust: sinks | Weight equal to upthrust: floats; Upthrust is also called buoyant force.
Archimedes' principle: $U pt h r u s t = w e i g h t o f l i q u i dd i s pl a ce d$; Greek scientist Archimedes.

How to solve Weight, Mass, Spring Balance and Floating and Sinking questions

Use this method for weight, spring balance and floating questions.

1For spring balance reading: find range, then smallest division = value between big marks ÷ number of divisions.
2Read the pointer: number of big marks plus (small divisions × smallest value).
3To find the weight on another body, use weight per kg from the table and multiply by the mass in kg (assuming the table values).
4Keep mass unchanged and change only the weight when moving between planets.
5For floating, compare gravity (down) with upthrust (up).
6Write the unit: N for weight, kg or g for mass.

Weight, Mass, Spring Balance and Floating and Sinking in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Reading a spring balance

Suppose a school spring balance has a range of 0 to 10 N, with big marks every 1 N and 5 small divisions between each pair. A stone hangs from it and the pointer rests 3 small divisions after the 4 N mark.

(a) Find the smallest reading. (b) Find the weight of the stone. (c) Can the balance weigh an object of 1.2 kg? (take 1 kg as 10 N). (d) How many small divisions are there in the whole scale?

Try it first, then show the solutionSolution

Smallest reading = 1 ÷ 5 = 0.2 N.
Weight = 4 + 3 × 0.2 = 4 + 0.6 = 4.6 N.
1.2 kg weighs 1.2 × 10 = 12 N, which is more than the 10 N range, so it cannot.
Divisions = 10 big intervals × 5 = 50.

Answer: (a) 0.2 N; (b) 4.6 N; (c) no, 12 N exceeds the range; (d) 50.

Memory hook: Big mark plus little steps.

Weight on other worlds

Imagine a rock sample of mass 5 kg is carried from the Earth to the Moon and to Jupiter. Use the chapter's per-kg values (Earth 10 N, Moon 1.6 N, Jupiter 25.4 N).

(a) Find its weight on each. (b) What is its mass on the Moon? (c) By what percentage is the Moon weight less than the Earth weight?

Try it first, then show the solutionSolution

Earth: 5 × 10 = 50 N. Moon: 5 × 1.6 = 8 N. Jupiter: 5 × 25.4 = 127 N.
Mass stays 5 kg.
(50 − 8) ÷ 50 × 100 = 42 ÷ 50 × 100 = 84%.

Answer: (a) 50 N, 8 N, 127 N; (b) 5 kg; (c) 84% less.

Memory hook: Mass travels unchanged; weight changes with the planet.

Tricks to remember Weight, Mass, Spring Balance and Floating and Sinking

Short memory aids. Say each one aloud once, then use it on a question the same day.

Mass = Matter, Weight = Pull
M for Matter and Mass; W for Weight and Whose pull (Earth's).
Use it when: Mass versus weight.
Equal floats, more sinks
Weight equals upthrust: floats. Weight beats upthrust: sinks.
Use it when: Floating question.
Division by divisions
Smallest reading = gap between big marks ÷ number of small divisions.
Use it when: Spring balance reading.

Weight, Mass, Spring Balance and Floating and Sinking: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Mass versus weight · 3 marks · Short answerBoard level

Distinguish between mass and weight with respect to definition, unit and change with place.

Try it first, then show the solutionSolution

Mass is the amount of matter; weight is the gravitational force with which the Earth (or another planet) pulls the object. (1)
Unit: mass in g or kg; weight in newton. (1)
Mass is the same everywhere; weight changes slightly over the Earth and a lot on other planets. (1)

Answer: Mass: amount of matter, kg, constant; weight: pull of gravity, N, varies with place.

Example 2: Spring balance smallest division · 1 mark · MCQBoard level

On a spring balance, the gap between 0 N and 1 N has 5 small divisions. The smallest value it can read is
(a) 0.1 N
(b) 0.2 N
(c) 0.5 N
(d) 5 N

Try it first, then show the solutionSolution

1 N ÷ 5 = 0.2 N.

Answer: (b)

Example 3: Weights on the Earth and the Moon · 5 marks · Long answerHarder board level

Use 10 N per kg on the Earth and 1.6 N per kg on the Moon.
(a) Find the weight of a 2.5 kg object on each. (2)
(b) What is its mass on the Moon? (1)
(c) An astronaut has weight 700 N on the Earth. Find the astronaut's mass and weight on the Moon. (2)

Try it first, then show the solutionSolution

(a) Earth: 2.5 × 10 = 25 N. Moon: 2.5 × 1.6 = 4 N. (2)
(b) 2.5 kg, because mass does not change. (1)
(c) Mass = 700 ÷ 10 = 70 kg. Moon weight = 70 × 1.6 = 112 N. (2)

Answer: (a) 25 N and 4 N; (b) 2.5 kg; (c) 70 kg; 112 N.

Example 4: Why do some objects float · 4 marks · Case-basedHarder board level

Read the passage and answer.

A coin dropped in water sinks, while a larger wooden block floats. A closed empty bottle pushed into water bounces back up. Pumice floats although it is a rock.
(a) Name the force acting upwards on each object. (1)
(b) Compare the weights and upthrusts for the sinking coin and the floating block. (1)
(c) How does Archimedes' principle describe the upthrust? (1)
(d) Why does pumice float? (1)

Try it first, then show the solutionSolution

(a) Upthrust or buoyant force from the water. (1)
(b) Coin: weight is greater than the upthrust, so it sinks. Block: the upthrust equals its weight, so it floats. (1)
(c) Upthrust equals the weight of the liquid displaced by the object. (1)
(d) Pumice has air pockets from trapped gas, making it less dense than water. (1)

Answer: (a) Upthrust; (b) coin weight > upthrust, block equal; (c) equals weight of displaced liquid; (d) air pockets make it less dense.

Example 5: Same size, different depths · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Three objects 1, 2 and 3 of the same size and shape are made of different materials. They float in water with object 1 dipping the least, object 2 more, and object 3 the most. A student argues that since 1 dips least it must be the heaviest.
(a) For a floating object, how do weight and upthrust compare? (1)
(b) If all three float, how does the weight of each compare with the weight of the water it displaces? (1)
(c) Arrange the three in increasing order of weight and give the reason. (1)
(d) Correct the student's argument. (1)

Try it first, then show the solutionSolution

(a) They are equal. (1)
(b) Each weighs the same as the water it displaces. (1)
(c) Same size and shape: the more it dips, the more water it displaces, so the heavier it is. Object 1 < 2 < 3 in weight. (1)
(d) Dipping least means displacing least water, so the lightest, not the heaviest. (1)

Answer: (a) Equal; (b) equal to displaced water's weight; (c) 1 < 2 < 3; (d) object 1 is the lightest.

Where students lose marks in Weight, Mass, Spring Balance and Floating and Sinking

The slips examiners see most often, and what to write instead.

Mistake: Giving the unit of weight as kilogram.
Everyday language mixes mass and weight.
Wrong working
Weight of the bag = 5 kg.
Right working
Mass of the bag = 5 kg; weight = 50 N on the Earth (10 N per kg).
Do this: kg for mass, N for weight.
Usually costs: 1 mark.
Mistake: Saying mass changes on the Moon.
The scale reading changes.
Wrong working
Mass on the Moon is 1.6 kg.
Right working
Mass stays 1 kg; weight is 1.6 N.
Do this: Matter does not change.
Usually costs: 1 mark.
Mistake: Misreading a spring balance by ignoring the least count.
Students count divisions as 1 N each.
Wrong working
4 N + 3 divisions = 7 N
Right working
4 N + 3 × 0.2 N = 4.6 N
Do this: Find the value of one division first.
Usually costs: 1 mark.
Mistake: Saying only light objects float.
Pumice and large ships surprise students.
Wrong working
Heavy objects always sink.
Right working
An object floats when upthrust equals its weight; pumice floats because it is less dense than water.
Do this: Compare weight and upthrust.
Usually costs: 1 mark.

Exam tips

Write the smallest reading of any instrument with its working.
Use N for weight and kg or g for mass, and say which you mean.
In planet problems keep mass the same and multiply by the planet's per-kg value.
For floating, name both forces and their directions.
Quote Archimedes' principle with 'fully or partially immersed' and 'weight of the liquid displaced'.

Practice questions on Weight, Mass, Spring Balance and Floating and Sinking

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
Name the SI unit of weight and the instrument used to measure it.
Hint
Weight is a force.
Answer
Newton; spring balance.
Q2 · 1 mark · MCQBoard level
The weight of an object on the Moon compared with the Earth is
(a) the same
(b) smaller because the Moon's gravitational pull is less
(c) greater
(d) zero
Hint
Chapter table: 1.6 N against 10 N.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A 1 kg object has the same mass on Mars and on Venus but different weights.
Reason (R): Weight depends on the gravitational force of the planet, while mass is the amount of matter.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Mars 3.8 N, Venus 9 N per kg.
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
A spring balance has range 0 to 20 N, with big marks every 2 N and 10 small divisions between them. (a) Find the smallest reading. (b) The pointer is at 6 N plus 7 small divisions: find the weight. (c) Find the mass of the object on the Earth (take 10 N per kg). (d) What would its weight be on the Moon (1.6 N per kg)?
Hint
Smallest reading = 2 ÷ 10.
Answer
(a) 2 ÷ 10 = 0.2 N. (b) 6 + 7 × 0.2 = 6 + 1.4 = 7.4 N. (c) 7.4 ÷ 10 = 0.74 kg. (d) 0.74 × 1.6 = 1.184 N, about 1.18 N (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A shopkeeper says 'This sack has a weight of 25 kg.' A student replies that the sack has a mass of 25 kg. On the Moon a spring balance marked for the Earth shows a different reading for the same sack.
(a) Who is using the correct scientific term? (b) What is the sack's weight on the Earth in N? (take 10 N per kg) (c) What would a spring balance show on the Moon, in N, for the same sack (1.6 N per kg)? (d) Why does the reading change but not the mass?
Hint
Kilogram is a unit of mass.
Answer
(a) The student. (b) 25 × 10 = 250 N. (c) 25 × 1.6 = 40 N. (d) Weight depends on the gravitational pull, which is less on the Moon; mass is the amount of matter and stays 25 kg (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is weight?Show answer
The force with which the Earth pulls an object towards itself.
2.What is the smallest reading in the chapter's spring balance?Show answer
0.2 N (1 N divided into 5 divisions).
3.What is another name for upthrust?Show answer
Buoyant force.
4.State Archimedes' principle.Show answer
An object fully or partly immersed in a liquid feels an upward force equal to the weight of the liquid it displaces.
5.Why does pumice float?Show answer
It has trapped air pockets and is less dense than water.

Weight, Mass, Spring Balance and Floating and Sinking: frequently asked questions

Why is the weight on a spring balance marked in both N and g?

The balance has one scale for weight in newton and another for the corresponding mass in gram, marked on the assumption that it is used on the Earth.

Can we weigh an object to find its mass?

For practical purposes yes, because weight is almost the same everywhere on the Earth, but science keeps the two ideas separate.

Why does the mug feel lighter in water?

Water exerts an upward force, the upthrust, on it.

What decides whether an object sinks?

If its weight is greater than the upthrust it sinks.