Exploring Forces Class 8: Effects of Force, Contact and Non-contact Forces, Weight and Floating

What Exploring Forces is about

Key concepts

Force as a push or pull, and what it can do

NCERT section 5.2 What Can a Force Do to the Objects on Which It Is Applied?

In science, a push or a pull on an object is a force (Activity 5.1: you can move a box by pushing, pulling, lifting or dragging it, always by a push or pull).

Effects of a force (5.2):

• make a still object move;
• change the speed of a moving object (speed up or slow down);
• change the direction of motion;
• change the shape of an object;
• or several of these together.

Examples: a friend holding your moving bicycle from behind (pull, slows it), a bat hitting a moving ball (push, changes direction), pressing a balloon (push, changes shape), rolling a chapati, turning an autorickshaw's handle.

Whenever speed, direction or shape changes, a force must be acting.

Worked example

State whether a push or pull is used and the effect of the force: (a) a goalkeeper stopping a moving football, (b) kneading dough, (c) a cyclist turning the handlebar.

Try it first, then show the solutionSolution

1. (a) Push; stops the ball (changes its speed to zero).
2. (b) Push (and pull); changes the shape of the dough.
3. (c) Push on one side and pull on the other; changes the direction of motion.

Answer: (a) stops motion (b) changes shape (c) changes direction.

Forces are interactions; the newton

NCERT section 5.3 Are Forces an Interaction Between Two or More Objects?

When you push a table, your hand and the table interact. A force appears only when at least two objects interact; when the interaction stops, the force disappears.

Definition: a force is a push or pull on an object resulting from its interaction with another object. The SI unit of force is the newton (N), written with a small 'n' in full.

• When two objects interact, each feels a force from the other: pushing a table, you feel the table push back on your hand.
• An object at rest is not necessarily free of forces: the forces on it may be balancing each other (more about this in higher grades).

Worked example

Name the two interacting objects in each case: (a) a magnet pulling a pin, (b) a person pushing a cart, (c) the Earth pulling a falling mango.

Try it first, then show the solutionSolution

1. (a) The magnet and the pin.
2. (b) The person's hands and the cart.
3. (c) The Earth and the mango.

Answer: Every force involves two objects: magnet–pin, hands–cart, Earth–mango.

Contact forces: muscular force

NCERT section 5.4.1 Contact forces

Contact forces act only when the objects touch, directly (hands) or through something (a stick or rope).

Muscular force is the force produced by the action of our muscles, which contract and relax, when we walk, run, lift, push, jump or stretch. Animals, birds, fish and insects use muscular force to move and survive, and people have long used the muscular force of animals such as bullocks, horses and camels to plough, pull carts and carry loads.

Inside the body, muscular force helps us chew, pushes food along the alimentary canal, and makes the heart contract and expand to circulate blood.

Worked example

Give two examples of muscular force acting inside the human body and say why each is important.

Try it first, then show the solutionSolution

1. Muscles of the alimentary canal push food along during digestion.
2. Heart muscles contract and expand to pump blood around the body, which is essential for survival.

Answer: Moving food in digestion; pumping blood by the heart.

Friction

NCERT section 5.4.1 Contact forces

A ball rolling on flat ground slows down and stops; a bicycle stops sooner on a rough road. A force must be acting.

Activity 5.3: a box gently pushed on a table slides and stops, whichever way it is pushed. A force between the box and the table acts opposite to its motion.

Friction is the force that arises when one surface slides, or is about to slide, over another. It always acts opposite to the direction of motion (or attempted motion), and it is a contact force.

Cause: even smooth-looking surfaces have tiny bumps (irregularities) that lock into each other and oppose sliding.

Activity 5.4: the same box pushed on glass, tile, wood, cloth and sand slides different distances: friction depends on the nature of the surfaces, and is greater on rougher surfaces.

Necessity or problem? Friction lets us walk and lets a bicycle stop when the brakes are applied; on very smooth or wet surfaces friction is small, so we slip. But it also slows down things we want to keep moving, such as a ball rolling on the ground. The book invites a class debate on whether friction is a necessity or a problem.

Worked example

The same box, pushed equally hard, slid 120 cm on glass, 90 cm on tile, 60 cm on wood, 25 cm on cloth and 10 cm on sand. Which surface gives the most friction? What does the data show?

Try it first, then show the solutionSolution

1. The shorter the distance, the greater the friction, so sand gives the most friction and glass the least.
2. Friction depends on the nature of the surfaces: rougher surfaces give more friction.
3. For a fair comparison, the same box and the same push must be used each time.

Answer: Sand; friction increases with roughness.

Non-contact forces: magnetic and electrostatic

NCERT section 5.4.2 Non-contact forces

Non-contact forces act even when objects do not touch.

Magnetic force (Activity 5.5): two ring magnets on a vertical wooden stick, with like poles facing, make the upper magnet float above the lower one; pressing it down, you feel a push back. Reversing one magnet makes them attract and stick. Like poles repel and unlike poles attract. The push or pull a magnet exerts on another magnet, or on something made of a magnetic material, is called magnetic force.

Electrostatic force:

• Activity 5.6: a plastic scale or straw rubbed with polythene attracts small bits of paper without touching them. Rubbing builds up static charges on surfaces; a charged object attracts some uncharged objects.
• Activity 5.7: two balloons rubbed with a woollen cloth repel each other; the woollen cloth attracts a rubbed balloon. Both the rubbing and the rubbed objects get charged, but with opposite kinds of charge.
• So there are two kinds of charge, positive and negative: like charges repel, unlike charges attract.
• The force exerted by a charged body on another charged or uncharged body is the electrostatic force. Moving charges form an electric current.

Worked example

A plastic straw rubbed with a polythene bag picks up small bits of paper. Two straws rubbed with the same bag are hung near each other and move apart. Explain both observations.

Try it first, then show the solutionSolution

1. Rubbing builds up static charge on the straw; a charged object attracts light uncharged bits of paper without touching them (electrostatic force).
2. Both straws were rubbed with the same material, so they carry the same kind of charge.
3. Like charges repel, so the two straws push each other apart.

Answer: The charged straw attracts paper; two straws with like charges repel.

Gravitational force

NCERT section 5.4.2 Non-contact forces

Activity 5.8: a ball thrown upwards always comes back down, however hard it is thrown. Fruits, leaves and rain all fall towards the ground.

The Earth attracts (pulls) all objects towards itself: this is the gravitational force, also called force of gravity or gravity. It is a non-contact force and is always attractive (magnetic and electrostatic forces can attract or repel).

Vertical motion:

• An object dropped from a height falls straight down, getting faster.
• An object thrown straight up slows down as it rises (gravity acts downwards, opposite to its motion), stops for an instant at the top, changes direction and comes down, speeding up.

This is why cyclists speed up going downhill without pedalling and must pedal harder uphill.

Worked example

A coconut falls from a tree, and a stone is thrown straight up. For each, give the direction of the gravitational force on it and say whether its speed increases or decreases while moving.

Try it first, then show the solutionSolution

1. Coconut: gravity acts downwards, in the direction of motion, so it speeds up as it falls.
2. Stone going up: gravity still acts downwards, opposite to its motion, so it slows down until it stops for an instant at the top.
3. The stone then falls back, and gravity (downwards, along the motion) speeds it up.

Answer: Gravity is always downwards: the coconut speeds up; the rising stone slows, then speeds up on the way down.

Weight, mass and the spring balance

NCERT section 5.5 Weight and Its Measurement

The weight of an object is the force with which the Earth pulls it. Being a force, weight is measured in newtons (N).

Activity 5.9: different objects hung from the same spring stretch it by different amounts, so the Earth pulls them with different forces: their 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 newtons (often with a second scale in grams).

• Range: the largest weight it can measure (for example 0 to 10 N). Never hang anything heavier, or the spring may be damaged.
• Smallest division (least count): the value between two big marks divided by the number of small divisions; for 1 N split into 5 divisions, $\frac{1}{5}=0.2$ N.

Mass versus weight:

• Mass is the amount of matter, measured in g or kg; it is the same everywhere.
• Weight is the gravitational pull, in N; it can change slightly on Earth and very much on other planets. The book's table for a 1 kg mass: Earth 10 N, Moon 1.6 N, Mars 3.8 N, Venus 9 N, Jupiter 25.4 N.
• In daily life we say a bag 'weighs 10 kg', but scientifically that is its mass; its weight on Earth is about 100 N.

Worked example

A spring balance has marks every 1 N, with 4 small divisions between them. What is its smallest reading? If an object stretches it to 3 big marks plus 3 small divisions, what is the weight?

Try it first, then show the solutionSolution

1. Smallest division $=\frac{1\text{ N}}{4}=0.25$ N.
2. 3 big marks $=3$ N; 3 small divisions $=3\times 0.25=0.75$ N.
3. Weight $=3+0.75=3.75$ N.

Answer: Least count 0.25 N; weight 3.75 N.

Upthrust, floating and sinking

NCERT section 5.6 Floating and Sinking

Activity 5.13: push a closed empty bottle into a bucket of water: you feel an upward push, and when released the bottle bounces up. Water (and every liquid) pushes upwards on objects in it: this force is the upthrust or buoyant force.

An object in a liquid has two forces on it:

• its weight (gravitational force) downwards;
• the buoyant force upwards.

If the weight is more than the buoyant force, the object sinks; if they are equal, it floats. A mug full of water feels lighter while still under water because the upthrust supports part of its weight.

Archimedes' principle: an object fully or partly immersed in a liquid feels an upward force equal to the weight of the liquid it displaces. If the displaced liquid weighs less than the object, it sinks; if equal, it floats.

The buoyant force also depends on the liquid's density (Chapter 9). Pumice, a volcanic rock full of gas bubbles, is less dense than water and floats.

Measuring upthrust: weigh an object in air with a spring balance, then while it is under water; the drop in reading is the buoyant force.

Worked example

A stone weighs 12 N on a spring balance in air and 8 N when fully under water. Find the buoyant force. What is the weight of the water it pushes aside?

Try it first, then show the solutionSolution

1. Buoyant force $=$ weight in air $-$ apparent weight in water $=12-8=4$ N.
2. By Archimedes' principle, the buoyant force equals the weight of water displaced.
3. So the displaced water weighs 4 N; the stone sinks because 12 N is more than 4 N.

Answer: 4 N upthrust; the displaced water weighs 4 N.

Science in real life: Exploring Forces

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.

Topics in this chapter

• Force, Its Effects and Types: Contact and Non-contact Forces
• Weight, Mass, Spring Balance and Floating and Sinking

Exploring Forces: formulas and key facts

Unit of force and weight

$\text{newton (N)}$

Effects of force

$\text{start motion}\mid \text{change speed}\mid \text{change direction}\mid \text{change shape}$

Least count of a spring balance

$\text{least count}=\frac{\text{value between big marks}}{\text{number of small divisions}}$

Weight of 1 kg (book table)

$\text{Earth }10\text{ N},\text{Moon }1.6\text{ N},\text{Mars }3.8\text{ N},\text{Jupiter }25.4\text{ N}$

Buoyant force from a spring balance

$F_{\text{up}}=W_{\text{air}}-W_{\text{water}}$

Archimedes' principle

$\text{upthrust}=\text{weight of liquid displaced}$

Float or sink

$W>F_{\text{up}}\Rightarrow \text{sinks};W=F_{\text{up}}\Rightarrow \text{floats}$

Charges

$\text{like charges repel, unlike charges attract}$

Exploring Forces: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Exploring Forces

The question patterns to expect, and the method that works for each.

Tricks to remember Exploring Forces

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

• SSDS

  A force can Start, change Speed, change Direction, change Shape.

  Use it when: Effects of force.

• Muscles and Friction touch; Magnets, Charges and Gravity reach

  Contact: muscular, friction. Non-contact: magnetic, electrostatic, gravitational.

  Use it when: Classifying forces.

• Gravity only pulls

  Magnets and charges push or pull; gravity only pulls.

  Use it when: Comparing non-contact forces.

• Mass stays, weight sways

  Mass is the same everywhere; weight changes with gravity.

  Use it when: Mass versus weight.

• Lighter by the water lifted

  An object in water loses (seems to lose) weight equal to the water it displaces.

  Use it when: Archimedes' principle.

Exploring Forces: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Exploring Forces

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

• ✗Mistake: Writing mass in newtons or weight in kilograms

  Everyday language mixes them.

  Wrong working

  The bag's weight is 10 kg.

  Right working

  The bag's mass is 10 kg; its weight on Earth is about 100 N.

  ✓Do this: Mass in kg or g; weight (a force) in N.

  Usually costs: 1 mark.

• ✗Mistake: Drawing friction in the direction of motion

  Friction is thought to help motion.

  Wrong working

  A box pushed to the right has friction acting to the right.

  Right working

  Friction acts to the left, opposite to the motion.

  ✓Do this: Friction always opposes motion or attempted motion.

  Usually costs: 1 mark in diagrams.

• ✗Mistake: Saying no force acts on an object at rest

  No motion is taken to mean no force.

  Wrong working

  A book resting on a table has no forces on it.

  Right working

  Gravity pulls it down and the table pushes it up; the forces balance.

  ✓Do this: At rest means balanced forces, not no forces.

  Usually costs: 1 mark.

• ✗Mistake: Reading only the big marks on a spring balance

  Least count is not worked out.

  Wrong working

  The pointer is between 3 N and 4 N, so the weight is 3 N.

  Right working

  Find the least count (say 0.2 N) and count small divisions: 3 N + 2 × 0.2 N = 3.4 N.

  ✓Do this: Always work out the value of one small division first.

  Usually costs: 1-2 marks.

• ✗Mistake: Thinking the rubbing cloth stays uncharged

  Only the rubbed object is considered.

  Wrong working

  Only the balloon gets charged when rubbed with wool.

  Right working

  Both get charged, with opposite kinds of charge, which is why they attract.

  ✓Do this: Rubbing charges both objects oppositely.

  Usually costs: 1 mark.

NCERT exercise guide: Exploring Forces

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Exploring Forces

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

1. Q1 · 1 mark · MCQExam level

   The SI unit of weight is
   (a) kilogram
   (b) gram
   (c) newton
   (d) metre

   Hint

   Weight is a force.

   Answer

   (c) newton

2. Q2 · 1 mark · MCQExam level

   Which force is always attractive?
   (a) Magnetic
   (b) Electrostatic
   (c) Gravitational
   (d) All of these

   Hint

   Earth only pulls.

   Answer

   (c) Gravitational

3. Q3 · 1 mark · Very short answerExam level

   Name the force that brings a rolling ball to rest on level ground.

   Hint

   Opposes motion.

   Answer

   Friction

4. Q4 · 2 marks · Very short answerExam level

   Why must at least two objects be involved for a force to act?

   Hint

   Interaction.

   Answer

   A force results from an interaction: one object pushes or pulls on another, and the force disappears when the interaction stops.

5. Q5 · 2 marks · Very short answerHarder exam level

   Two ring magnets on a stick: the upper one floats. What happens if one magnet is turned over? Explain.

   Hint

   Like and unlike poles.

   Answer

   Unlike poles now face each other, so the magnets attract and the upper one drops onto the lower one.

6. Q6 · 2 marks · Very short answerHarder exam level

   A spring balance reads 0 to 5 N with 10 small divisions between each 1 N mark. What is its least count, and what does a reading of 4 big marks and 7 small divisions mean?

   Hint

   1 N ÷ 10.

   Answer

   Least count 0.1 N; reading $4+0.7=4.7$ N.

7. Q7 · 3 marks · Short answerHarder exam level

   A plastic ball pushed under water shoots back up when released, but a glass marble of the same size sinks. Explain using forces.

   Hint

   Weight versus upthrust.

   Answer

   Both push aside the same amount of water, so they feel the same upthrust. The light plastic ball weighs less than this upthrust, so it is pushed up; the heavier marble weighs more than the upthrust, so it sinks.

8. Q8 · 2 marks · Very short answerHarder exam level

   A box is given the same push on glass, sand, cloth and wood. Arrange the surfaces from least to most friction, and describe how you would check your order.

   Hint

   Activity 5.4.

   Answer

   Least to most friction: glass, wood, cloth, sand (smoother surfaces give less friction). Check: give the same box the same push on each surface (for example by releasing it from the same point on a ramp), measure how far it slides, repeat, and compare the distances: the shorter the distance, the greater the friction.

9. Q9 · 2 marks · Very short answerHarder exam level

   A 2 kg packet is hung from a spring balance on Earth and on Mars. Using the book's values for 1 kg (Earth 10 N, Mars 3.8 N), find both readings. What is its mass on Mars?

   Hint

   Weight changes, mass does not.

   Answer

   Earth: $2\times 10=20$ N; Mars: $2\times 3.8=7.6$ N. Its mass is still 2 kg.

10. Q10 · 5 marks · Long answerHarder exam level

    Plan an experiment to show that friction depends on the type of surface. Include what you change, keep the same and measure, and a results table.

    Hint

    Same object, same push.

    Answer

    Change: surface (glass, wood, cloth, sand). Keep the same: the object, the push (for example release it from the same point on the same ramp), and the start position. Measure: distance travelled before stopping, three times each, and find the mean. Table: surface | trial 1 | trial 2 | trial 3 | mean distance. Expect shorter distances on rougher surfaces.

11. Q11 · 2 marks · Very short answerHOTS / competency

    A satellite moves round the Earth at a steady speed. Is any force acting on it? Explain.

    Hint

    Direction of motion.

    Answer

    Yes. Its direction of motion keeps changing as it goes round, and a change of direction needs a force; the Earth's gravitational pull provides it.

12. Q12 · 3 marks · Short answerHOTS / competency

    A student says the Earth pulls a 10 kg bag and a 1 kg bag with the same force because 'gravity is the same for everything'. Correct the student using an activity from the chapter.

    Hint

    Spring stretch.

    Answer

    In Activity 5.9, different objects stretch the same spring by different amounts, so the Earth pulls them with different forces. Weight depends on mass: the 10 kg bag has about ten times the weight (about 100 N against 10 N).

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. 1.What is a force?Show answer

   A push or pull on an object resulting from its interaction with another object.

2. 2.What is the SI unit of force?Show answer

   The newton (N).

3. 3.List the effects a force can have.Show answer

   Start motion, change speed, change direction, change shape.

4. 4.Name two contact forces.Show answer

   Muscular force and friction.

5. 5.Name three non-contact forces.Show answer

   Magnetic, electrostatic and gravitational forces.

6. 6.In which direction does friction act?Show answer

   Opposite to the direction of motion or attempted motion.

7. 7.What causes friction?Show answer

   Interlocking of tiny irregularities on the two surfaces in contact.

8. 8.What happens between like charges?Show answer

   They repel.

9. 9.What is weight?Show answer

   The force with which the Earth pulls an object; measured in newtons.

10. 10.State Archimedes' principle.Show answer

    An object in a liquid feels an upward force equal to the weight of the liquid it displaces.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give an example for each effect of force?
• ☐Can you name the two interacting objects in any force?
• ☐Can you explain why a rolling ball stops?
• ☐Can you say how friction depends on the surface?
• ☐Can you explain the floating ring magnet?
• ☐Can you explain why rubbed balloons repel but a balloon and wool attract?
• ☐Can you describe vertical motion under gravity?
• ☐Can you find the least count of a spring balance and read it?
• ☐Can you explain why weight changes on the Moon but mass does not?
• ☐Can you find upthrust from two spring balance readings?

Exploring Forces: frequently asked questions