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Contact Force
- Exerted only through physical touch
- Two objects must touch each other
- Muscular force is a contact force
- Frictional force is a contact force
- No touch means no contact force
Example 1 — Lifting a book
- hand touches the book
- muscles apply force
- a contact force lifts it
Example 2 — Ball on ground
- ball touches the ground
- friction acts between them
- a contact force slows it
Important Points
- The two examples of contact forces are muscular force and frictional force.
- Contact can also be through a stick or rope, like hitting a ball with a hockey stick.
- Without physical touch, a contact force cannot act.
Definition — Contact Force
A contact force means a force which can be exerted by an object on another object only through physical touching.
Muscular Force
- Force exerted by the body's muscles
- Used in walking, lifting, kicking
- Humans and animals exert it
- Needs the body in contact with object
- So it is a contact force
Example 1 — Ox pulling a plough
- the ox uses its muscles
- pulls the plough
- helps a man do hard work
Example 2 — Inside the body
- muscles push food in digestion
- lungs expand in breathing
- the heart beats by muscular force
Important Points
- Muscular force acts inside the body too — in digestion, breathing and heartbeat.
- Animals like ox, horse, camel and reindeer use muscular force to do hard jobs.
- It is a contact force because the body must touch the object.
Definition — Muscular Force
Muscular force means the force exerted by the muscles of the body.
Frictional Force
- It always opposes the motion of a body
- Acts between two surfaces in contact
- Caused by surface irregularities
- Rougher surfaces give more friction
- It is a contact force
Example 1 — Ball stopping
- a ball rolls on the ground
- friction opposes its motion
- the ball slows and stops
Example 2 — Drag in fluids
- fluids exert friction called drag
- streamlined shapes face less drag
- helps cars and birds move
Important Points
- Friction is caused by the interlocking of irregularities in the two surfaces in contact.
- The frictional force always acts opposite to the direction of motion.
- Drag is the frictional force from a fluid like air or water; streamlined shapes reduce it.
Definition — Frictional Force
Frictional force means the force which always opposes the motion of one body over another body.
Materials needed
A book and a table.
Procedure
Place a book on the table and give it a push towards the right side.
Then give it a push towards the left side.
Then give it a push towards the left side.
Observation
The book moves some distance and stops on its own each time.
Explanation
Friction acts opposite to the motion — towards the left when the book moves right, and towards the right when it moves left — so it stops the book.
◆ Summary
- Push right: friction acts left.
- Push left: friction acts right.
- Friction opposes motion.
Friction — A Necessary Evil
- Friction has useful advantages
- It lets us walk without slipping
- It lets vehicles move without skidding
- It also has disadvantages
- It wears out shoes and machine parts
Example 1 — Walking and writing
- friction grips our shoes to ground
- friction holds pencil tip to paper
- so we walk and write
Example 2 — Wear and tear
- friction wears shoe soles
- it wears out machine parts
- and reduces machine efficiency
Important Points
- Friction lets us light a matchstick by rubbing it on the rough side of a matchbox.
- Friction wastes energy as heat and reduces the efficiency of machines.
- It is called a "necessary evil" because it is both useful and harmful.
Definition — Drag
Drag means the frictional force exerted by a fluid like air or water that opposes an object moving through it.
Objective Type Questions Q1(d) - Page 92 Lakhmir Singh
Question
Name the following: Two contact forces and two non-contact forces.
Show answer
Contact: muscular and frictional force. Non-contact: magnetic and gravitational (or electrostatic) force.
Explanation
- Contact forces act only by touching — muscular and frictional force.
- Non-contact forces act from a distance — magnetic, electrostatic and gravitational force.
Objective Type Questions Q1(g) - Page 92 Lakhmir Singh
Question
Name the following: The force which always opposes motion.
Show answer
Frictional force (friction)
Explanation
- Friction acts opposite to the direction of motion between two surfaces in contact.
Objective Type Questions Q3(b) - Page 93 Lakhmir Singh
Question
State whether true or false: Due to friction, the speed of the ball rolling on a flat ground increases. (NCERT Book Question)
Show answer
False
Explanation
- Friction opposes motion, so it slows the ball down and finally stops it.
Subjective Type Questions - Short Answer Q4(a) - Page 94 Lakhmir Singh
Question
Which force do the animals apply while moving, chewing and doing other activities?
Show answer
Muscular force
Explanation
- Animals use the force of their muscles to move, chew and do work.
Subjective Type Questions - Short Answer Q5(a) - Page 94 Lakhmir Singh
Question
Give reason: When we stop rowing a boat which is moving in water, it slows down and then stops.
Show answer
The frictional force of water opposes the boat's motion and stops it.
Explanation
- Water exerts a frictional force on the moving boat.
- This friction opposes the motion of the boat.
- So the boat slows down and finally stops.
Subjective Type Questions - Short Answer Q6 - Page 94 Lakhmir Singh
Question
Give two examples to show that friction is necessary in our lives.
Show answer
Friction lets us walk without slipping and lets vehicles move without skidding.
Explanation
- Friction between our shoes and the ground stops us from slipping while walking.
- Friction between tyres and road lets a car move without skidding.
Subjective Type Questions - Short Answer Q8 - Page 94 Lakhmir Singh
Question
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain. (NCERT Book Question)
Show answer
Smooth surfaces have very little friction, which is not enough to stop us from slipping.
Explanation
- Friction between our feet and the ground normally prevents slipping.
- Ice and polished floors are very smooth, so the friction is much less.
- This small friction is not enough to grip our feet, so we slip.
Subjective Type Questions - Long Answer Q1(a) - Page 94 Lakhmir Singh
Question
What is meant by a contact force? Explain with the help of an example.
Show answer
A contact force acts only through physical touch between two objects.
Explanation
- A contact force can be exerted only when two objects touch each other.
- Muscular force and frictional force are contact forces.
- Example: when we lift a book, our hand touches the book and the muscles apply force.
Subjective Type Questions - Long Answer Q2(a) - Page 94 Lakhmir Singh
Question
(a) Define frictional force (or friction). (b) Explain why frictional force is said to be a contact force.
Show answer
Friction opposes motion between surfaces, and it acts only when surfaces are in contact, so it is a contact force.
Explanation
- (a) Frictional force is the force which always opposes the motion of one body over another.
- (b) Friction arises only when the surfaces of two objects are in touch with each other.
- Since it needs physical contact between surfaces, it is a contact force.
Multiple Choice Questions Q1 - Page 95 Lakhmir Singh
Question
Which of the following is not an example of muscular force?
- (a) a porter carrying a load on a wheel-barrow
- (b) an apple falling from a tree
- (c) a child riding a bicycle
- (d) a person drawing water from a well
Show answer
(b) an apple falling from a tree
Explanation
- An apple falls due to gravity, not muscular force.
Multiple Choice Questions Q4 - Page 95 Lakhmir Singh
Question
Which of the following are contact forces? A. friction B. gravitational force C. magnetic force D. muscular force
- (a) A and B
- (b) B and C
- (c) A and D
- (d) B and D
Show answer
(c) A and D
Explanation
- Friction and muscular force need physical contact; gravity and magnetism act from a distance.
