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Non-Contact Force
- Exerted even from a distance
- No physical touch is needed
- Magnetic force is one example
- Electrostatic force is another
- Gravitational force is the third
- magnet brought near iron pins
- pins jump to it without touch
- a non-contact force
- Earth pulls the stone down
- without touching it
- gravity is a non-contact force
- The three non-contact forces are magnetic, electrostatic and gravitational force.
- These forces act even without the two objects being in contact.
- Gravitational force is always attractive, unlike magnetic and electrostatic forces.
Magnetic Force
- The force exerted by a magnet
- Acts even from a distance
- Attracts iron, steel, nickel, cobalt
- Like poles repel, unlike poles attract
- So it is a non-contact force
- magnet brought near iron nails
- nails are pulled towards it
- force of attraction acts
- like poles repel each other
- unlike poles attract each other
- without touching
- A magnet does not exert force on copper, wood or glass.
- Magnetic force is used in toys, fridge magnet strips and electromagnets to lift scrap iron.
- It acts from a distance, so it is a non-contact force.
Then reverse the second magnet so unlike poles face each other.
- Same poles: repulsion, magnet floats.
- Unlike poles: attraction.
- Force acts without touch.
Electrostatic Force
- Force exerted by a charged object
- Charges come from rubbing
- Like charges repel, unlike attract
- It can act from a distance
- So it is a non-contact force
- comb rubbed in dry hair
- it gets electric charge
- attracts tiny pieces of paper
- balloon rubbed with cloth
- gets electrically charged
- sticks to the wall
- Electric charges are of two types — positive and negative.
- The force between two like charges is repulsion; between unlike charges it is attraction.
- Electrostatic force is used to remove dust, soot and fly-ash from factory chimney smoke.
Rub each balloon with the woollen cloth and observe; then bring the cloth near one balloon.
- Balloons gain same charge.
- They repel and move apart.
- Cloth attracts the balloon.
Gravitational Force
- The pull exerted by objects with mass
- Every object pulls every other
- It is always a force of attraction
- The Earth's pull is the force of gravity
- It is a non-contact force
- a fruit drops from a tree
- Earth pulls it down
- this is the force of gravity
- gravity acts across space
- holds the Moon in its orbit
- no contact needed
- Newton gave the idea of gravity on seeing an apple fall from a tree.
- Gravity between two small objects is very weak; it is strong only if one body is huge like the Earth.
- Gravity acts from a distance and is always attractive, so it is a non-contact force.
Name the following: The force which attracts tiny pieces of paper to a plastic pen when rubbed in dry hair.
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Explanation
- Rubbing charges the pen, which then attracts paper by electrostatic force.
Name the following: The force that can be used to gather iron pins scattered on the floor.
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Explanation
- A magnet attracts iron pins by magnetic force.
Name the following: The force with which an object is attracted towards the centre of Earth.
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Explanation
- The Earth pulls all objects towards its centre by the force of gravity.
Fill in the blank: A charged body ________ an uncharged body towards it.
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Explanation
- The electrostatic force between a charged and an uncharged object is always attraction.
Fill in the blank: The north pole of a magnet ________ the north pole of another magnet.
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Explanation
- Like poles of two magnets repel each other.
Fill in the blank: The ________ force holds the Earth in its orbit around the Sun.
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Explanation
- The gravitational force between the Sun and the Earth keeps the Earth in orbit.
State whether true or false: There is no force between two charged objects placed at a small distance apart. (NCERT Book Question)
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Explanation
- Electrostatic force acts between charged objects even from a distance, attracting or repelling them.
Match Column A with Column B: (a) Muscular force (b) Magnetic force (c) Frictional force (d) Gravitational force (e) Electrostatic force — with (i) A cricket ball stopping on its own just before touching the boundary line (ii) A child lifting a school bag (iii) A fruit falling from a tree (iv) Balloon rubbed on woollen cloth attracting hair strands (v) A compass needle pointing North. (NCERT Book Question)
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Explanation
- Muscular force → a child lifting a school bag.
- Magnetic force → a compass needle pointing North.
- Frictional force → a cricket ball stopping on its own.
- Gravitational force → a fruit falling from a tree.
- Electrostatic force → balloon attracting hair strands.
Which of the following are non-contact forces? Magnetic force, Frictional force, Gravitational force, Muscular force, Electrostatic force.
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Explanation
- These act from a distance without touch.
- Frictional and muscular force need contact, so they are contact forces.
What is meant by gravitational force (or force of gravity)? Give one example.
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Explanation
- Every object with mass pulls every other object towards it.
- The Earth's pull on objects is called the force of gravity.
- Example: a fruit falling from a tree to the ground.
Which force is responsible for raising our body hair when we try to take off a terylene or polyester shirt in the dry weather?
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Explanation
- The shirt gets charged by rubbing and the electrostatic force raises our body hair.
Give reason: Any object thrown upwards always comes down.
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Explanation
- The Earth exerts a gravitational pull on every object.
- This pull acts downwards towards the centre of the Earth.
- So a thrown object slows, stops and then falls back down.
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why? (NCERT Book Question)
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Explanation
- Rubbing with wool gives both balloons the same type of charge.
- Like charges repel each other by electrostatic force.
- So the two balloons move away from each other.
Give one practical application of magnetic force.
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Explanation
- Magnetic force is used in electromagnets to lift and separate iron objects.
- It is also used in toys and fridge magnet strips.
What is meant by a non-contact force? Explain with the help of an example.
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Explanation
- A non-contact force can be exerted without physical touch between objects.
- Magnetic, electrostatic and gravitational forces are non-contact forces.
- Example: a magnet attracts iron pins towards it even before touching them.
Explain why, magnetic force is said to be a non-contact force.
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Explanation
- When a magnet is brought near iron pins, the pins jump towards it even before contact.
- A magnet can attract or repel another magnet from a distance.
- Since no physical touch is needed, magnetic force is a non-contact force.
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad (ignore the frictional force due to air resistance).
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Explanation
- The thrust produced by burning fuel pushes the rocket upwards.
- The gravitational force of the Earth pulls the rocket downwards.
Which of the following is not an example of the force of gravity?
- (a) a leaf falling from a tree
- (b) a boy pushing a cart on a level plane
- (c) a diver jumping into a swimming pool
- (d) a stone falling from the top of a cliff
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Explanation
- Pushing a cart uses muscular force; the others involve objects falling due to gravity.
Which of the following force is used in reducing air pollution by removing dust, soot and fly-ash particles from the smoke coming out of chimneys of factories?
- (a) magnetic force
- (b) gravitational force
- (c) electrostatic force
- (d) frictional force
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Explanation
- Charged plates use electrostatic force to attract and remove dust and soot from smoke.
A comb attracts tiny bits of paper due to:
- (a) friction
- (b) electrostatic force
- (c) gravitational force
- (d) magnetic force
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Explanation
- A rubbed comb gets charged and attracts paper by electrostatic force.
A magnetic force is exerted on objects made of:
- (a) plastic
- (b) rubber
- (c) steel
- (d) clay
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Explanation
- A magnet exerts force on magnetic materials like iron, steel, nickel and cobalt.
The force which causes all objects to fall on Earth:
- (a) magnetic
- (b) muscular
- (c) electrostatic
- (d) gravitational
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Explanation
- The Earth's gravitational force pulls all objects towards its centre.
