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๐ฌ Think about it
A magnet can pull a pin across a gap. A rubbed balloon makes your hair stand up without touching it. An apple falls with nothing pushing it. These forces reach across empty space. How can a force act without touching? Let's explore the three non-contact forces.
What are non-contact forces?
- Forces whose effect is felt even when objects are not in contact.
- They act across a distance, through empty space.
- Magnetic, electrostatic and gravitational forces are non-contact forces.
Example:
A magnet picks up a pin before it even touches it — a non-contact force at work.
What is magnetic force?
- The force a magnet exerts on another magnet or a magnetic material.
- Like poles (N–N, S–S) repel; unlike poles (N–S) attract.
- It acts from a distance, so it is a non-contact force.
In this Activity, we will make one ring magnet float above another to feel the magnetic force.
Materials needed
Two ring magnets and a wooden stick.
Procedure
1. Hold the stick vertically over a wooden table and insert one ring magnet onto it.
2. Insert the second ring magnet above it so the like poles face each other. Does it float?
3. Gently push the second magnet down. Do you feel a force?
4. Now reverse the poles of both magnets. Does the second magnet still float?
2. Insert the second ring magnet above it so the like poles face each other. Does it float?
3. Gently push the second magnet down. Do you feel a force?
4. Now reverse the poles of both magnets. Does the second magnet still float?
Observation
With like poles facing, the second magnet floats above the first and resists being pushed down. With unlike poles facing, it does not float.
Explanation
A magnet can exert a force on another magnet without touching it. This magnetic force is a non-contact force.
โ Summary
- Like poles repel and float
- Force felt without contact
- Magnetic force is non-contact
What is electrostatic force?
- When some materials are rubbed, electric charges build up on them.
- These are static charges, and such an object is a charged object.
- The force a charged body exerts on another charged or uncharged body is electrostatic force.
- It acts even without contact, so it is a non-contact force.
What are the two kinds of static charges?
- The two kinds are called positive and negative.
- Like (similar) charges repel each other.
- Unlike (opposite) charges attract each other.
- A rubbing object and the rubbed object get opposite charges.
Two Kinds of Charge
Like charges
Two balloons charged the same way repel each other
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Unlike charges
A charged balloon and the woollen cloth attract each other
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Charges are of two kinds: positive and negative
In this Activity, we will charge a plastic scale by rubbing and use it to attract paper bits.
Materials needed
A plastic scale or plastic straw, a piece of polythene, and small pieces of paper.
Procedure
1. Rub the plastic scale or straw vigorously with the polythene.
2. Do not touch the rubbed part with your hand or any metal object.
3. Bring it close to the small paper pieces without touching them. What do you notice?
2. Do not touch the rubbed part with your hand or any metal object.
3. Bring it close to the small paper pieces without touching them. What do you notice?
Observation
The paper pieces get pulled towards the scale and stick to it.
Explanation
Rubbing builds up static charges on the scale. The charged scale attracts the uncharged paper pieces even without contact — an electrostatic force.
โ Summary
- Rubbing makes static charge
- Charged scale attracts paper
- Acts without contact
In this Activity, we will charge two balloons and see whether they attract or repel.
Materials needed
Two balloons, a length of thread, and a woollen cloth.
Procedure
1. Inflate the two balloons and hang them so they do not touch.
2. Rub both balloons with the woollen cloth and release them (do not touch the rubbed parts). What do you observe?
3. Now bring the woollen cloth close to one of the rubbed balloons. What happens?
2. Rub both balloons with the woollen cloth and release them (do not touch the rubbed parts). What do you observe?
3. Now bring the woollen cloth close to one of the rubbed balloons. What happens?
Observation
The two balloons move away from each other (repel). The balloon and the woollen cloth move towards each other (attract).
Explanation
The balloons got the same kind of charge, so they repel (like charges repel). The cloth got the opposite charge, so it attracts the balloon (unlike charges attract).
โ Summary
- Same charge balloons repel
- Balloon and cloth attract
- Two kinds of charge
โก A step further
- When charges move, they form an electric current in a circuit.
- It is the same current that lights a lamp.
- It also produces the heating effect and the magnetic effect.
What is gravitational force?
- The force with which the Earth attracts objects towards itself.
- It is also called the force of gravity, or simply gravity.
- It acts without contact, so it is a non-contact force.
- It is always an attractive force (unlike magnetic or electrostatic force).
What is vertical motion?
- A dropped object falls straight down to the ground.
- A ball thrown up slows, stops at the top, then falls straight down.
- Motion in a vertical direction under gravity is called vertical motion.
In this Activity, we will throw a ball up and see that it always comes back down.
Materials needed
A ball.
Procedure
1. Throw the ball vertically upwards. Does it come down?
2. Throw it again, harder this time. Does it still fall back to the ground?
3. Think of other situations where an object thrown up always returns to the ground.
2. Throw it again, harder this time. Does it still fall back to the ground?
3. Think of other situations where an object thrown up always returns to the ground.
Observation
However hard you throw it, the ball always falls back to the ground.
Explanation
The Earth pulls every object towards itself with the gravitational force, so thrown objects always come back down.
โ Summary
- Thrown ball comes back
- Earth pulls it down
- This pull is gravity
โ
Test Yourself
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What is a non-contact force?
View Answer
A force whose effect is felt even when the objects are not in contact. -
Name the three non-contact forces.
View Answer
Magnetic force, electrostatic force, and gravitational force. -
How do like and unlike charges behave?
View Answer
Like charges repel each other; unlike charges attract each other. -
Is gravitational force attractive or repulsive?
View Answer
Always attractive. -
What is electrostatic force?
View Answer
The force a charged body exerts on another charged or uncharged body.
Important Definitions
- Non-contact force — A force felt even when objects are not in contact.
- Magnetic force — The force a magnet exerts on another magnet or a magnetic material.
- Static charge — Electric charge built up on a rubbed object that does not move by itself.
- Electrostatic force — The force a charged body exerts on another charged or uncharged body.
- Gravitational force — The force with which the Earth attracts objects towards itself; always attractive.
- Vertical motion — Motion in a vertical direction under the influence of gravity.
๐ NCERT Question 1 — Match items in Column A
Match each type of force in Column A (muscular, magnetic, frictional, gravitational, electrostatic) with the correct example in Column B.
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๐ NCERT Question 2 — State whether the following statements
True or False: a force is always needed to change speed; friction increases a ball's speed; there is no force between two charged objects placed apart.
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๐ NCERT Question 3 — Two balloons rubbed with
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
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