Important Concepts and notes

1. Force

A force is a push or pull resulting from the interaction between two or more objects. The SI unit of force is newton (N).

2. Effects of Force

• Force can make an object move from rest.
• It can increase or decrease the speed of a moving object.
• It can change the direction of motion.
• It can change the shape of an object.

3. Contact and Non-contact Forces

4. Muscular Force

Muscular force is the force produced by the action of muscles. It is a contact force. Humans and other living beings use it for walking, running, lifting, pushing, jumping and other activities.

5. Friction

Friction is the force that acts when an object moves or tries to move over another surface. It always acts opposite to the direction of motion or attempted motion.

• Friction is a contact force.
• It is caused by minute irregularities on surfaces in contact.
• Rough surfaces generally produce greater friction.
• Friction can also act when objects move through liquids and gases.

6. Magnetic Force

Magnetic force is the force exerted by a magnet on another magnet or a magnetic material. It is a non-contact force.

• Like magnetic poles repel each other.
• Unlike magnetic poles attract each other.

7. Electrostatic Force

Electrostatic force is the force exerted by a charged body on another charged or uncharged body. It can act without physical contact.

• Rubbing certain materials can produce static charges.
• Like charges repel each other.
• Unlike charges attract each other.
• The two kinds of static charges are positive and negative.

8. Gravitational Force

Gravitational force is the force with which the Earth attracts objects towards itself. It is a non-contact force and is always attractive.

When an object is thrown upwards, gravity causes its speed to decrease until it stops momentarily at the highest point. It then falls back towards the Earth.

9. Weight

Weight is the force with which the Earth pulls an object towards itself. Since weight is a force, its SI unit is newton (N).

10. Spring Balance

A spring balance is used to measure the weight of an object. When an object is suspended from it, the spring stretches and the scale shows the weight, usually in newtons.

11. Mass and Weight

12. Upthrust or Buoyant Force

A liquid exerts an upward force on an object placed in it. This force is called upthrust or buoyant force.

• If gravitational force is greater than buoyant force, the object sinks.
• If gravitational force and buoyant force are equal, the object floats.
• Buoyant force depends, among other factors, on the density of the liquid.

13. Archimedes' Principle

According to Archimedes' Principle, an object fully or partially immersed in a liquid experiences an upward force equal to the weight of the liquid displaced by the object.

Quick Revision

• Force → Push or pull.
• SI unit of force → Newton (N).
• Contact forces → Muscular force and friction.
• Non-contact forces → Magnetic, electrostatic and gravitational forces.
• Friction → Opposes motion.
• Gravity → Always attractive.
• Weight → Gravitational force acting on an object.
• Spring balance → Measures weight.
• Buoyant force → Upward force exerted by a liquid.
• Mass → Remains the same, while weight can vary with location.

NCERT Solutions

Find clear, step-by-step solutions to the questions from this chapter.

Exam Tips

1. Learn the Definitions Precisely

Questions often ask for definitions. Learn these important terms in clear scientific language: force, friction, muscular force, magnetic force, electrostatic force, gravitational force, weight, buoyant force and Archimedes' Principle.

2. Remember the Effects of Force

For a question such as “What can a force do?”, remember:

• Make an object move from rest.
• Change its speed.
• Change its direction of motion.
• Change its shape.

3. Contact vs Non-contact Forces

This is an important classification for exams.

4. Do Not Confuse Mass and Weight

This is one of the most important points of the chapter. Mass is the amount of matter in an object and is measured in g or kg. Weight is the gravitational force acting on an object and is measured in newtons (N).

5. Remember the Direction of Friction

Friction always acts opposite to the direction in which an object is moving or trying to move.

Exam example: If a box is moving towards the right, friction acts towards the left.

6. Learn Attraction and Repulsion

7. Be Ready for Daily-Life Examples

The chapter uses many everyday situations to explain forces. In an exam, you may be asked to identify the force involved.

• Lifting a school bag → Muscular force
• Ball slowing down on the ground → Friction
• Compass needle pointing North → Magnetic force
• Fruit falling from a tree → Gravitational force
• Rubbed balloon attracting hair → Electrostatic force

8. Understand Floating and Sinking

Remember the relationship between the two forces acting on an object in a liquid:

• Gravitational force > buoyant force → object sinks.
• Gravitational force = buoyant force → object floats.

9. Remember Archimedes' Principle

For questions based on floating and sinking, remember that an immersed object experiences an upward force equal to the weight of the liquid displaced by it.

10. Spring Balance Questions

Remember that a spring balance measures weight (force) and its scale is marked in newtons. Also pay attention to the range and smallest division of the scale when answering numerical or reading-based questions.

11. For Assertion, Reason and MCQ Questions

• Force can act with or without contact.
• An object at rest does not necessarily mean that no force is acting on it; forces may be balanced.
• Friction decreases the speed of a moving object.
• Gravity acts towards the Earth.
• Mass remains unchanged when an object is taken to another place, while its weight may change.

12. How to Write Better Answers

• Start with the correct scientific definition.
• Use important keywords such as push or pull, interaction, contact, non-contact, opposite direction and gravitational force.
• Give a suitable daily-life example when the question asks for one.
• Always write the correct unit: force and weight → N; mass → g or kg.
• For questions involving directions, clearly mention the direction of the force.

Last-Minute Revision

Before the exam, make sure you can answer these questions without looking at your notes:

• What is force?
• What are the effects of force?
• What is the difference between contact and non-contact forces?
• What is friction and in which direction does it act?
• What are magnetic and electrostatic forces?
• Why do objects fall towards the Earth?
• What is the difference between mass and weight?
• How does a spring balance measure weight?
• What is buoyant force?
• When does an object float or sink?
• State Archimedes' Principle.

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