Exploring Forces

5.1 What Is a Force?

A force is a push or pull caused by an interaction between objects. Push a box, pull a drawer, or lift a bag: each involves a force. Its SI unit is the newton (N). A force has magnitude and direction. If an object is at rest, forces may still act on it but balance one another; rest does not imply no force.

5.2 What Can a Force Do?

A force can start an object moving, change its speed, change its direction, stop it or deform it. Kicking a stationary ball starts motion; a goalkeeper stops a ball; squeezing a balloon changes shape. The effect depends on the combined forces, not simply the presence of one force.

5.3 Force as an Interaction

When your hand pushes a table, the table also pushes back on your hand. Forces arise from interactions. The force acting on each object is not the same as the net force on either object; balanced forces can leave an object stationary.

5.4.1 Contact Forces: Muscular Force

Muscles exert forces in lifting, walking, pushing and chewing. Humans and animals use muscular forces to move objects and themselves. A rope or stick can transmit contact forces indirectly. Muscular force is a contact force.

5.4.1 Contact Forces: Friction

Friction opposes relative sliding or the tendency to slide between contacting surfaces. A sliding book slows because friction acts opposite its motion. Roughness and the normal contact force influence friction; polished floors may provide less grip. Air and water resistance are forms of fluid friction. Friction is helpful for walking and braking but can cause wear and energy loss.

5.4.2 Non-contact Force: Magnetic

Magnets exert forces without touching each other. Like poles repel and unlike poles attract. A magnet attracts suitable magnetic materials such as iron; it does not strongly attract all metals. Ring magnets facing like poles can repel while constrained on a non-magnetic rod.

5.4.2 Non-contact Force: Electrostatic

Rubbing some materials transfers electrical charge. A charged plastic scale can attract tiny neutral paper pieces through charge redistribution. Two balloons rubbed similarly often repel because like charges repel. Opposite charges attract. Keep experiments away from sockets, electrical devices and flammable vapours.

5.4.2 Non-contact Force: Gravity

Earth attracts objects gravitationally without touching them. A ball thrown vertically upwards slows, momentarily reaches zero vertical speed at its highest point and then descends. Gravity acts downward throughout its flight, including at the top. Air resistance may also act opposite the ball's motion.

5.5 Weight and Its Measurement

Mass describes the amount of matter and is measured in kg or g; weight is gravitational force and is measured in newtons. Weight W=mg (extension), where g near Earth's surface is about 9.8 N/kg; the textbook often uses 10 N/kg for simple comparisons. Mass stays the same on the Moon while weight is approximately one-sixth its Earth value. A spring balance measures force through spring extension. The textbook spring balance ranges from 0 to 10 N and has five small divisions per 1 N, so least count=1/5=0.2 N.

5.6 Floating and Sinking

Water exerts an upward buoyant force on immersed objects. Archimedes' principle: upthrust equals the weight of displaced fluid. A floating object at rest has buoyant force equal to its weight; if an object cannot displace enough fluid to balance its weight, it sinks. Shape, density and displaced volume matter. A hollow boat can float even when made of dense metal; porous pumice can float because its bulk density may be below water.

Types of forces

Mass and weight across worlds

Activities 5.1–5.13: guided observations

5.1 Push, pull and lift a cardboard box. 5.2 Record whether everyday forces change speed, direction or shape. 5.3 Slide an object in opposite directions and observe friction. 5.4 Compare stopping distances on different surfaces, controlling starting push. 5.5 Observe repulsion of ring magnets on a rod. 5.6 Rub a plastic scale and attract paper. 5.7 Rub two balloons and observe repulsion. 5.8 Throw a soft ball vertically in a clear area. 5.9 Compare spring stretching with different masses. 5.10 Read the 0–10 N spring-balance range. 5.11 Calculate least count of 0.2 N from five subdivisions. 5.12 Measure weights without exceeding the instrument range. 5.13 Gently push a sealed empty bottle into water and observe upthrust. Never invent measurements; students should enter their actual observations.

Figures and practical safety

Fig. 5.1 shows pushing, pulling and lifting; Figs. 5.5–5.6 show friction; Fig. 5.7 repelling ring magnets; Figs. 5.8–5.9 electrostatic attraction/repulsion; Fig. 5.11 vertical ball motion; Figs. 5.12–5.14 spring measurements; Fig. 5.15 floating bottle; Fig. 5.16 ball moving from a slope onto a flat surface; Fig. 5.17 three differently immersed objects; Fig. 5.18 an electroscope. Use teacher supervision for springs, magnets and electroscope construction. Avoid small magnets near young children and keep them away from implanted medical devices.

Common mistakes and exam revision

Do not say a ball at the top of its flight experiences zero gravity; its speed is briefly zero, not the gravitational force. Do not use kilograms for weight in scientific answers. Do not assume friction always acts opposite an object's overall motion; it opposes relative slipping or impending slipping at the contact. Do not infer that larger objects always sink. The official chapter uses descriptive reasoning, not advanced force-vector calculations.

NCERT Keep the Curiosity Alive — questions with full solutions

Original additional practice (not official NCERT exercises)

MCQs, HOTS and quick revision

MCQ: Which is non-contact: muscular, frictional, magnetic? Answer: magnetic. MCQ: What is the SI unit of weight? Answer: newton. HOTS: Can a heavy ship float while a tiny coin sinks? Yes, because floating depends on displaced water and average density, not simply mass or size. Exam tip: At the top of a thrown ball’s path, its velocity is momentarily zero but gravity still acts downward.

Discover, design and debate

Investigate which rubbed materials attract paper; compare friction as helpful or harmful; design a teacher-supervised spring balance; make a simple electroscope using the textbook diagram; and write a creative story about gravity. Record actual measurements rather than made-up observations. Never use electrical mains, fragile glass or strong magnets unsupervised.

Source and diagram verification

Consult the official NCERT Curiosity Chapter 5 PDF for Figs. 5.1–5.18. Question 10 is solved using the visually checked Fig. 5.17; object 1 is most deeply immersed, followed by 2 and 3. All ten textbook numbered questions are covered, and extra questions are clearly identified as original practice.