Force and Pressure
A force is any push or pull arising from the interaction of two objects that can change an object's state of motion or shape, and when spread over an area it gives rise to pressure.
Basic concepts of force, its types and effects are staple general-science material for Prelims, usually tested as contact-vs-non-contact classification, the four effects of a force, and SI units. As the foundation of mechanics, the chapter underpins applied Science & Technology themes in GS-III — from friction and simple machines to the magnetism, electrostatics and gravitation behind modern devices and space technology.
Understand the chapter
Force — A Push or a Pull
In science, any push or pull exerted on an object is called a force. Everyday actions like kicking, lifting, pushing, pulling and flicking are all applications of force that can set an object moving, slow it, speed it up or change its direction. Force is the single agent behind every change in an object's state of motion or shape.
- Force = a push or a pull on an object.
- SI unit of force is the newton (N).
- An object cannot move, stop, speed up, turn or deform by itself — only a force does it.
Forces Arise from Interaction; Magnitude, Direction and Net Force
A force never exists in isolation: at least two objects must interact for a force to come into play. Every force has both a magnitude (its strength) and a direction, so changing either changes its effect. When several forces act together, only the net force decides the outcome.
- Two interacting objects are needed for a force (e.g., a man pushing a car).
- Forces in the same direction add up; in opposite directions they subtract (net = difference).
- Equal and opposite forces give zero net force — the tug-of-war rope stays still.
Force Changes the State of Motion
The state of motion of a body is described by its speed and its direction of motion, with rest being simply the state of zero speed. A force applied along the motion increases speed, while a force opposite to the motion decreases it. A force can also redirect a moving object, so a change in speed, direction, or both counts as a change in the state of motion.
- State of motion = speed + direction; rest = zero speed.
- Force along motion speeds it up; force opposing motion slows it down.
- Force can change direction (e.g., a ball striking a ruler, a hockey flick).
Force Changes the Shape of an Object
Besides motion, a force can deform an object — pressing dough, squeezing an inflated balloon, stretching a rubber band or compressing a spring all change shape. Whether motion or shape changes depends on the object and how it is held. Importantly, a force may produce one effect, several, or — as with pushing a wall — none at all.
- Force can change shape (dough into chapati, pressed rubber ball, stretched rubber band).
- Applied force does not always change the state of motion (a heavy box, a wall).
Contact Forces — Muscular Force and Friction
Forces that act only when bodies touch are contact forces. Muscular force, produced by the action of muscles, lets humans and animals (bullocks, horses, camels) push, lift and bend, and internally it drives digestion and breathing. Friction, arising at the contact between two surfaces, always acts on a moving object opposite to its direction of motion and is what brings rolling balls, cycles and boats to rest.
- Contact force: the agent must touch the object (directly or via a stick/rope).
- Muscular force = force due to the action of muscles (also moves food in the alimentary canal, aids breathing).
- Friction always opposes motion and arises due to surface contact.
Non-contact Forces — Magnetic, Electrostatic and Gravitational
Some forces act even without physical contact — these are non-contact forces. A magnet exerts force across a gap (like poles repel, unlike poles attract), a charged body such as a straw rubbed with paper exerts an electrostatic force on other bodies, and gravity pulls every object — coins, leaves, fruits — towards the Earth. Since attraction is a pull and repulsion a push, magnetic and electrostatic interactions are still pushes and pulls.
- Magnetic force: like poles repel, unlike poles attract; acts at a distance.
- Electrostatic force: force by a charged body on another charged or uncharged body.
- Gravitational force: the Earth's pull that makes released objects fall down.
From Force to Pressure
The chapter extends force to pressure — the force acting on a unit area of a surface. The same force concentrated on a smaller area produces greater pressure, which is why sharp and pointed tools work so well. Pressure is therefore force per unit area.
- Pressure = force / area.
- SI unit of pressure is the pascal (Pa).
- Smaller area under the same force gives larger pressure.
Key terms
- Force
- A push or a pull exerted on an object; SI unit newton (N).
- Net force
- The single resultant of all forces acting on a body; it decides the actual effect.
- Magnitude
- The strength or size of a force, considered apart from its direction.
- State of motion
- An object's condition described by its speed and direction; rest = zero speed.
- Contact force
- A force that acts only when the applying body touches the object (e.g., muscular, friction).
- Non-contact force
- A force that acts without physical contact (e.g., magnetic, electrostatic, gravitational).
- Muscular force
- Force produced by the action of muscles in humans and animals; a contact force.
- Friction
- Contact force at touching surfaces that always opposes the motion of an object.
- Electrostatic force
- Force exerted by a charged body on another charged or uncharged body; a non-contact force.
- Gravitational force
- The attractive pull exerted by the Earth (and all masses) that makes objects fall.
Must-know facts exam-ready
- Force = a push or a pull; SI unit is the newton (N).
- A force needs at least two objects interacting — nothing moves itself.
- A force has both magnitude and direction; changing either changes its effect.
- Forces in the same direction add; in opposite directions they subtract (net force).
- A force can change state of motion (speed and/or direction) and/or shape — or do nothing.
- State of motion = speed + direction; rest is the state of zero speed.
- Contact forces in the chapter: muscular force and friction.
- Friction always acts opposite to the direction of motion and is a contact force.
- Non-contact forces: magnetic, electrostatic and gravitational.
- Magnets: like poles repel, unlike poles attract.
- A charged body (e.g., straw rubbed with paper) exerts electrostatic force without contact.
- Pressure = force per unit area; SI unit is the pascal (Pa).
Memory tricks remember it for good
Traps to avoid
- Force is not just 'strength' — it has both magnitude AND direction; ignoring direction changes the answer.
- Zero net force does NOT mean no force acts — equal, opposite forces (tug-of-war) still act but cancel.
- Friction is a CONTACT force, not non-contact, even though nothing visibly pushes the slowing object.
- Magnetic, electrostatic and gravitational forces are NON-contact — don't group them with muscular force or friction.
- Applying a force does not always cause motion — a wall or heavy box may not move; the effect can instead be a shape change, or none.
- Both attraction (a pull) and repulsion (a push) are forces — a magnet's repulsion is not 'no force'.
Exam focus
🧠 Prelims angles
- Contact vs non-contact classification (muscular/friction vs magnetic/electrostatic/gravitational).
- The four effects of force — spotting which option is NOT an effect.
- SI units: force = newton (N), pressure = pascal (Pa).
- Direction of friction — always opposite to the motion.
- Magnetic poles (like repel, unlike attract) and charging by rubbing (straw–paper).
- Pressure–area relationship — why pointed/sharp tools work better.
✍️ Mains angles GS-III
- Friction as both essential and wasteful in technology and daily life.Balance its utility (walking, braking, gripping) against losses (wear, heat) and link to energy efficiency in GS-III.
- How foundational forces underpin modern science and technology.Trace magnetism to motors/maglev, gravitation to space launches and electrostatics to printing, showing basic physics driving applied S&T.
Last-minute revision tick as you recall
- Force = push or pull; SI unit newton (N).
- Needs at least two objects interacting.
- Has both magnitude and direction.
- Same direction add, opposite subtract — net force decides effect.
- Force changes state of motion (speed/direction) and/or shape.
- State of motion = speed + direction; rest = zero speed.
- Contact: muscular, friction. Non-contact: magnetic, electrostatic, gravitational.
- Friction always opposes motion; like poles repel, unlike attract.
- Pressure = force / area; SI unit pascal (Pa).
Distilled from NCERT Class 8 · Science (Class 8) for UPSC. Always cross-check facts with the original NCERT.