Force and Pressure | Science Grade 8 CBSE

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The six ideas of force and pressure

From a simple push to the weight of the whole atmosphere — tap each term to see what it means and where you meet it.

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The three big ideas

A force is a push or a pull. We cannot see a force, but we see what it does.
A force can start motion, stop motion, speed up or slow down, change direction, or change the shape of an object (squeezing dough, stretching a rubber band).
When two forces act together in the same direction they add up; in opposite directions they partly cancel. The leftover is the net force.
If forces are balanced (net force zero), motion does not change; if they are unbalanced, the object speeds up, slows down or changes direction.

Common mistake: thinking a moving object must have a force pushing it along. A ball rolling on smooth ground keeps moving on its own; it slows only because friction (an unbalanced force) acts against it.

Contact forces need touching: muscular force (lifting a bag), and friction (the force that opposes sliding).
Non-contact forces act across a gap: gravitational force (the Earth pulling everything down), magnetic force (a magnet attracting iron), and electrostatic force (a charged comb lifting bits of paper).
Non-contact forces are why a dropped object falls without anything touching it, and why a magnet moves a pin before they meet.

Worked example. You rub a plastic scale on dry hair and bring it near tiny paper bits — they jump up to it. Which force is this, and is it contact or non-contact?

Rubbing gives the scale an electric charge, so it attracts the paper by electrostatic force. The paper moves before touching the scale, so it is a non-contact force.

Pressure = force ÷ area. The same force on a smaller area gives a greater pressure — that is why sharp tools cut and broad feet do not sink.
The SI unit of pressure is the pascal (Pa), equal to one newton per square metre (N/m²).
Liquids and gases (fluids) exert pressure too — in all directions, not just downwards. Liquid pressure increases with depth.
Atmospheric pressure is the push of the weight of the air above us. It is large, but we do not feel crushed because the same pressure acts inside our bodies too.

Worked example. A box weighs 200 N. Resting on a face of area 4 m² it gives pressure P₁; balanced on a peg of area 0.01 m² it gives P₂. Compare them.

P₁ = 200 ÷ 4 = 50 Pa. P₂ = 200 ÷ 0.01 = 20 000 Pa. The same weight on a tiny area gives 400 times the pressure — the principle behind nails, pins and knife edges.

Where you'll meet it

Force and pressure at work

Sharp tools, broad supports

A knife edge, a needle and a nail tip are sharpened to a tiny area so a small push becomes a huge pressure. The opposite trick — wide tractor tyres, broad camel feet, flat foundation slabs — spreads weight over a big area so nothing sinks or sags.

Drinking straws and droppers

A straw, a dropper and a syringe all work by lowering the pressure inside so the atmosphere pushes liquid in. You are not really "pulling" the juice up — the air outside is pushing it.

Dams and deep diving

Because water pressure rises with depth, dam walls are thickest at the base and deep-sea divers must guard against the crushing pressure far below the surface. The deeper you go, the harder the water pushes.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and case studies, testing whether you can use the ideas, not just recall them.

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