CBSE Class 8 Science Force and Pressure Notes with Questions Set 02

Force

A force is a push or a pull. It arises when two objects interact. Lifting, bending, stretching, twisting and squeezing all involve a push or a pull. Force has both magnitude (strength) and direction. Its SI unit is the newton (N); 1 N is the force that gives a mass of 1 kg an acceleration of 1 m/s². We cannot see a force, only its effects.

Effects of Force

Net Force and Measuring Force

When two forces act in the same direction, the net force is their sum (25 N + 20 N = 45 N). When they act in opposite directions, the net force is their difference (12 N − 10 N = 2 N, in the direction of the larger force). Changing the magnitude changes the effect: a ball filled with too much air bursts. Force is measured with a spring balance. In an extension spring balance the spring stretches when pulled; in a compression spring balance it is compressed.

Contact and Non-contact Forces

Muscular force: force applied with our muscles, such as pushing a door or lifting a bag. Friction: the force that opposes the motion between two surfaces in contact; it helps us hold a pen, write, walk and run.

Magnetic force: the force exerted by a magnet on other magnets and on iron, cobalt and nickel. Maglev (magnetic levitation) trains float above the track on magnets, so there is almost no friction and they can move very fast. Cranes with powerful magnets lift scrap iron.

Gravitational force: the force of attraction between objects. The Earth pulls objects towards its centre, which is why a thrown ball falls back and a coconut drops. It keeps the planets in orbit around the Sun and the Moon around the Earth.

Electrostatic force: the force of attraction or repulsion between charged bodies. A plastic comb rubbed on dry hair picks up small bits of paper, while two rubbed balloons with the same charge repel each other. Static charge is used in pollution control to collect dust particles from air.

Weight and Mass

Weight is the gravitational force with which the Earth pulls an object. Weight = mass × acceleration due to gravity (W = m × g), where g = 9.8 m/s² (about 10 m/s²). It is a force, so it is measured in newtons using a spring balance. Mass stays the same everywhere, but weight changes with gravity, so we weigh more on Earth than on Mars.

Example: a person of mass 54 kg has weight 54 × 9.8 = 529.2 N.

Pressure

Pressure is the force acting per unit area of a surface: Pressure = Force / Area. Its SI unit is newton per square metre (N/m²), called the pascal (Pa). For the same force, a smaller area gives more pressure, which is why a push pin enters a board easily and stiletto heels sink into sand while flat shoes do not. A camel's broad flat feet spread its weight, so it does not sink in sand.

Example 1: a sail exerts a force of 50,000 N on an area of 250 m². Pressure = 50,000 / 250 = 200 N/m².

Example 2: an animal weighing 4000 N stands on one foot of area 100 cm² = 0.01 m². Pressure = 4000 / 0.01 = 4 × 10&sup5; Pa.

Pressure in Fluids

Liquids and gases are called fluids. A fluid exerts pressure on the walls of its container and on objects in it, and this pressure acts equally in all directions. Water pushes a submerged ball upwards with an upward force called the buoyant force. The pressure in a liquid increases with depth, so water from the lowest hole of a container travels farthest, deep-sea divers wear special suits, and dams are built thicker at the bottom. Pressure in a liquid also depends on its density. Hydraulic lifts and hydraulic brakes use the pressure of liquids. Pascal's law: pressure applied to a confined liquid is transmitted equally in all directions.

Atmospheric Pressure

The air around the Earth has weight and exerts a pressure called atmospheric pressure. At sea level it is about 100 kPa (more exactly 101.3 kPa). We are not crushed because the pressure inside our body is equal to the pressure outside. Evangelista Torricelli, an Italian physicist, discovered air pressure in the 17th century and made the mercury barometer. Atmospheric pressure decreases with altitude, because the air becomes thinner higher up. Astronauts wear pressurised suits as there is no air in space.

Instruments for Measuring Pressure

Applications of Pressure

Knives and scissors have a very small cutting edge, so a small force gives a large pressure. Spikes on football shoes give grip. When we sip through a straw, the pressure inside drops and the higher outside air pressure pushes the liquid up. When a syringe plunger is pulled back, blood or medicine is pushed in by the outside pressure. A snowboard has a large area, so the pressure on snow is low and the rider does not sink.

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