Class 8 Physics — Important Questions with Answers

What it is

Pressure is how concentrated a force is - the same push on a smaller area presses much harder.

Answer

Pressure is the force acting on a unit area, P = F/A. A sharp knife has a very small area at its edge, so for the same force the pressure it exerts is much greater, and it cuts easily. A blunt knife has a larger edge area, so the same force produces less pressure and cuts poorly.

What it is

Pressure is the force acting on unit area of a surface.

Answer

Pressure = force / area = 100 / 2 = 50 pascal (Pa). The same force pressed onto a smaller area produces a much higher pressure, which is why a sharp edge cuts easily.

What it is

A force is a push or a pull on an object that arises from an interaction between two objects.

Answer

A force is a push or a pull acting on an object due to its interaction with another object. A force can make a stationary object move, stop a moving object, change its speed, change its direction of motion, and change the shape or size of an object.

What it is

When forces on an object cancel out they are balanced; when they do not, the net (unbalanced) force changes the object's motion.

Answer

Balanced forces are equal in size and act in opposite directions, so their net force is zero and the object's state of motion does not change - for example, a book resting on a table. Unbalanced forces do not cancel out, so there is a net force that changes the object's speed or direction - for example, when one team in a tug of war pulls harder, the rope moves towards them.

What it is

Contact forces act only on touching (muscular, friction); non-contact forces act from a distance (magnetic, electrostatic, gravitational).

Answer

Contact forces act only when two objects are in physical contact, such as muscular force and frictional force. Non-contact forces act even without contact, over a distance, such as magnetic force, electrostatic force and gravitational force.

What it is

Pressure is the force acting per unit area on a surface.

Answer

Pressure = force / area = 100 / 2 = 50 N/m^2 (pascal). If the area is halved to 1 m^2 with the same force, pressure = 100 / 1 = 100 Pa, so halving the area doubles the pressure.

What it is

Liquids exert pressure on the container walls and base, and this pressure increases with depth.

Answer

A liquid exerts pressure in all directions on the walls and base of its container. The pressure increases as the depth increases, and at the same depth it is the same in all directions. This is why the walls of a dam are made much thicker at the bottom than at the top.

What it is

The weight of the air column above us presses on every surface; this is atmospheric pressure.

Answer

Atmospheric pressure is the pressure exerted by the weight of the column of air above a surface. It is very large, but we are not crushed because the pressure of the blood and fluids inside our body is equal to the atmospheric pressure and balances it from inside.

What it is

Muscular force comes from the action of muscles; gravitational force is the pull of the Earth on every object.

Answer

Gravitational force is the force with which the Earth pulls every object towards its centre; it gives an object its weight. The force used to lift a bucket is muscular force, a contact force produced by our muscles.

What it is

Friction is the grip between two surfaces that resists sliding.

Answer

Friction is called a necessary evil because it is both useful and harmful. It is necessary as it lets us walk, hold objects, write and stop vehicles. It is an evil because it wears out machine parts and shoe soles and produces heat that wastes energy. So we increase it where needed (treads, brakes) and reduce it where it wastes energy (lubricants, ball bearings).

What it is

Friction is the force that opposes the relative motion between two surfaces in contact.

Answer

Friction is the force that opposes the relative motion between two surfaces that are in contact. It always acts in the direction opposite to the motion (or attempted motion) of the object. Friction is caused by the interlocking of tiny irregularities on the two surfaces.

What it is

Friction depends on how rough the surfaces are and on how hard they are pressed together.

Answer

Friction depends on the nature (roughness or smoothness) of the two surfaces in contact - rougher surfaces produce more friction - and on how hard the surfaces are pressed together, that is, the force pressing them, so a heavier object experiences more friction.

What it is

Static friction acts before motion starts, sliding friction during sliding, and rolling friction when a body rolls; rolling is the smallest.

Answer

The three types are static friction (acts when the object is at rest and about to move), sliding friction (acts when one surface slides over another) and rolling friction (acts when a body rolls over a surface). In decreasing order they are static friction > sliding friction > rolling friction, so rolling friction is the smallest.

What it is

Friction is useful (walking, gripping, braking) but also wastes energy and wears out parts.

Answer

Advantages: friction lets us walk without slipping and lets vehicles stop when brakes are applied; it also helps us write and hold objects. Disadvantages: friction wears out the soles of shoes, tyres and machine parts, and it produces heat that wastes useful energy.

What it is

Friction is reduced using lubricants, ball bearings, streamlined shapes and polished surfaces.

Answer

Friction can be reduced by applying lubricants such as oil and grease, which form a thin layer between surfaces; by using ball bearings that convert sliding friction into much smaller rolling friction; and by polishing surfaces or giving vehicles a streamlined shape to reduce the friction of air and water.

What it is

Sound is a vibration that travels by shaking the particles of a medium.

Answer

Sound is produced by vibrating objects; the vibrations push the particles of the surrounding medium, creating compressions and rarefactions that travel outward as a wave. Sound needs a material medium (solid, liquid or gas) because it moves by making particles vibrate. In a vacuum there are no particles, so sound cannot travel.

What it is

Sound is produced by vibrating objects and travels as a disturbance through a medium.

Answer

Sound is produced when an object vibrates, that is, moves rapidly to and fro. For example, a drum makes sound when its stretched membrane vibrates, and in humans sound is produced by the vibration of the vocal cords in the voice box (larynx). When the vibration stops, the sound stops.

What it is

Sound needs a material medium (solid, liquid or gas) to travel and cannot pass through a vacuum.

Answer

No, sound cannot travel through a vacuum because it needs a material medium (solid, liquid or gas) whose particles vibrate and pass on the disturbance. In a vacuum there are no particles to carry the vibrations. This is shown by the bell-jar experiment, where the sound of a ringing bell fades as the air is pumped out.

What it is

Amplitude is the maximum displacement of a vibration, frequency is the number of oscillations per second, and time period is the time for one oscillation.

Answer

Amplitude is the maximum distance a vibrating object moves from its central rest position. Frequency is the number of complete oscillations made in one second, measured in hertz (Hz). Time period is the time taken to complete one oscillation. Frequency and time period are reciprocals of each other.

What it is

Loudness depends on the amplitude of a sound, while pitch depends on its frequency.

Answer

The loudness of a sound depends on the amplitude of the vibration - a larger amplitude produces a louder sound. The pitch (shrillness) of a sound depends on its frequency - a higher frequency produces a higher pitch, like a whistle, while a lower frequency produces a lower pitch, like a drum.

What it is

Humans hear sounds with frequencies from about 20 Hz to 20000 Hz.

Answer

The audible range for a normal human ear is from about 20 Hz to 20000 Hz. Sounds with frequencies below 20 Hz are called infrasound and those above 20000 Hz are called ultrasound; both are inaudible to humans, though animals such as dogs and bats can hear ultrasound.

What it is

Unpleasant, unwanted, loud sound is noise; too much of it in the environment is noise pollution.

Answer

The presence of excessive or unwanted loud sound in the environment is called noise pollution. Its harmful effects include loss of hearing, and stress-related problems such as lack of sleep, anxiety and high blood pressure. It can be reduced by planting trees along roads, which absorb sound, and by using silencers in vehicles and machines.

What it is

Electroplating uses electricity to coat one metal with a thin layer of another.

Answer

Electroplating is the process of depositing a layer of one metal over another using electricity (the chemical effect of electric current). When current passes through a solution of a metal salt, metal ions are deposited on the object connected to the negative terminal. It is used to coat iron with chromium or nickel to prevent rusting and to make cheap ornaments look like gold or silver.

What it is

Liquids that conduct electricity (like salt or acid solutions) do so through the movement of ions.

Answer

No, not all liquids conduct electricity. Liquids such as solutions of common salt, acids and bases conduct electricity because they contain freely moving ions that carry the current. Liquids such as distilled water, oil and alcohol are poor conductors because they have very few free ions.

What it is

Electroplating is depositing a layer of one metal on another using electricity.

Answer

Electroplating is the process of depositing a thin layer of a desired metal over another material using electric current. It is an application of the chemical effect of electric current. It is used to coat iron taps and car parts with shiny chromium to prevent rusting and improve appearance, and to coat iron cans with tin for storing food.

What it is

Reflection is light bouncing off a surface following two fixed rules.

Answer

First law: the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. Second law: the angle of incidence is always equal to the angle of reflection (i = r).

What it is

When light reflects, the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal lie in the same plane.

Answer

First law: the angle of incidence is equal to the angle of reflection, where both angles are measured from the normal (the line perpendicular to the mirror at the point of incidence). Second law: the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

What it is

Reflection from a smooth surface is regular and forms images; reflection from a rough surface is diffused and does not.

Answer

In regular reflection, light falling on a smooth surface such as a plane mirror is reflected in one direction, so parallel incident rays stay parallel and a clear image is formed. In diffused (irregular) reflection, light falling on a rough surface such as paper or a wall is reflected in many different directions, so no clear image is formed. In both cases the laws of reflection are still obeyed at each point.

What it is

Lightning is a huge electric spark between charged clouds or between a cloud and the ground.

Answer

During a thunderstorm, air currents make the top of a cloud positively charged and the bottom negatively charged. When the charge grows large enough it jumps as a giant spark - lightning - to another cloud or to the ground. A lightning conductor is a metal rod fixed at the top of a building and connected to the earth; it safely carries the lightning's charge into the ground and protects the building.

What it is

Rubbing certain objects transfers charge; there are two kinds of charge - positive and negative.

Answer

When two objects are rubbed together, such as a plastic comb through dry hair, charge is transferred from one to the other, so both become electrically charged. There are two kinds of charge, positive and negative. Like charges (both positive or both negative) repel each other, while unlike charges (one positive and one negative) attract each other.

What it is

Lightning is a huge electric discharge between charged clouds or between a cloud and the ground; a lightning conductor safely carries it to earth.

Answer

Lightning is caused by the movement of accumulated electric charges - an electric discharge - between clouds or between a charged cloud and the earth. A lightning conductor is a metal rod fixed on the top of a tall building and connected to a metal plate buried in the ground. It provides an easy conducting path so that the lightning's charge passes safely into the earth without damaging the building.

What it is

Speed tells how fast something moves - the distance covered in unit time.

Answer

Speed = distance / time = 60 / 10 = 6 metres per second (m/s). If the same distance is covered in less time the speed is higher; if it takes more time the speed is lower.

What it is

Distance travelled is the product of speed and the time for which the body moves.

Answer

Distance = speed x time = 10 x 30 = 300 metres (m). This is simply the speed formula rearranged: since speed = distance / time, distance = speed x time.

What it is

Speed tells how fast an object moves; it is the distance travelled per unit time.

Answer

Average speed = total distance / total time = 240 / 4 = 60 km/h. To convert to m/s, multiply by 1000/3600 (that is, divide by 3.6): 60 / 3.6 = 16.67 m/s approximately.