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

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

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

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

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

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.