Basic Physics in Daily Life: Units, Motion, Energy, Heat, Light, Sound and Electricity

Physics explains everyday happenings such as why a bus passenger falls forward when the driver brakes, why rice takes longer to cook in the hills and why we see lightning before we hear thunder. Lok Sewa exams often ask short questions on units, laws and simple formulas, so learn the tables and formulas in this note well.

1. SI base units

The SI (International System of Units) is the worldwide measuring system. It has seven base units, from which all other units are made.

Note: mass is in kilogram, not gram, and temperature in kelvin, not degree Celsius.

2. Common derived units

Derived units are formed by combining base units.

3. Motion and Newton’s laws

Speed = distance ÷ time; its SI unit is m/s. Speed has size only, so it is a scalar quantity.

Velocity is speed in a stated direction (displacement ÷ time); having size and direction, it is a vector.

Acceleration is the rate of change of velocity (change in velocity ÷ time). Its unit is m/s². Negative acceleration, when a vehicle slows down, is called retardation.

To change km/h into m/s, multiply by 5/18. Example: 72 km/h = 72 × 5/18 = 20 m/s.

Newton’s three laws of motion:

1. First law (law of inertia): a body stays at rest or keeps moving in a straight line at the same speed unless an outside force acts on it. Example: when a bus stops suddenly, standing passengers lean forward because their bodies tend to keep moving.
2. Second law: force equals mass times acceleration, F = m × a. Example: a force of 10 N on a 2 kg trolley gives an acceleration of 5 m/s².
3. Third law: for every action there is an equal and opposite reaction. Examples: a gun moves back (recoil) when fired, and a rocket goes up as gases rush down.

4. Gravity, mass and weight

Every object pulls every other object (gravitation); the pull of the Earth is called gravity. Near the Earth’s surface a falling body gains speed at about 9.8 m/s², the acceleration due to gravity (g). On the Moon, g is about one-sixth of that on Earth.

Weight = mass × g. On the Moon a body keeps the same mass but weighs only about one-sixth as much.

5. Work, energy and power

Work = force × distance moved in the direction of the force; its unit is the joule. If you push a wall and it does not move, no work is done in physics.

Energy is the ability to do work, also measured in joules.

• Kinetic energy is energy due to motion: KE = ½ × m × v².
• Potential energy is energy due to position or height: PE = m × g × h. Water stored behind a dam has potential energy.

Law of conservation of energy: energy can neither be created nor destroyed; it only changes from one form to another. In a hydropower plant, stored water’s potential energy becomes kinetic energy and then electrical energy.

Power is the rate of doing work: power = work ÷ time, in watts. Example: 600 J of work in 3 seconds is 600 ÷ 3 = 200 W.

6. Pressure

Pressure = force ÷ area; its SI unit is the pascal. The same force gives more pressure on a small area and less on a large area. That is why:

• a sharp knife cuts better than a blunt one;
• school bags have wide straps so they do not hurt the shoulders.

Atmospheric pressure, the push of the air, is highest at sea level and decreases as we go higher.

7. Heat and temperature

Heat is energy that flows from a hotter body to a colder one, measured in joules. Temperature tells how hot or cold a body is and is measured with a thermometer.

Conversion formulas:

• F = (9/5 × C) + 32
• K = C + 273 (more exactly 273.15)

Worked example: normal body temperature is 37 °C. In Fahrenheit, 9/5 × 37 + 32 = 66.6 + 32 = 98.6 °F. In kelvin, 37 + 273 = 310 K.

The boiling point of water depends on air pressure. In the high mountains of Nepal air pressure is lower, so water boils below 100 °C and food cooks slowly. A pressure cooker raises the pressure inside and so raises the boiling point.

8. Light

Light travels in straight lines and can move through a vacuum at about 3 × 10^8 m/s.

Reflection is the bouncing back of light; the angle of incidence equals the angle of reflection. A plane mirror forms an upright, same-size, laterally inverted (left and right swapped) image.

Refraction is the bending of light as it passes from one medium to another; it makes a pencil in water look bent.

A convex lens (thicker at the middle) converges light; a concave lens (thinner at the middle) diverges it.

Defects of vision:

Dispersion is the splitting of white light into seven colours (VIBGYOR: violet, indigo, blue, green, yellow, orange, red), as in a prism. A rainbow forms when raindrops disperse sunlight through refraction and reflection inside the drops.

9. Sound

Sound is produced by vibrating objects. It needs a medium (solid, liquid or gas), so it cannot pass through a vacuum.

Sound travels fastest in solids, slower in liquids and slowest in gases. In air it moves at about 340 m/s, far slower than light, so we see lightning before we hear thunder.

The human ear can hear sounds from 20 Hz to 20,000 Hz. Sound below 20 Hz is infrasonic; sound above 20,000 Hz is ultrasonic.

An echo is sound heard again after reflecting from a hard surface; for a clear echo in air the surface should be at least about 17 m away.

10. Electricity basics

Electric current is the flow of charge, measured in amperes with an ammeter. Voltage (potential difference) pushes the current and is measured in volts with a voltmeter. Resistance opposes current; its unit is the ohm.

Ohm’s law: at a constant temperature, the current through a conductor is directly proportional to the voltage across it. So V = I × R. Example: a 12 V battery across a 4 Ω resistor gives a current of 12 ÷ 4 = 3 A.

Conductors such as silver, copper and aluminium let current pass easily; insulators such as rubber, plastic, glass and dry wood do not.

The commercial unit of electrical energy is the kilowatt-hour (kWh), called one unit on the bill (1 kWh = 3.6 × 10^6 J). Example: a 100 W bulb used for 10 hours uses 100 × 10 = 1000 Wh = 1 kWh.