Std. 9
Physics

Physics uses SI units so measurements are consistent. Length is measured in metre (m), mass in kilogram (kg) and time in second (s).

Least count is the smallest measurement an instrument can read reliably. A smaller least count generally allows finer measurement.

Vernier callipers measure small lengths and diameters more precisely than an ordinary ruler. Understand the main scale, Vernier scale and zero error.

A screw gauge measures very small dimensions such as wire diameter. For the prescribed screw gauge, pitch is 1 mm and least count is 0.01 mm.

T=2π√(l/g). T is time period, l is pendulum length and g is acceleration due to gravity. The syllabus requires the relation and simple numerical problems, not its derivation.

Plotting length l against T² gives a straight-line relationship. The slope helps interpret the proportionality between l and T².

A scalar has magnitude only, such as distance and speed. A vector has magnitude and direction, such as displacement and velocity.

Distance is total path length. Displacement is the directed change from initial to final position and can be zero even when distance is not.

speed=distance/time velocity=displacement/time. Speed has magnitude only; velocity has direction.

a=(v−u)/t. Acceleration is rate of change of velocity; retardation acts opposite to the direction of motion.

v=u+at s=ut+½at² s=½(u+v)t v²=u²+2as

The slope of a distance-time graph gives speed. The slope of a velocity-time graph gives acceleration; the area under a velocity-time graph represents displacement.

Force is an interaction that can change motion or shape. Contact forces include friction, tension and normal reaction; non-contact forces include gravitational, electric and magnetic forces.

An object remains at rest or in uniform straight-line motion unless acted on by an external unbalanced force. This introduces inertia.

F=ma for constant mass. The force is related to the rate of change of momentum and acts in its direction.

For every action there is an equal and opposite reaction. The two forces act on different bodies.

p=mv. Momentum depends on mass and velocity and is a vector quantity.

Know SI and CGS units of force and the prescribed relation with gravitational units.

P=F/A. Pressure is normal force acting per unit area.

P=hρg for gauge pressure at depth h in a liquid. Pressure increases with depth and depends on density.

The atmosphere exerts pressure because air has weight. Atmospheric pressure changes with altitude.

Pressure applied to an enclosed fluid is transmitted equally in all directions. Hydraulic machines use this principle.

Hydraulic brakes, lifts and presses use pressure transmission through liquids.

A fluid exerts an upward force on an immersed object. This buoyant force is called upthrust.

A body immersed wholly or partly in a fluid experiences upthrust equal to the weight of fluid displaced.

An object floats when buoyant force balances its weight. A floating body's average density is less than that of the fluid.

RD=density of substance/density of water. Relative density has no unit.

Ships, hydrometers and submarines use buoyancy and displacement principles.

Temperature indicates degree of hotness; heat is energy transferred because of a temperature difference.

Q=mcΔT. Specific heat capacity is heat needed to raise unit mass by one degree.

C=Q/ΔT. It is heat required to raise the temperature of the whole body by one degree.

During a change of state, supplied heat can change internal arrangement without changing temperature at the phase-change point.

Q=mL. Latent heat is energy absorbed or released during change of state without temperature change.

Mechanical, electrical, chemical and thermal energy can change form; total energy is conserved.

Reflection is the return of light into the same medium after striking a surface.

i=r. The incident ray, reflected ray and normal lie in the same plane.

A plane mirror forms a virtual, erect image of the same size; the image appears as far behind the mirror as the object is in front.

Concave mirrors converge parallel rays; convex mirrors diverge them. Know pole, principal axis, centre of curvature and focus.

1/f=1/v+1/u with the appropriate sign convention.

Use principal rays accurately and identify whether the image is real/virtual, erect/inverted and magnified/diminished.

Sound is produced by vibrating bodies and requires a material medium. In air it travels as a longitudinal wave.

f=1/T. Frequency is vibrations per second and determines pitch.

v=fλ, where v is wave speed, f frequency and λ wavelength.

Amplitude is maximum displacement from mean position. Greater amplitude is associated with greater loudness.

Sound can reflect from surfaces. An echo is reflected sound heard separately from the original under suitable conditions.

A typical human audible range is about 20 Hz to 20 kHz. Ultrasound is above it; infrasound is below it.

I=Q/t. Current is rate of flow of electric charge.

V=W/Q. Potential difference is work done per unit charge.

V=IR. For a conductor at constant physical conditions, current is proportional to potential difference.

R=ρl/A. Resistance depends on material, length, cross-sectional area and temperature.

P=VI=I²R=V²/R. Power tells how quickly electrical energy is transferred or converted.

In series the same current flows through each resistor. In parallel the potential difference across each branch is the same.

A magnetic field is the region where a magnetic pole or moving charge experiences magnetic influence. Field direction is shown by field lines.

Earth behaves approximately like a giant magnet and its field helps a compass align approximately north-south.

A current-carrying coil produces a magnetic field. A soft iron core can make the field much stronger.

Increasing current, increasing turns and using a suitable magnetic core can strengthen a simple electromagnet.

Electromagnets are used in electric bells, relays, lifting devices and other equipment.

Magnetic field lines should have correct direction and should never cross one another.

Radioactivity is spontaneous disintegration of unstable atomic nuclei accompanied by radiation.

Alpha particles are helium nuclei; beta radiation consists of fast charged particles; gamma radiation is electromagnetic radiation.

Alpha has the least penetrating power and gamma the greatest among the three; their ionising abilities show the opposite general trend.

N=N₀(1/2)^(t/T½). Half-life is the time required for half the radioactive nuclei in a sample to decay.

Radioisotopes have applications in medicine, industry, agriculture and scientific research.

Ionising radiation can damage tissue, so exposure time, distance, shielding and controlled handling are important.