Lets have a look at some of the most important physics formula and concepts from class 9 NCERT book. Do let me know if I have missed something.
CBSE Class 9 Physics Formulas and Concepts
Chapter 7: Motion
Distance and Displacement:
Distance: Scalar quantity representing the total path covered.
Displacement: Vector quantity representing the shortest distance between the initial and final position.
Speed and Velocity:
Speed: Speed = Distance / Time
Velocity: Velocity = Displacement / Time
Acceleration:
Acceleration: a = (v – u) / t, where:
u = initial velocity, v = final velocity, t = time.
Equations of Motion (for uniform acceleration):
v = u + at
s = ut + ½at²
v² = u² + 2as
Derivation of s = ut + ½at²:
We know: s = Average Velocity × Time
Average Velocity = (u + v) / 2
Substitute v = u + at into the equation:
s = [(u + (u + at)) / 2] × t
s = ut + ½at²
Uniform Circular Motion:
Speed in circular motion: v = 2πr / T, where r = radius, T = time period.
Chapter 8: Force and Laws of Motion
First Law of Motion:
An object remains in its state of rest or uniform motion unless acted upon by an external unbalanced force.
Second Law of Motion:
Force: F = ma, where F = force, m = mass, a = acceleration.
Derivation: F = Rate of change of momentum = (mv – mu) / t
F = m(v – u) / t = ma
Third Law of Motion:
For every action, there is an equal and opposite reaction.
Momentum:
Momentum: p = mv, where p = momentum, m = mass, v = velocity.
Change in momentum: Δp = F × t
Chapter 9: Gravitation
Universal Law of Gravitation:
F = G × (m₁ × m₂) / r², where:
F = gravitational force, G = gravitational constant, m₁ and m₂ = masses of two objects, r = distance between them.
Acceleration Due to Gravity:
g = GM / R², where:
G = gravitational constant, M = mass of Earth, R = radius of Earth.
Weight:
W = mg, where W = weight, m = mass, g = acceleration due to gravity.
Density:
Density = Mass / Volume
Relative Density:
Relative Density = Density of a substance / Density of water
Chapter 10: Work and Energy
Work:
W = F × d × cos(θ), where:
F = force, d = displacement, θ = angle between force and displacement.
Kinetic Energy (KE):
KE = ½mv², where m = mass, v = velocity.
Derivation of KE:
Work done: W = F × d
F = ma and d = v² / 2a (from equations of motion)
W = ma × (v² / 2a) = ½mv²
Potential Energy (PE):
PE = mgh, where m = mass, g = acceleration due to gravity, h = height.
Power:
P = W / t, where P = power, W = work, t = time.
Energy:
Energy = Power × Time
Chapter 11: Sound
Speed of Sound:
v = λ × f, where v = velocity, λ = wavelength, f = frequency.
Echo:
Distance: d = v × t, where v = speed of sound, t = time.
Laws of Reflection of Sound:
Angle of incidence = Angle of reflection
Intensity and Loudness:
Intensity ∝ Amplitude²
