Physics Formula Hkdse

Physics Formulas for HKDSE (Hong Kong Diploma of Secondary Education)

The HKDSE Physics exam covers a wide range of topics, from mechanics to electromagnetism. Understanding key physics formulas is essential for success in the exam. Below is a guide to some of the most important physics formulas you need to know, along with examples of how to use them.

1. Mechanics

Newton’s Second Law of Motion

Formula:

F = ma

Where:
- F is the force (in newtons, N)
- m is the mass (in kilograms, kg)
- a is the acceleration (in meters per second squared, m/s²)

Example:

A car with a mass of 1,000 kg accelerates at 2 m/s². What is the force acting on the car?

Solution:

F = 1000 × 2 = 2000 N

Kinematic Equations for Uniform Acceleration

v = u + at
s = ut + ½at²
v² = u² + 2as

Where:
- v is the final velocity (in meters per second, m/s)
- u is the initial velocity (in meters per second, m/s)
- a is the acceleration (in m/s²)
- t is the time (in seconds, s)
- s is the displacement (in meters, m)

Example:

A car starts from rest and accelerates at 3 m/s² for 5 seconds. What is its final velocity?

Solution:

v = u + at = 0 + (3 × 5) = 15 m/s

2. Energy

Kinetic Energy (KE)

Formula:

KE = ½mv²

Where:
- m is the mass (in kg)
- v is the velocity (in m/s)

Example:

What is the kinetic energy of a 50 kg object moving at 10 m/s?

Solution:

KE = ½ × 50 × 10² = 2500 J

Gravitational Potential Energy (GPE)

Formula:

GPE = mgh

Where:
- m is the mass (in kg)
- g is the acceleration due to gravity (9.8 m/s²)
- h is the height (in meters, m)

Example:

What is the gravitational potential energy of a 10 kg object 5 meters above the ground?

Solution:

GPE = 10 × 9.8 × 5 = 490 J

3. Electromagnetism

Ohm’s Law

Formula:

V = IR

Where:
- V is the voltage (in volts, V)
- I is the current (in amperes, A)
- R is the resistance (in ohms, Ω)

Example:

If a resistor has a resistance of 5 ohms and the current flowing through it is 2 amperes, what is the voltage across the resistor?

Solution:

V = 2 × 5 = 10 V

Power in an Electric Circuit

Formula:

P = VI

Where:
- P is the power (in watts, W)
- V is the voltage (in volts, V)
- I is the current (in amperes, A)

Example:

What is the power dissipated by a device that operates at 12 volts and draws a current of 3 amperes?

Solution:

P = 12 × 3 = 36 W

4. Waves and Optics

Wave Speed

Formula:

v = fλ

Where:
- v is the wave speed (in meters per second, m/s)
- f is the frequency (in hertz, Hz)
- λ is the wavelength (in meters, m)

Example:

A wave has a frequency of 50 Hz and a wavelength of 2 meters. What is the speed of the wave?

Solution:

v = 50 × 2 = 100 m/s

Snell’s Law

Formula:

sin i / sin r = v₁ / v₂

Where:
- i is the angle of incidence
- r is the angle of refraction
- v₁ and v₂ are the wave speeds in the respective mediums

Example:

If the angle of incidence is 30 degrees and the wave speed changes from 300 m/s to 200 m/s, what is the angle of refraction?

Solution:

sin r = (2/3) × sin 30° ≈ 0.333 → r ≈ 19.5°

Conclusion

Understanding these key formulas is crucial for tackling the physics questions in the HKDSE exam. Practice applying these formulas to a variety of problems to strengthen your grasp of the concepts and improve your performance on the test.