Physics

Junior Science 2025-2026: Physics - Forces, Energy and Energy Resources - Sound

Learning Intentions

Forces
  - Recall that forces can be balanced or unbalanced.
  - Appreciate that balanced forces keep stationary objects stationary; moving objects will continue moving at a constant speed.
  - Understand
    - Mass: Amount of matter measured in kilograms (kg) or grams (g).
    - Weight: A force measured in Newtons (N).
    - Earth's gravitational pull: 10 N acting on every kilogram (w = mg).
Mass and Weight
- Mass remains the same on different planets; however, weight changes.
Air and Pressure
- Moving air has a speed; fast-moving air results in low pressure.
- Shape affects airflow.

Using Forces

Types of Forces

Balanced Forces: When two forces acting in opposite directions are equal, resulting in a stable condition (e.g., a book on a table).
Unbalanced Forces: When two forces acting in opposite directions are not equal, resulting in acceleration or movement.

Examples

Parachutist: Moves at a constant speed when the downward force of gravity is balanced by the upward force of air resistance.
Book on a table: Remains stationary because the weight (downward force) is balanced by the table's upward force.

Resultant Force

When resultant force is balanced:
- Stationary object remains stationary.
- Moving object continues at the same speed.
When forces are unbalanced, the object will change its state of motion.

Gravity and Weight

Definitions

Weight: Force caused by gravity, measured in Newtons (N).
Mass: Measure of matter in an object; units are grams (g) or kilograms (kg).

Gravitational Force on Earth

On Earth, 1 kg of mass has a weight of 10 N due to gravitational pull.
To find weight: $w = m imes g$ ,
where $g = 10 N/kg$ on Earth.

Weight Variation on Other Celestial Bodies

The Moon has a gravitational pull of approximately 1/6th of that on Earth.
  Therefore:
  - A mass of 50 kg on Earth has a weight of 500 N.
  - The same mass on the Moon has a weight of approximately 83.3 N.

Gravitational Pull and Distance

Gravity depends not just on mass but also on the distance from the mass.
- In deep space, gravitational effects weaken due to distance from celestial bodies.

Astronauts and Weightlessness

Astronauts experience weightlessness on the International Space Station due to continuous free-fall towards Earth, despite still having mass.

Measurements and Experiments

Measuring Mass and Weight

Use top pan balance to measure mass in kilograms.
Use a Newton-meter to measure weight in Newtons.

Example Experiment

Measure different objects:
  - Example:
    | Object | Mass (kg) | Weight (N) |
    |--------------|-----------|-------------|
    | Pencil case | 0.5 | 5 |
    | Mobile phone | 0.15 | 1.5 |

Air Pressure and Movement

How Gas Particles Move

Gas particles move rapidly and collide, creating pressure.
Pressure increases with more collisions (more particles).

Air Pressure Movement

Pressure moves from high to low areas.
The marshmallow experiment shows how decreasing pressure affects objects (e.g., a marshmallow expands in lower pressure).
Air has mass and exert pressure, which affects objects submerged in it.

Bernoulli’s Principle

Explanation

Fast-moving air results in lower pressure; slower air has higher pressure.
When you blow across a piece of paper, the paper rises due to higher pressure below and lower pressure above (Bernoulli Effect).

Application to Flight

Airplane wings are designed to create lower pressure above and higher pressure below, resulting in lift.

Forces Acting on Aircraft

Thrust: Forward force produced by engines.
Drag: Resistance force acting against thrust.
Lift: Upward force generated due to pressure difference above and below the wing.
Gravity: Downward force acting on the airplane.

Energy and Energy Resources

Learning Intentions

Recognize different energy types and fuels.
Understand energy transfer diagrams.
Appreciate the sun as the ultimate energy source.
Differentiate between renewable and non-renewable resources.

Types of Energy

Chemical Energy: Stored in fuels, batteries, and food (e.g., petrol).
Elastic Potential Energy: Found in compressed springs or stretched materials.
Gravitational Potential Energy: Possessed by an object due to its height (e.g., at the top of a mountain).

Energy Resources: Fossil Fuels Formation

Coal, oil, and gas formed from ancient plants and marine organisms over millions of years through pressure and heat conversion.
They are non-renewable and release chemical energy when burned.

Renewable Energy Sources

Include wind, solar, tidal, geothermal, and hydroelectric energies.
They can be replenished within a human lifetime.

Sound and its Properties

Learning Intentions

Understand how sound is produced, transmitted, and perceived.
Recognize the range of human hearing and impairments.

How Sound Travels

Sound is caused by vibrations and travels through solids, liquids, and gases, but not in a vacuum.

Anatomy of the Ear

Pinna: Collects sound waves.
Ear Canal: Funnels sound to the eardrum.
Eardrum: Vibrates to enter sound into the middle ear.
Inner Ear Components: Cochlea converts vibrations into neural signals.

Hearing Range

Human hearing range: from 20 Hz to 20 kHz.
- Frequencies lower than 20 Hz: Infrasound.
- Frequencies higher than 20 kHz: Ultrasound.

Sound Pollution

Noise is defined as unwanted or disturbing sound, contributing to discomfort and potential hearing damage.
Control measures include ear protection in noisy environments.

Key Physics Concepts

Definitions

Mass: Amount of matter (kg or g).
Weight: Gravitational force acting on an object (N).
Speed of Sound: Faster in solids than in liquids, and slowest in gases.

Important Relationships

Weight calculation via: $w = m imes g$
Gravitational field strengths vary by location.

Conclusion

Understanding forces, energy types, and the mechanics of sound enhances comprehension of physical sciences and their applications in technology and environmental considerations.