Application of Forces & Transfer of Energy
9.1 Forces in Nature
Learning Objective: Show curiosity about the destructive power of forces in nature such as earthquakes, tsunamis, volcanic eruptions, and tropical cyclones.
Tropical Cyclones
Cause: Climate change leads to increased sea surface temperatures, resulting in longer and more intense storms.
Origin: Begin over warm oceans in tropical regions.
Characteristics: Accompanied by heavy rain and strong winds.
Impact: Causes flooding and landslides.
Wind Speed: Can reach up to 200 \text{ km/h}.
Example: 2024 Hurricane Beryl in the Caribbean.
Earthquakes
Mechanism: The Earth's surface consists of solid rock plates that continuously move very slowly.
Occurrence: When one plate slides over or against another plate, the exerted force results in earthquakes.
Example: 2024 Hualien Earthquake in Taiwan.
Volcanic Eruption
Mechanism: Deep within Earth, rocks melt to form magma.
Occurrence: The Earth moves violently when magma is pushed out towards the surface, leading to a volcanic eruption.
Release: Releases magma, volcanic ash, and gases.
Example 1: 2018 Volcán de Fuego eruption in Guatemala.
Example 2: The ancient city of Pompeii, Italy, was buried under 4 to 6 \text{ m} of volcanic ash during the eruption of Mount Vesuvius in AD 79.
Tsunami
Cause: Earthquakes and volcanic eruptions occurring beneath the ocean can trigger powerful waves called tsunamis.
Height: Tsunamis can reach heights of up to 40 \text{ m}.
Example: 2004 Indian Ocean Tsunami, affecting Thailand and Indonesia.
Constructive Power of Nature's Forces
Forces in nature can also shape the physical environment and create beautiful landforms.
Example: The 12 Apostles in Australia, formed by harsh ocean waves and strong winds.
9.2 Types of Forces
Learning Objectives:
Understand that a force can be a contact force (e.g., friction) or a non-contact force (e.g., magnetic force, gravitational force).
Measure force, using newton (\text{N}) as the SI unit.
Compare weight and mass.
Definition of Force
A force is either a 'push' or a 'pull'.
Can be produced by touching an object with our body or with another object (e.g., gripping poles in a bus).
Can also act when objects do not touch (e.g., attractive force between magnets).
Two types of forces: Contact forces and Non-contact forces.
9.2.1 Contact Forces
Definition: Forces acting between two objects that are in physical contact with each other.
Examples:
Friction
Elastic force
a) Friction
Definition: The force that opposes motion between two surfaces in contact.
Examples:
Pushing an object along the floor: Friction opposes the motion.
Walking: Rough soles of shoes/feet rub against the ground; friction pushes the body forward.
Writing on paper: Pencil tip rubs against the paper's rough surface, friction transfers carbon lead, and allows us to hold the pencil.
Lighting a matchstick: Friction between the matchstick head and the rough box sides causes heat, igniting the match.
Parachuting: The large parachute creates a large frictional force with the air (air resistance), reducing the parachutist's fall speed.
b) Elastic Force
Definition: A force acting on a stretched or compressed elastic object to return it to its original shape.
Examples:
Rubber band
Bow string
Spring
Catapult slingshot
Bungee jumping rope
9.2.2 Non-Contact Forces
Definition: Forces acting between two objects that are not touching each other.
Examples:
Gravitational force
Magnetic force
a) Gravitational Force
Definition: The force that attracts two objects towards each other.
Mechanism: Stars and planets exert gravitational force, pulling objects towards their center.
Weight: When a planet exerts gravitational force on an object, the object is said to have weight.
b) Magnetic Force
Definition: The force exerted between a magnet and another magnet or magnetic material (e.g., iron, steel).
Poles: Every magnet has a North pole and a South pole.
Like poles repel: North and North, South and South.
Unlike poles attract: North and South.
Example: Maglev trains operate on the principle of magnetic repulsion between the train and the track, allowing the train to levitate.
9.2.3 Mass vs. Weight
Mass
Definition: The amount of matter in an object.
SI Unit: Kilogram (\text{kg}).
Measuring Instrument: Beam balance or electronic balance.
Location: Mass of an object remains the same regardless of location (e.g., on Earth, Mars, or Jupiter).
Weight
Definition: The measure of the gravitational force acting on an object.
SI Unit: Newton (\text{N}) (same as force).
Measuring Instrument: Spring balance (extension spring balance for pulling force, compression spring balance for pushing force).
Location: Weight changes depending on the strength of the planet's gravity (gravitational field strength).
Stronger gravity results in higher weight.
Example: Weight on Earth (600 \text{ N} for 60 \text{ kg} mass) $>$ weight on Moon (100 \text{ N} for 60 \text{ kg} mass) because Earth has stronger gravity.
Comparison Table: Mass vs. Weight
Feature | Weight | Mass |
|---|---|---|
What is it? | Gravitational force exerted on an object | Amount of matter in an object |
How does it change? | May change with location | Remains constant regardless of location |
SI Unit | Newton (\text{N}) | Kilogram (\text{kg}) |
Measuring Instrument | Spring balance | Electronic balance, beam balance |
9.3 Interactions between Objects
Learning Objectives: Recognize that interactions between two or more objects result in energy transfer which can cause changes to:
The state of rest or motion of an object.
Turning effects in objects (e.g., spanners, levers to open tins).
The size and/or shape of an object.
Pressure on objects.
9.3.1 Effects of Forces
A force can:
a) Move a stationary object.
* Example: Taking a penalty shot – a stationary soccer ball starts moving after being kicked.
b) Change the speed of a moving object.
* Example: A cyclist riding a bicycle – applying larger/smaller force on pedals changes speed.
c) Change the direction of a moving object.
* Example: Hitting a shuttlecock – the shuttlecock changes direction when hit by a badminton racket.
d) Stop a moving object.
* Example: Goalkeeper saving a shot – the ball stops moving when caught.
e) Change the shape of an object.
* Example: Kneading plasticine – force application changes the plasticine's shape.
f) Change the size of an object.
* Example: Moulding clay – moulding a lump of wet clay changes its size and shape.
* Consequence: The effect of forces on a car's size and shape after impact has consequences for its safety.
g) Cause a turning effect.
* Example 1: Opening cans with a spoon – applying force creates a turning effect to open the lid.
* Example 2: Tightening nuts with a spanner – applying force to turn the spanner tightens nuts.
* Example 3: Opening a door – a force at the doorknob makes the door rotate at the hinges, producing a turning effect.
* Example 4: Using a crowbar – a force on the handle turns the crowbar to pull out a nail.
* Example 5: Using a fishing rod – a large force on the handle lifts the rod, lifting the fish over a large distance.
* Example 6: Using a wheelbarrow – an upward force on the handles makes a heavy load easier to move.
9.3.2 Pressure
Learning Objectives:
Investigate pressure using the formula, pressure = force/area.
Show an appreciation of daily life phenomena associated with pressure (e.g., high-heeled shoes, cutting edge of a knife).
[G3 only] Atmospheric pressure (e.g., use of suction cups, drinking from straws).
[G3 only] Pressure due to liquid (e.g., submarines have depth limits).
Definition: Pressure is an effect of a force acting on an object; it is the force acting per unit contact area.
Dependence: The amount of pressure exerted depends on the amount of force and the contact area on which the force acts.
Formula:
p = \frac{F}{A}, where p is pressure, F is Force, and A is Contact area.
Therefore, F = p \times A and A = \frac{F}{p}.
Units:
Force is measured in newton (\text{N}).
Contact area is measured in \text{m}^2 or \text{cm}^2.
SI Unit of pressure: Pascal (\text{Pa}) or \text{N/m}^2.
Other common unit: \text{N/cm}^2.
Effect of Contact Area: The smaller the contact area, the greater the pressure.
Balloon Experiment: A balloon on one nail bursts (small contact area, high pressure), but on many nails it doesn't (large contact area, low pressure).
Lorry Example: A 2000 \text{ kg} lorry with 4 wheels will sink deeper into a muddy field than a 2000 \text{ kg} lorry with 6 wheels, because the 4-wheel lorry has a smaller contact area and thus exerts a larger pressure.
Factors Affecting Pressure
Force:
Increase Force: Carrying many durians in a canvas bag causes fingers to hurt due to large weight (force) creating large pressure.
Decrease Force: Carrying fewer durians reduces pain as less weight (force) means less pressure.
Conclusion: When the force acting on a surface decreases, the pressure decreases.
Area:
Small Area: Narrow and thin handles of a canvas bag cause large pressure on fingers.
Large Area: Padding the handles with a thick cloth increases the area, reducing pain as larger area means less pressure.
Conclusion: When the area of the surface increases, the pressure decreases.
Pressure in Everyday Life
Pins: Are narrow and thin (small contact area) to exert a larger pressure, making it easier to pierce wooden boards.
Knives and Scissors: Have very small cutting edges (small contact area) to exert larger pressure, cutting through objects easily.
Soccer Boots: Have studs to reduce the contact area with the field, exerting larger pressure and increasing grip.
9.3.2a Liquid Pressure (G3 only)
Observation: Pressure increases as you go deeper underwater due to the increased amount of water above.
Example 1: Dam Walls: The base of a dam has a much thicker wall than its top because water pressure is greater at the base, requiring a thicker wall to withstand it.
Example 2: Submarines: Submarines need strong bodies to withstand very high pressure at great depths. Diving beyond a certain depth can damage the submarine as pressure becomes too high. The deeper a submarine dives, the greater the underwater pressure.
9.3.2b Atmospheric Pressure (G3 only)
Definition: Earth is surrounded by a thick layer of air consisting of air particles with mass. This layer of air particles pushes down and exerts pressure on objects on Earth.
Altitude: As altitude increases (e.g., on a mountain), there are fewer air particles pushing down, so atmospheric pressure decreases.
Atmospheric Pressure in Daily Lives
Drinking from a Straw: Sucking through a straw lowers the air pressure inside it. The higher atmospheric pressure outside the straw then pushes the liquid up into the straw.
Suction Cups: When a suction cup is pressed against a wall, air is pushed out from underneath it, lowering the pressure inside. The higher atmospheric pressure outside then pushes the suction cup against the wall, holding it in place.
9.3.3 Work Done
Learning Objectives:
State the SI unit of work and energy as the joule (\text{J}).
Identify that work done is an example of energy transfer that occurs when an object moves in the direction of a force.
Energy: The ability to do work.
SI Unit: Joule (\text{J}) for both energy and work done. One joule of energy is needed to do one joule of work.
Conditions for Work to be Done
Two conditions must be met:
A force is applied on the object.
The object moves in the direction of the applied force.
If any of these conditions are not met, no work is done.
Examples of Work Done
Example 1: Marie rearranging equipment.
Scenario: Marie picks up a box from the floor and places it on a high shelf (2 \text{ m} above, applying 40 \text{ N} upward force).
Analysis:
Upward force of 40 \text{ N} is applied: Condition 1 met.
Box moves upwards for 2 \text{ m} (in the direction of force): Condition 2 met.
Conclusion: Work is done.
Example 2: Marie holding a basket.
Scenario: Marie holds a basket of tennis balls (50 \text{ N} upward force) for a while without moving, holding it 1 \text{ m} above the floor.
Analysis:
Upward force of 50 \text{ N} is applied: Condition 1 met.
Basket does not move: Condition 2 not met.
Conclusion: No work is done.
Example 3: Marie carrying a basket horizontally.
Scenario: Marie carries the basket (50 \text{ N} upward force) for 20 \text{ m} along a corridor, keeping it at the same height.
Analysis:
Upward force of 50 \text{ N} is applied: Condition 1 met.
Basket moves to the right (different direction than the upward force): Condition 2 not met for the 50 \text{ N} upward force.
Conclusion: No work is done by the 50 \text{ N} upward force.
9.4 Conservation of Energy
Learning Objectives:
Recognize that energy cannot be created nor destroyed; it is conserved when it is transferred (from one object to another) and/or converted (from one form to another).
Infer that energy can be converted from one form to another.
SI Unit of energy: Joule (\text{J}).
Types of Energy: Electrical energy, Thermal energy (heat), Light energy, Sound energy, Nuclear energy.
Kinetic Energy (KE)
Definition: Energy a body possesses due to its motion.
Relationship: Any objects that move possess kinetic energy; the faster the speed, the greater the kinetic energy.
Potential Energy (PE)
Definition: Energy stored in an object due to its position, state, or shape.
Forms: Elastic Potential Energy, Chemical Potential Energy, Gravitational Potential Energy.
Elastic Potential Energy (EPE)
Definition: Energy stored in a compressed or stretched elastic object.
Examples: Compressed spring, stretched rubber band, stretched bow string.
Chemical Potential Energy (CPE)
Definition: Energy stored in fuels (e.g., petrol, coal), batteries, bombs, food, and in our body.
Gravitational Potential Energy (GPE)
Definition: Energy possessed by an object due to its height above the ground.
Reference Point: GPE at ground level is fixed to be 0 \text{ J}.
Relationship: The greater the height from ground level, the greater the GPE.
Energy Conversion
Energy can be converted from one form to another.
Example 1: Archer and Bow:
Archer applies force, pulls bowstring (work done).
Energy Conversion: Chemical potential energy (in arm muscles) to Elastic potential energy (in the bowstring).
Archer releases arrow (work done on arrow).
Energy Conversion: Elastic potential energy (in bowstring) to Kinetic energy (in the moving arrow).
Example 2: Light bulb: Electrical energy converted to light energy and thermal energy.
Example 3: Battery-operated loudspeaker: Chemical potential energy (in battery) converted to electrical energy, which is then converted to sound energy.
Example 4: Electrical Fan: Electrical energy converted to kinetic energy, thermal energy, and sound energy.
Example 5: Handphone: Chemical potential energy (in battery) converted to electrical energy, which is then converted to light energy, sound energy, and thermal energy.
Example 6: Roller Coaster (moving down): Gravitational potential energy converted to kinetic energy, thermal energy, and sound energy.
Example 7: Man jogging: Chemical potential energy (in the body) converted to kinetic energy.
Principle of Conservation of Energy
Statement: Energy cannot be created or destroyed. It can only be converted from one form to another or transferred from one object to another, but the total energy in the system will remain constant.
Example: Energy Conversion of a Swing
Highest points (A and E): The swing has gravitational potential energy only.
Moving from A to C (lowest point): Some GPE is converted to KE. At the lowest point C, all GPE from A is converted to KE.
Moving from C to D: Some KE is converted back to GPE.
Moving from D to E: The remaining KE moves the swing to E, where it again has potential energy only.
Cycle: The same energy conversions (GPE $\leftrightarrow$ KE) occur as the swing moves from E back to A.
Conservation: The total amount of energy remains constant at all points during the swinging motion.
The gravitational potential energy 'lost' between A and C is converted into kinetic energy at C. This continuous conversion allows the swing to move to and fro, ideally forever if no other energy conversion (like to thermal energy due to air resistance) takes place.
Example: Energy Conversion of a Roller Coaster
Highest point (A): The car has maximum Gravitational Potential Energy (GPE).
Moving from A to B: GPE is converted to Kinetic Energy (KE).
Moving from B to C: Some KE is converted back to GPE.
Lowest point (D): All GPE (from its initial height at C relative to D) is converted to KE. The car has maximum KE at this point.
Throughout the ride, energy also converts to Thermal Energy (TE) and Sound Energy due to friction and air resistance.
9.5 Sources of Energy
Learning Objectives: Show an appreciation of the uses of various sources of energy (e.g., fossil fuels, solar, hydroelectric, wind, geothermal, biofuels, and nuclear energy) and their impact on the environment.
Types of Energy Sources
Type of Energy Source | Characteristic | Examples |
|---|---|---|
Non-renewable sources | Limited; will run out | a) Fossil Fuels |
Renewable sources | Can be sustained or renewed indefinitely | b) Solar energy, c) Hydroelectric energy, d) Wind energy, e) Geothermal energy, f) Biofuels, g) Nuclear energy |
a) Fossil Fuels
Origin: Most non-renewable energy sources are derived from fossil fuels, formed over millions of years from the remains of dead plants and animals.
Types: Crude oil, natural gas, coal.
Energy Conversion in Power Stations: Chemical potential energy (in fuel) $\rightarrow$ Heat (in water) $\rightarrow$ Kinetic energy (of turbines) $\rightarrow$ Electrical energy.
Environmental Impact:
Negative: Burning fossil fuels produces air pollutants and greenhouse gases (e.g., carbon dioxide).
Carbon dioxide traps heat, causing global warming.
Global warming leads to melting polar ice, rising sea levels, and erratic, destructive weather patterns.
b) Solar Energy
Mechanism: Photovoltaic or solar cells directly convert light energy from sunlight into electrical energy.
Energy Conversion: Light energy (from Sun) $\rightarrow$ Electrical energy.
Example: Singapore installs solar panels on many buildings, including Loyang View Secondary School.
Environmental Impact:
Positive: Does not produce harmful substances or pollutants during operation.
Negative: Production and disposal of solar panels generate toxic waste, requiring careful management.
c) Hydroelectric Energy
Mechanism: A hydroelectric power station stores water in a reservoir behind a dam. The flow of water from the reservoir turns the blades of a turbine to generate electrical energy.
Energy Conversion: Gravitational potential energy (of water) $\rightarrow$ Kinetic energy (of water) $\rightarrow$ Kinetic energy (of turbines) $\rightarrow$ Electrical energy.
Process:
Reservoir water stores gravitational potential energy.
Water runs down passageway at high speed, converting GPE to KE.
Moving water turns turbine blades, converting water's KE to turbine's KE.
Turbine's KE is converted to electrical energy in the generator.
Environmental Impact:
Positive: Does not produce pollutants during operation.
Negative: Building dams causes extensive flooding, destroying forests, wildlife habitats, and farmland.
Kills plants and forces animals to relocate.
Dams prevent the natural flow of sediments and nutrients down rivers, affecting surrounding ecosystems.
d) Wind Energy
Mechanism: Converts the kinetic energy of moving air (wind) into electricity by rotating one or more turbines.
Energy Conversion: Kinetic energy (of air) $\rightarrow$ Electrical energy.
Example: Royd Moor wind farm in England.
Environmental Impact:
Positive: Does not produce pollutants during operation.
Negative: Requires large land areas for wind farms, destroying natural habitats and causing wildlife loss.
Rotating turbines can cause deaths of birds and bats.
Produces low-frequency noise, potentially causing nausea and headaches in some people.
e) Geothermal Energy
Mechanism: Derived from hot rocks deep underground in volcanic areas. Water is drilled deep into the Earth, heated into steam, which then powers a turbine to produce electricity.
Energy Conversion: Heat $\rightarrow$ Kinetic energy (of steam) $\rightarrow$ Kinetic energy (of turbines) $\rightarrow$ Electrical energy.
Process:
Cooled water is pumped into hot rocks.
Water is heated by geothermal energy, turning into steam.
Hot water and steam are pumped to the surface.
Steam powers turbines and generators to produce electrical energy.
Example: Geothermal power station in Indonesia.
Environmental Impact:
Negative: Geothermal sites often require clearing large land areas, destroying wildlife habitats and affecting biodiversity.
Traces of toxic elements buried underground can be drawn out and may harm the environment if not handled properly.
f) Biofuels
Composition: Usually made from animal waste or plant materials unsuitable for human consumption. Can also be produced from recycling food waste (e.g., used cooking oil).
Use: Burnt to produce energy, similar to fossil fuels.
Examples: Ethanol from sugarcane and corn can be converted into biodiesel fuel for motor vehicles.
Carbon Cycle Relevance: Plants grown for biofuels absorb carbon dioxide during photosynthesis, balancing the CO_2 released when biofuels are burnt.
Environmental Impact:
Negative: Produces greenhouse gases, contributing to global warming, similar to fossil fuels.
Positive: Plants used can be grown quickly, and their CO_2 absorption helps balance emissions, making them potentially carbon-neutral.
Positive: Recycles and reduces waste by utilizing used cooking oil and food waste.
g) Nuclear Energy
Mechanism: Energy harnessed from the nucleus of an atom through nuclear reactions (e.g., when a heavy atom like uranium splits, releasing a large amount of heat energy). This heat is used to heat water into steam, which spins turbines to generate electricity.
Environmental Impact:
Positive: Requires less land space to generate the same amount of energy compared to other alternative sources.
Negative: Potential risks to the health and safety of communities living near nuclear power plants.
Negative: Requires careful disposal and storage of nuclear waste to prevent leakage.
Famous Nuclear Accidents:
1986 Chernobyl disaster.
2011 Fukushima nuclear power meltdown.