forces and energy
2026 Secondary 2 Science - Chapter 1: Forces and Energy
Learning Outcomes
Understanding Forces:
A force can be classified as:
Contact Force: e.g., frictional force.
Non-contact Force: e.g., magnetic and gravitational forces.
Understanding Speed:
Speed can be calculated using the formula:
ext{speed} = rac{ ext{distance}}{ ext{time}}
Measuring Force:
The SI unit for force is the newton (N).
Weight vs. Mass:
Understand the difference between weight and mass.
Weight is the force due to gravity, while mass is the amount of matter in an object.
Energy Transfer and Work:
Interaction between objects results in a transfer of energy, causing changes:
State of rest or motion of an object.
Turning effects (e.g., spanners and levers).
Size and/or shape of an object.
Pressure exerted on objects.
Pressure Measurement:
Investigate pressure using:
ext{pressure} = rac{ ext{force}}{ ext{area}}
Real-Life Applications of Forces:
Appreciate daily life phenomena related to pressure (e.g., high-heeled shoes, cutting edges, atmospheric pressure, straws, and submarines).
Work and Energy:
SI unit of work and energy is the joule (J).
Conservation of Energy:
Energy cannot be created or destroyed; it can only be transferred or converted.
Significant Figures: Answers should be expressed to the correct significant figures.
Curiosity About Natural Forces:
Understanding destructive forces such as tsunamis and volcanoes and their environmental impact.
What is a Force?
A force is defined as an interaction that occurs between objects. By exerting a force, one can:
Make a stationary object move.
Stop a moving object.
Make a moving object speed up or slow down.
Change the direction of a moving object.
Change the shape and size of an object.
Create a turning effect to make an object rotate.
Note: A force cannot change the mass of an object.
The SI unit for force is newton (N).
Force is measured using a force meter or newton meter (e.g., spring balance).
What is Speed?
Speed measures how fast an object moves, defined as distance traveled per unit time:
ext{speed} = rac{ ext{distance moved}}{ ext{time taken}}Units for speed include:
Metres per second (m/s)
Kilometres per hour (km/h)
A larger force results in a greater change in speed:
Example: Larger braking forces result in shorter stopping distances.
When the mass of an object increases, the force needed for speed changes also increases.
Newton's Laws of Motion: Sir Isaac Newton developed laws based on these principles.
Types of Forces
1. Non-contact Forces:
Act on objects that are not in physical contact.
Examples: Gravitational force, magnetic force.
2. Contact Forces:
Act on objects that are touching.
Example: Friction.
Representation of Forces
Forces can be represented using arrows:
Arrow Direction: Indicates the direction of the force.
Arrow Length: Indicates the magnitude of the force.
Gravitational Force
Gravitational force exists between objects that possess mass.
Example: Earth's gravitational force pulls and holds objects to its surface.
Weight is defined as the gravitational force acting on an object: W = mg Where:
W: weight (N)
m: mass (kg)
g: gravitational field strength (N/kg)
Gravitational field strengths vary:
Moon: g = rac{10}{6} ext{ N/kg}
Sun: g = 28 imes 10 ext{ N/kg}
Near Earth's surface: g ext{ is approximately } 10 ext{N/kg}
The gravitational force decreases with distance from Earth's surface.
Difference Between Mass and Weight
Mass | Weight |
|---|---|
Measure of matter in an object | Measure of gravitational force acting on the object |
Depends on the number of particles | Depends on both mass and gravitational strength |
SI unit: kilogram (kg) | SI unit: newton (N) |
Measured by electronic or beam balance | Measured by spring balance |
Quick Check Questions
Quick Check 1: Weight Calculation
An object has a mass of 25 kg. Gravitational field strengths:
Moon: 1.6 N/kg
Mars: 3.7 N/kg a. Calculate weight on Moon:
W_{ ext{Moon}} = 25 ext{ kg} imes 1.6 ext{ N/kg} = 40 ext{ N}
b. Calculate weight on Mars:W_{ ext{Mars}} = 25 ext{ kg} imes 3.7 ext{ N/kg} = 92.5 ext{ N}
A spring balance reads 15.4 N; calculate mass:
m = rac{W}{g} = rac{15.4 ext{ N}}{10 ext{ N/kg}} = 1.54 ext{ kg} (or 1540 g.
Magnetic Force
Magnets exert a non-contact force known as magnetic force, which can attract
magnetic materials.Magnets can attract or repel each other:
Like poles repel, unlike poles attract.
Friction (Frictional Force)
Friction opposes the movement of an object:
It always acts in the opposite direction to the object’s motion.
States:
An object remains stationary if the force applied is less than maximum static friction.
Friction occurs when surfaces rub against one another (solids, liquids, gases).
Example: Air resistance is a form of friction that acts against moving objects in air.
Characteristics of Friction
Increases with:
Rougher surfaces.
Heavier objects.
Useful Applications of Friction
Prevents slipping while walking.
Enables grip on objects (e.g., holding items).
Lighting matchsticks.
Negative Impacts of Friction
Causes heating of surfaces.
Wears down objects.
Slows down movements.
Solutions to Overcome Friction
Apply lubricants between surfaces.
Utilize wheels or ball bearings.
Smoothen surfaces.
Streamlining.
Quick Check 2:
What cannot occur when a force is applied?
Identify types of forces in given scenarios:
(a) Rolling a ball: Contact
(b) Stone falling: Gravitational
(c) Magnet on a refrigerator: Magnetic
(d) Plane heating during flight: Frictional / Air resistance
State of Motion and Resultant Forces
State of motion describes the movement behavior of an object.
Resultant force (or net force) is determined by summing all acting forces:
Consider both magnitude and direction.
Examples of Resultant Forces
Parallel forces in the same direction:
Resultant force = 3 N + 5 N = 8 N to the right.
Equal and opposite forces:
Resultant force = 3 N + (– 3 N) = 0 N,
Unequal parallel forces in opposite directions:
Resultant force = 5 N + (– 3 N) = 2 N to the right.
Effects of Resultant Forces on Objects
A resultant force influences the state of motion, defined as:
Balanced Forces:
Resultant force = 0; object remains at rest or maintains constant speed.
Unbalanced Forces:
Resultant force ≠ 0; object experiences acceleration (change in speed/direction).
Situational Examples
Resultant Force Acting on an Object:
If two forces of 3 N and 5 N act to the right, the resultant = 8 N.
Effect of Resultant Force on Speed and Direction
Effect of Resultant Forces:
Start movement (object accelerates).
Stop movement (deceleration).
Change speed (increase/decrease speed).
Change direction (motion direction varies).
Quick Check 4
Identify the direction for forces on a basketball.
Calculate the resultant force of a parachutist descending at constant speed.
Estimate driving force of a bus moving at constant speed against friction.
Analyzing box movements and effects of applied forces:
(a) When maximum static friction equals applied force: No movement occurs.
(b) If the applied force exceeds friction, the box begins to move.
(c) At constant pushing force equal to friction: Continue moving with constant speed.
(d) Sliding into the box: Box slows down and friction overcomes the applied force.
Pressure
Definition: Pressure is the force applied perpendicular to the surface area of an object: p = rac{F}{A} Where:
p: Pressure (N/m² or Pa)
F: Force (N)
A: Area (m²)
Units of Pressure
SI unit is Pascal (Pa) or N/m²; can also be expressed in N/cm².
Pressure increases as contact area decreases for constant force.
Applications of Pressure
Sharp knife: Focuses force on small area to cut effectively.
Bulldozer with large tracks reduces pressure on soft ground to prevent sinking.
Quick Check 5
If a force of 2.5 N acts over an area of 4.0 m², calculate pressure:
p = rac{2.5 ext{ N}}{4.0 ext{ m}^2} = 0.625 ext{ Pa}
If force of 104 N/m² acts over 7.0 m², find the magnitude of force:
F = p imes A = 104 ext{ N/m}^2 imes 7.0 m^2 = 728 ext{ N}
Calculate area of a box base (6.0 cm) and determine pressure on the table using weight 15.0 N:
Area = 6.0 imes 6.0 = 36 ext{ cm}^2 = 0.0036 m^2
Pressure = rac{15.0 N}{0.0036 m^2} = 4166.67 Pa
Fluid Pressure
Liquid exerts pressure due to its weight, increasing with depth and density.
Example:
Water pressure in a dam increases with depth leading to thicker dam walls.
Atmospheric Pressure
Earth has a surrounding atmosphere that exerts pressure due to air weight.
Example:
Drinking with a straw involves lowering the air pressure inside it, allowing atmospheric pressure to push liquid upwards.
Turning Effect of a Force
A force applied off-center can make an object rotate about a pivot.
Max Turning Effect: When force applied at the greatest perpendicular distance from the pivot.
Example:
Opening a door requires less force if the handle is farther from the hinge.
Energy
Energy is defined as the ability to do work; its SI unit is the joule (J).
Different forms of energy can convert from one form to another:
Potential Energy: Energy stored.
Gravitational Potential Energy (GPE): Energy based on height from a reference point due to gravity:
GPE = mghElastic Potential Energy: Stored in stretched or compressed materials (e.g., rubber bands).
Chemical Potential Energy: Stored in chemical bonds (e.g., food, fuels).
Kinetic Energy: Energy of a moving object; depends on speed and mass:
KE = rac{1}{2}mv^2Thermal Energy: Energy transfer from hot to cold objects.
Other forms: Electrical, light (radiant), and sound energy.
Conservation of Energy
Energy transfers do not create or destroy energy; only changes in form:
Principle of Conservation of Energy states:
Energy cannot be created or destroyed.
Energy can convert from one form to another.
The total amount of energy in an isolated system remains constant.
Example: Energy Transfer in Vertical Motion
Upward Motion:
KE converted to GPE; speed decreases, height increases.
Downward Motion:
GPE converted to KE; speed increases, height decreases.
Examples of Energy Conservation
Ball Dropped from Height: Without air resistance, achieve a final speed of 20 m/s.
Pendulum Motion: Energy transforms between KE and GPE while maintaining overall energy balance, assuming minimal air resistance.
Work and Energy
Work is done when a force causes an object to move in the force direction:
ext{Work} = F imes dThe SI unit of work is joule (J); same unit as energy because doing work transfers energy to the object.
Factors Affecting Work Done
No work is done if:
Object does not move with applied force.
Object moves with constant speed (no force acting).
Object moves perpendicular to the direction of the force.
Example of Work Transferring Energy
When lifting an object, energy is transferred; when slided against friction, energy is converted into thermal energy.
Quick Check 8
Determine work done in lifting tasks.
Consider stationary forces against a wall.
Apply calculations for force when work is known.
Sources of Energy
Renewable vs Non-renewable energy:
Renewable energy: Supplied faster than usage.
Non-renewable energy: Used faster than re-supplied (e.g., fossil fuels).
Fossil Fuels: Coal, oil, and natural gas—causing environmental issues (e.g., greenhouse gas emissions).
Nuclear Energy: Comes from radioactive materials, with significant safety and environmental concerns.
Energy Sources and Their Impact
Non-renewable: Fossil fuels, nuclear energy.
Advantages: High energy yield, availability.
Disadvantages: Environmental hazards, finite supply.
Renewable: Biofuels, wind, hydro, geothermal, solar.
Advantages: Sustainable, lesser environmental harm.
Disadvantages: Intermittent availability, initial costs.
Conclusion: Forces of Nature and Their Effects
Natural forces can have catastrophic impacts (e.g., earthquakes, tsunamis, volcanoes). Earthquakes occur due to tectonic plate movements, creating ground shaking.
Tsunamis are formed from shifts in fault lines under the ocean, leading to devastating waves.
Understanding these forces can help mitigate their effects on the environment and human safety.