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Topic 1: Motion forces and energy
Topic 1: Motion forces and energy
Study guide
Study guide
Complete topic summary
Complete topic summary
1. Centre of Gravity:
Theme: Stability and Balance
Key Idea: The centre of gravity (CG) is the point where all of an object's weight appears to act. An object balanced at its CG will not tip. The position of the CG significantly affects an object's stability. A lower CG and a wider base increase stability.
Quote: "The centre of gravity (CG) of an object is the point where all of its weight appears to act."
Applications: Sports car design (low CG for cornering), bus design (high CG can lead to tipping), balancing acts (tightrope walkers adjusting CG).
Experimental Determination: Irregular objects require experimental determination of the CG using a plumb line. Suspend the object from multiple points, draw vertical lines using the plumb line; the intersection is the CG.
2. Density:
Theme: Mass per Unit Volume and Buoyancy
Key Idea: Density (ρ) is defined as mass (m) per unit volume (V): ρ = m/V. Density determines whether an object floats or sinks. If an object's density is less than the fluid's density, it floats; otherwise, it sinks.
Quote: "Density is defined as the mass per unit volume of a substance."
Determining Density: Procedures vary for liquids, regular solids, and irregular solids (displacement method).
Applications: Understanding buoyancy, liquid layering in density columns.
3. Elasticity:
Theme: Forces Affecting Size and Shape, Hooke's Law
Key Idea: Forces can cause stretching, compression, or deformation. Load-extension graphs illustrate the response of elastic materials to applied forces. Hooke's Law (within the limit of proportionality) states that extension is directly proportional to force.
Quote: "A load-extension graph is used to describe how an elastic solid responds when a force (load) is applied."
Spring Constant: The spring constant (k) quantifies stiffness: F = kx. A larger k means a stiffer spring.
Limit of Proportionality: The point beyond which Hooke's Law no longer applies; the graph of load vs. extension curves.
4. Energy:
Theme: Energy Stores and Transfers, Conservation of Energy
Key Idea: Energy exists in different stores (kinetic, gravitational potential, chemical, elastic, nuclear, electrostatic, internal/thermal). Energy is transferred between stores (mechanical work, electrical work, heating, radiation). The principle of conservation of energy states that energy cannot be created or destroyed, only transferred.
Quote: "Energy cannot be created or destroyed, only transferred from one store to another."
Key Equations: Kinetic Energy: Ek = 1/2 * m * v^2. Gravitational Potential Energy: ΔEp = m * g * Δh.
Applications: Falling ball, car braking, pendulum swinging, flow diagrams of energy transfers.
Sankey Diagrams: Visually represent energy transfers, illustrating useful vs. wasted energy.
5. Energy Resources:
Theme: Renewable and Non-Renewable Energy Sources
Key Idea: Energy is obtained from various natural sources. Different energy sources have advantages and disadvantages based on renewability, availability, reliability, scale, and environmental impact.
Examples: Fossil fuels, biofuels, hydroelectric power, tidal power, wave power, geothermal energy, nuclear power, solar cells, wind power.
Efficiency: Not all input energy is converted into useful energy; some is wasted. Efficiency = (Useful Energy Output / Total Energy Input) * 100%.
Quote: "Energy is obtained from various natural sources and converted into useful forms such as electricity."
6. Friction and Drag:
Theme: Opposing Motion
Key Idea: Friction is a force that opposes motion between surfaces in contact. Drag forces act on objects moving through fluids (liquids and gases).
Types of Friction: Static (prevents initial movement) and kinetic (acts on moving objects).
Drag Forces: Depend on speed, surface area, and fluid density.
Applications: Reducing drag in swimming, using parachutes.
7. Mass and Weight:
Theme: Understanding the Difference Between Mass and Weight
Key Idea: Mass is the quantity of matter (scalar, kg). Weight is the gravitational force on an object (vector, N), depending on the gravitational field strength (g). W = mg
Gravitational Field Strength: "g" is approximately 9.8 N/kg on Earth.
Quote: "Mass is a measure of the quantity of matter in an object...Weight is the gravitational force exerted on an object that has mass."
Important Note: Mass remains constant regardless of location; weight changes based on gravitational field strength.
8. Momentum:
Theme: Measure of Motion
Key Idea: Momentum (p) is a measure of an object's motion: p = mv. Impulse is the change in momentum caused by a force acting over time: FΔt = Δ(mv). The law of conservation of momentum states that the total momentum of a system remains constant if no external forces act on it.
Newton's Second Law: Force is equal to the rate of change of momentum.
Applications: Car safety (airbags, crumple zones), sports, rocket propulsion.
Quote: "Momentum is a measure of an object's motion, which depends on both its mass and velocity."
9. Motion:
Theme: Describing Motion: Speed, Velocity, and Acceleration
Key Idea: Speed is distance travelled per unit time (scalar). Velocity is speed in a given direction (vector). Acceleration is the change in velocity per unit time (vector).
Key Equations: Speed = Distance/Time, Acceleration = Change in Velocity/Time.
Graphs: Distance-time and speed-time graphs are used to represent motion.
Circular Motion: Requires a centripetal force acting perpendicular to the direction of motion.
10. Physical Quantities and Measurements:
Theme: Accurate Measurement and Vector vs. Scalar
Key Idea: Accurate measurement is fundamental in physics. Scalar quantities have magnitude only; vector quantities have both magnitude and direction.
Measurement Techniques: Rulers, measuring cylinders, clocks, digital timers.
Scalar Examples: Distance, speed, time, mass, energy, temperature.
Vector Examples: Force, weight, velocity, acceleration, momentum.
Vector Resultants: Determined by calculation (Pythagorean theorem for right angles) or graphical methods (tip-to-tail drawing).
11. Pressure:
Theme: Force Distributed Over an Area
Key Idea: Pressure (p) is defined as force (F) per unit area (A): p = F/A. Pressure in liquids increases with depth and density.
Equation for Pressure Change in a Liquid: Δp = ρ * g * Δh
Applications: Hydraulic systems, blood pressure, airplane cabins, syringes, scuba diving.
12. Resultant Forces:
Theme: Net Force and Newton's Laws of Motion
Key Idea: The resultant force is the net force acting on an object. Newton's First Law states that an object remains at rest or in constant motion unless acted upon by an external force. Newton's Second Law relates force, mass, and acceleration: F = ma.
Effects of Resultant Force: Changes the velocity of an object (speed or direction).
13. Turning Effect of Forces (Moments):
Theme: Rotation Around a Pivot
Key Idea: A moment is the turning effect of a force around a pivot. Moment = Force x Perpendicular Distance from Pivot. For an object to be in equilibrium, the sum of clockwise moments must equal the sum of anticlockwise moments.
Equilibrium: No resultant force and no resultant moment.
Quote: "A moment occurs when a force is applied to an object in such a way that it causes it to rotate about a fixed point, known as a pivot or fulcrum."
14. Work and Power:
Theme: Work Done and Rate of Energy Transfer
Key Idea: Work is done when a force moves an object. The amount of work done is equal to the energy transferred. Power is the rate at which work is done or energy is transferred.
Key Equations: Work Done (W) = Fd, Power (P) = W/t = ΔE/t
Applications: Lifting a box, pushing a car, running up stairs, electrical appliances.
Common Themes Across Topics:
Measurement and Units: Emphasis on correct units (kg, m, s, N, J, W, Pa) and accurate measurement techniques.
Formulas and Calculations: Importance of understanding and applying relevant formulas.
Real-World Applications: Connecting physics concepts to everyday life and engineering.
Energy Transfer and Conservation: Underlying principle governing many physical phenomena.
Scalar vs. Vector Quantities: Recognising the difference and dealing with them appropriately.
Example short answer questions
Example short answer questions
Explain how a low centre of gravity contributes to the stability of an object, providing a real-world example.
A low centre of gravity increases stability because it requires a greater force to topple the object. When the CG is lower, the object must be tilted further before its weight acts outside of its base of support. A sports car with a low CG is more stable around corners.
2. Describe the displacement method for determining the volume of an irregularly shaped solid and explain why it is necessary.
The displacement method involves submerging the irregular solid in a measuring cylinder filled with a known volume of water. The difference between the initial and final water levels gives the volume of the solid, which is necessary because irregular shapes lack simple geometric formulas for volume calculation.
3. What is the spring constant, and how does it relate to the stiffness of a spring? What are the units for the spring constant?
The spring constant (k) is a measure of a spring's stiffness, representing the force required to extend or compress the spring by a unit length. A larger spring constant indicates a stiffer spring, and the units are Newtons per meter (N/m).
4. State the principle of conservation of energy and give an example of energy transformation in a closed system.
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one form to another. An example is a falling object, where gravitational potential energy is converted into kinetic energy as it falls.
5. Compare and contrast two different renewable energy resources, outlining one advantage and one disadvantage of each.
Solar energy is renewable, with the advantage of no emissions, but the disadvantage of being unreliable, as it only works in sunlight. Wind energy is also renewable and pollution-free, but it is unreliable as well, depending on wind speed, and can be noisy and visually intrusive.
6. Describe the difference between static and kinetic friction, and provide an everyday example of each.
Static friction is the force that prevents an object from starting to move, while kinetic friction is the force that opposes the motion of an object already in motion. An example of static friction is the force that prevents a box from sliding when you initially push it, while kinetic friction is the force that slows down the box as it slides across the floor.
7. Explain the difference between mass and weight and why an object's weight can change depending on its location in the universe, but its mass remains the same.
Mass is a measure of the quantity of matter in an object, while weight is the gravitational force exerted on that mass. Weight depends on the gravitational field strength, so it varies with location, whereas mass remains constant regardless of location.
8. Define momentum and impulse and briefly explain how they are related.
Momentum is a measure of an object's motion, defined as the product of its mass and velocity (p=mv). Impulse is the change in momentum caused by a force acting over a period of time (Impulse = FΔt).
9. Explain the difference between speed and velocity, providing a real-world example to illustrate the distinction.
Speed is the rate at which an object is moving, regardless of direction, while velocity is the speed in a specific direction. For example, a car travelling at 60 mph has a speed of 60 mph, but its velocity is 60 mph north if it's travelling north.
10. Explain the principle of moments, and how the equation enables us to achieve balance.
The principle of moments states that for an object to be in rotational equilibrium, the total clockwise moments about a pivot point must equal the total anticlockwise moments about the same pivot point. This allows for calculating balancing forces.
11. How do you determine the density of different materials?
Density is mass per unit volume (ρ = m/V). To determine density:
Liquids: Measure mass with a balance, volume with a measuring cylinder, then calculate density.
Regular Solids: Calculate volume using geometric formulas (e.g., length x width x height for a cube), measure mass, then calculate density.
Irregular Solids: Measure mass. Use the displacement method (submerge the object in water in a measuring cylinder and find the volume difference) to find the volume, then calculate density.
12. What is elasticity and how is it represented graphically?
Elasticity is the ability of an object to return to its original shape after a force is applied and removed. A load-extension graph shows how an elastic material responds to a force. Initially, the graph is a straight line, representing Hooke's Law (extension is directly proportional to force). Beyond the elastic limit, the graph curves, indicating the material will no longer return to its original shape. The spring constant (k = F/x) quantifies a spring’s stiffness.
13. What are the different types of energy stores and how is energy transferred?
Energy can be stored as kinetic, gravitational potential (GPE), chemical, elastic (strain), nuclear, electrostatic, or internal (thermal) energy. Energy is transferred through mechanical work (force moving an object), electrical work (electric current), heating (temperature difference), and radiation (waves). The total energy in a closed system remains constant (conservation of energy), even as it is transferred between stores.
14. What is the difference between static and kinetic friction, and how do drag forces work?
Friction opposes motion between surfaces in contact. Static friction prevents initial movement, while kinetic (or sliding) friction acts on moving objects. Drag forces are friction forces that act on objects moving through fluids (liquids or gases). Drag depends on speed, surface area, and fluid density. A parachute uses a large surface area to increase air resistance (drag).
15. How do force, momentum, and impulse relate to each other?
Momentum (p = mv) is a measure of an object's motion. Impulse (FΔt) is the change in momentum caused by a force acting over time. Newton’s Second Law states that force is equal to the rate of change of momentum (F = Δp/Δt). The law of conservation of momentum states that the total momentum of a system remains constant if no external forces act on it. Applications include car safety features (airbags, crumple zones) and sports equipment.
16. How are mass and weight different, and what is gravitational field strength?
Mass is the quantity of matter in an object and is measured in kilograms (kg). It's a scalar quantity and is constant regardless of location. Weight is the gravitational force on an object, measured in newtons (N). It's a vector quantity and depends on gravitational field strength (g). Gravitational field strength is the force per unit mass (g = W/m). On Earth, g is approximately 9.8 N/kg.
Key terms
Key terms
Acceleration: The rate at which the velocity of an object changes over time; a vector quantity.
Air Resistance: The force of friction acting on an object moving through air.
Balance: A device used to measure mass by comparing it to known standard masses.
Buoyancy: The tendency of an object to float or sink in a fluid, determined by the object's density relative to the fluid's density.
Centre of Gravity (CG): The point where all of an object's weight appears to act.
Chemical Energy: Energy stored in the bonds of chemical compounds, released during chemical reactions.
Clockwise Moment: The rotational effect of a force that causes an object to rotate in the same direction as the hands of a clock.
Compression: The act of applying force to reduce the size or volume of an object.
Conservation of Energy: The principle that energy cannot be created or destroyed, only transferred from one form to another within a closed system.
Conservation of Momentum: The total momentum of a system remains constant if no external forces act.
Density (ρ): The mass per unit volume of a substance, typically measured in kilograms per cubic meter (kg/m³).
Deformation: A change in the shape or size of an object due to applied forces.
Displacement Method: A method to determine the volume of an irregularly shaped solid by measuring the volume of water it displaces when submerged.
Distance: The total length of the path travelled by an object, a scalar quantity.
Drag Force: A force that opposes the motion of an object through a fluid (liquid or gas).
Elastic Limit: The point beyond which a material will no longer return to its original shape after a force is removed.
Elastic Region: The section of a load-extension graph where the material obeys Hooke’s Law.
Elasticity: The ability of a material to return to its original shape after being stretched or compressed.
Electrostatic Energy: Energy stored due to interactions between electric charges.
Energy: The capacity to do work.
Equilibrium: A state in which the net force and net torque acting on an object are zero, resulting in no change in motion.
Force: An interaction that, when unopposed, will change the motion of an object; measured in newtons (N).
Fossil Fuels: Non-renewable energy resources formed from the remains of ancient organisms, including coal, oil, and natural gas.
Friction: A force that opposes motion between surfaces in contact.
Fulcrum: The fixed point around which a lever pivots; also known as the pivot.
Generator: A device that converts mechanical energy into electrical energy.
Geothermal Energy: Energy derived from the Earth's internal heat.
Gravitational Field Strength (g): The force per unit mass exerted by gravity, approximately 9.8 N/kg on Earth.
Gravitational Potential Energy (GPE): Energy stored in an object due to its height above the ground.
Hooke's Law: The law stating that the extension of a spring is directly proportional to the force applied, within the elastic limit.
Hydraulic Systems: Systems that use liquids under pressure to transmit force and do work.
Impulse: The change in momentum of an object.
Inelastic Region: The section of a load-extension graph where the material has passed the limit of proportionality and does not obey Hooke’s Law.
Inertia: The tendency of an object to resist changes in its state of motion.
Internal (Thermal) Energy: Energy stored due to the temperature of an object, consisting of both kinetic and potential energy of particles.
Kinetic Energy: The energy of an object due to its motion.
Kinetic Friction: The friction that acts on an object that is already in motion.
Lamina: A thin, flat object.
Limit of Proportionality: The point at which an elastic object has reached its elastic limit and will no longer return to its original shape when the force is removed
Load-Extension Graph: A graph that shows how the extension of an elastic material varies with the applied load.
Mass: A measure of the quantity of matter in an object; a scalar quantity measured in kilograms (kg).
Measuring Cylinder: A graduated container used to measure the volume of liquids.
Mechanical Work: Energy transferred when a force moves an object.
Meniscus: The curved surface of a liquid in a container, read at eye level for accurate volume measurement.
Moment of a Force: The turning effect of a force around a pivot; calculated as force multiplied by the perpendicular distance from the pivot.
Momentum: A measure of an object's motion, defined as the product of its mass and velocity.
Motion: The act or process of moving or being moved.
Net Force: The overall force acting on an object, taking into account all individual forces and their directions.
Newton (N): The unit of force.
Newton’s First Law of Motion: An object will remain at rest or continue moving in a straight line at constant speed unless acted upon by an external force.
Newton’s Second Law of Motion: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
Nuclear Energy: Energy stored within the nucleus of an atom, released during nuclear reactions.
Pascal (Pa): The unit of pressure, equal to one newton per square meter (N/m²).
Pendulum: A weight suspended from a pivot point that swings freely under the influence of gravity.
Period of Oscillation: The time taken for one complete cycle of a pendulum's swing.
Physical Quantities: Measurable aspects of the physical world, such as length, mass, time, and temperature.
Pivot: The fixed point around which an object rotates; also known as the fulcrum.
Plumb Line: A weight suspended from a string, used to establish a vertical reference line.
Potential Energy: Stored energy that an object has due to its position or condition.
Power: The rate at which work is done or energy is transferred, measured in watts (W).
Pressure (p): The amount of force exerted per unit area, measured in pascals (Pa).
Radiation: The emission or transmission of energy in the form of waves or particles.
Renewable Energy: Energy sources that are naturally replenished, such as solar, wind, and hydroelectric power.
Resultant Force: The net force acting on an object when two or more forces are acting along a straight line.
Scalar Quantity: A quantity that has magnitude only (e.g., mass, speed).
Speed: The distance travelled per unit time; a scalar quantity.
Static Friction: The friction that acts between two surfaces that are not moving relative to each other.
Strain Energy: Energy stored in stretched or compressed objects that can return to their original shape.
Stretching: The act of applying force to increase the length of an object.
Tidal Power: Energy harnessed from the movement of tides.
Time: The duration between two points.
Turbine: A rotary engine that converts the kinetic energy of a fluid (such as steam or water) into mechanical energy.
Vector Quantity: A quantity that has both magnitude and direction (e.g., velocity, force).
Velocity: The speed in a given direction; a vector quantity.
Volume (V): The amount of space occupied by a substance or object, typically measured in cubic meters (m³).
Watt (W): The unit of power, equal to one joule per second (J/s).
Wave Power: Energy harnessed from the movement of ocean waves.
Weight (W): The gravitational force exerted on an object; a vector quantity measured in newtons (N).
Work Done (W): The energy transferred when a force moves an object over a distance; measured in joules (J).
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