Physics物理思维导图

Physics（物理）是研究物质、能量、空间和时间及其相互作用的自然科学学科，以下是关于Physics的基础物理学习提纲。

Physics

• 1.6 Momentum
  • The velocity is -0.1ms_1 , with the minus sign showing direction because it is a vector. So the recoil speed is (the magnitude of the velocity vector only
  • ● Momentum is the product of mass and velocity: 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚 = 𝑚𝑎𝑠𝑠 × 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑝 = 𝑚𝑣 units = kg m/s
  • ● Impulse is the product of force and time, equal to the change in momentum: 𝑖𝑚𝑝𝑢𝑙𝑠𝑒 = 𝑓𝑜𝑟𝑐𝑒 𝑥 𝑡𝑖𝑚𝑒 𝑓𝑜𝑟 𝑤ℎ𝑖𝑐ℎ 𝑓𝑜𝑟𝑐𝑒 𝑎𝑐𝑡𝑠 = ∆(𝑚𝑣) units = Ns
  • In a collision, momentum is always conserved (principle of the conservation of momentum). The total momentum before is equal to the total momentum afterwards. ● This can be used to solve problems in one dimension.
• 1.7 Energy, work and power
  • 1.7.1 Energy Energy is stored in different forms including kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal).
    • Energy can be transferred between the different forms (stores) during events and processes. Energy is always conserved. The total energy before is equal to the total energy after.
    • ● Forces e.g. when gravity accelerates an object downwards and gives it kinetic energy. ● Electrical currents e.g. when a current passes through a lamp and it emits light and heat. ● Heating e.g. when a fire is used to heat up an object. ● Waves (electromagnetic/ sound) e.g. vibrations cause waves to travel through air as sound.
    • The change in gravitational potential energy as you raise the ball can be calculated using: 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 = 𝑚𝑎𝑠𝑠 𝑥 𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑥 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 ℎ𝑒𝑖𝑔ℎ𝑡 ∆𝐸 𝑝 = 𝑚𝑔∆ℎ
  • 1.7.2 Work
    • Work done is equal to the energy transferred ● Mechanical work is done when energy is transferred by a force, which moves something a distance. ● Electrical work is done when energy is transferred through electrical currents. 𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 = 𝑓𝑜𝑟𝑐𝑒 × 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 𝑊 = 𝐹𝑑 = ∆𝐸
  • 1.7.3 Energy Resources
    • Energy can be used to generate electrical power: ● A turbine is turned using energy from an energy resource. ● The turbine turns coils, in a magnetic field, in a generator. This generates electrical power.
    • ● Renewability - the ability to replenish energy as quickly as it is used. ● Availability - the ease of accessing the resource. ● Reliability - the ability to provide a consistent supply of energy. ● Scale - the amount of energy that can be produced using this resource. ● Environmental impact - the effects on the environment of obtaining and using the resource.
    • Fossil fuels as an energy resource: Fossil fuels are a source of chemical energy. They are formed from the decomposition and compression of organism remains over millions of years.
      • Fossil fuels are burnt to generate electrical power: ● Their chemical energy is transferred to heat energy. ● The heat energy is used to boil water, creating steam. ● The steam turns the turbine.
    • Biofuels as an energy resource: Biofuels are another source of chemical energy. They are produced from plants and animal waste. Like fossil fuels, they are burnt to generate electrical power: ● Their chemical energy is transferred to heat energy. ● The heat energy is used to boil water, creating steam. ● The steam turns a turbine.
    • Nuclear fuels as an energy resource: Nuclear fuels are the nuclei of radioactive isotopes which release energy when split in two (nuclear fission). They are another non-renerable resource, so share many advantages and disadvantages with fossil fuels. Nuclear fuels undergo nuclear fission to generate electrical power: ● Heat energy is released by nuclear fission. ● The heat energy is used to boil water, creating steam. ● The steam turns a turbine.
    • Water as an energy resource: Water’s waves and tides can generate electrical power: ● The kinetic energy as waves and tides move is used to turn underwater turbines. Water can also be released from behind hydroelectric dams to generate electrical power: ● The water behind the dam is above ground level, so has gravitational potential energy. ● This energy is transferred to kinetic energy when water is released down a slope. ● The flowing water turns the turbine.
    • Radioactive elements as an energy resource: Radioactive elements are a source of geothermal energy. They are found deep in the earth. Decaying radioactive elements can be used to generate electricity: ● As radioactive elements decay, their geothermal energy heats the surrounding rock. ● Water is poured into shafts in the hot rock. ● The heat energy boils the water, creating steam, which is returned via another shaft. ● Steam turns a turbine.
    • The Sun as an energy resource: The Sun’s light (electromagnetic waves) can be used to generate electricity: ● Solar energy from sunlight is turned into an electrical current by solar cells. The Sun’s light can also be used to heat water: ● Infrared waves of the Sun’s light heat water, contained within solar panels. ● The water goes to a tank and is stored for later use. ● A boiler may be needed to heat the water further. The Sun heats the atmosphere, creating wind which can also generate electricity: ● As the wind blows, it transfers kinetic energy to the blades of wind turbines
      • The original source of energy for most energy resources is the sun, (apart from for geothermal, nuclear and tidal). ● The sun’s energy is released by nuclear fusion. ● Research is being done to investigate how nuclear fusion could be used to produce electrical energy on a large scale.
  • 1.7.4 Power Power is the rate at which energy is transferred or the rate at which work is done. 𝑝𝑜𝑤𝑒𝑟 =𝑒𝑛𝑒𝑟𝑔𝑦 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑/𝑡𝑖𝑚𝑒. 𝑃 =∆𝐸/𝑡. 𝑝𝑜𝑤𝑒𝑟 =𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒/𝑡𝑖𝑚𝑒. 𝑃 =𝑊/𝑡
• 1.8 Pressure
  • Example: lying down on a bed of nails compared to a single nail. ● The force applied is the weight of your body. ● The total area is either a single pinpoint or many points spread out over a larger area ○ So on a bed of nails, the pressure is lower as the area is greater.
  • Pressure is the force per unit area. units = pascals. 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =𝑓𝑜𝑟𝑐𝑒/𝑎𝑟𝑒𝑎 P=F/A
  • The change in pressure beneath the surface of a liquid can be calculated: 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 = 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑥 𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑥 ℎ𝑒𝑖𝑔ℎ𝑡 ∆𝑝 = ρ𝑔∆ℎ
  • An object in a fluid experiences pressure at right angles to all its surfaces. ● The pressure beneath a liquid’s surface increases with depth and density because it is caused by the gravitational force of the fluid above that point.
• 1.5 Forces
  • Forces can change the size and shape of an object: Elastic solids can be extended when force is applied and will return to their original shape and size when the force is removed. Forces can change the speed or direction of an object
  • 1.5.1 Effects of forces
    • ● Load-extension graphs for an elastic object should be linear and pass through the origin. ○ The gradient of the linear section is the spring constant, k. ○ The point where the graph stops being linear is the limit of proportionality. Beyond which,is no longer true, and an object stretches irreversibly. 𝑘 =𝐹/𝑥
    • ● A resultant force is a single force that describes the combined action of all forces acting on an object.
      • If forces balance one another out when combined, there is no resultant force
    • ● Without a resultant force, an object either remains at rest or continues in a straight line at a constant velocity. (Newton’s First Law) ● With a resultant force, an object's velocity will change (acceleration) either by changing speed or direction of motion. ● The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object's mass. Where force and acceleration act in the same direction: 𝑓𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 × 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝐹 = 𝑚𝑎 (Newton’s Second Law)
    • ● To move in a circular motion, a resultant force is needed. ○ Objects moving in a circle are always changing direction (so velocity is always changing ○ This requires a force to continually act perpendicular to the object’s direction of motion (eg. the force of gravity on something orbiting Earth) ○ If mass and radius are constant, when force increases, speed increases. ○ If mass and speed are constant, when force increases, radius decreases. ○ To keep speed and radius constant in the case that mass increases, an increased force is required.
    • Friction: ● Friction (drag) is a force between two surfaces which can impede motion and result in heating. ○ It can act on an object moving through liquid or gas (e.g. air resistance)
  • 1.5.2 Turning effect of forces
    • ● When the clockwise moment equals the anticlockwise moment there is no resultant force, no resultant moment, and the object is in equilibrium (balanced).
    • ● The pivot point is the point which the object can rotate about. If a force is applied in the same line as the pivot the object will not rotate, and remains stationary. If the force applied is in a different line to the pivot, it will rotate in the direction of the force. ● If perpendicular to the object, perpendicular distance is the length of the object. ● If not perpendicular to the object, perpendicular distance to the pivot is found using trigonometry . ● The moment of a force is a measure of its turning effect: 𝑚𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝑎 𝑓𝑜𝑟𝑐𝑒 = 𝑓𝑜𝑟𝑐𝑒 × 𝑝𝑒𝑟𝑝𝑒𝑛𝑑𝑖𝑐𝑢𝑙𝑎𝑟 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑚𝑒𝑛𝑡 = 𝐹d
  • 1.5.3 Centre of gravity
    • ● The centre of gravity of a body is the point at which all of its weight can be considered to act.
    • calculate the centre of gravity of an irregularly shaped plane lamina: ● Hang up the lamina and suspend a plumb line (thread) from the same place. ● Mark the position of the plumb line. ● Repeat with the lamina suspended from different places. ● Where these lines intersect is the centre of gravity.
• 1.4 Density
  • ● Density is the mass per unit volume of an object: units = kg/m3 𝑑𝑒𝑛𝑠𝑖𝑡𝑦/𝑚𝑎𝑠𝑠/𝑣𝑜𝑙𝑢𝑚𝑒. ρ =𝑚/𝑉
  • To find the density of a liquid:
    • ● Calculate the mass - mass is equal to the difference in mass of a measuring cylinder when empty and after filling it with the liquid, measured using a balance. 𝑙𝑖𝑞𝑢𝑖𝑑'𝑠 𝑚𝑎𝑠𝑠 = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 𝑓𝑢𝑙𝑙 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑚𝑝𝑡𝑦 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 ● Measure the volume - volume is read from the cylinder. ● Calculate the density by plugging mass and volume into the density equation.
  • To find the density of a solid:
    • 𝑠𝑜𝑙𝑖𝑑'𝑠 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝑤𝑎𝑡𝑒𝑟'𝑠 𝑣𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 𝑤𝑖𝑡ℎ 𝑜𝑏𝑗𝑒𝑐𝑡 − 𝑤𝑎𝑡𝑒𝑟'𝑠 𝑣𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑜𝑏𝑗𝑒𝑐𝑡
    • ● Measure the mass - mass is measured using a balance. ● Calculate the volume: ○ Of regularly shaped solids - volume is found by measuring the solid’s dimensions and using the appropriate equation for the object’s shape:
    • ● Density of water is 1g/cm3; objects with density greater than this sink in water - those with a density less than 1g/cm3, float. ● The same is true for two liquids (if they do not mix): one liquid will float because it is less dense than the other liquid.
• 1.3 Mass and weight
  • ● Mass measures- how much matter is in an object (when the object is at rest relative to theobserver). 𝑢𝑛𝑖𝑡𝑠 = 𝑘𝑔
  • ● Weight is the gravitational force that acts on objects with mass. 𝑢𝑛𝑖𝑡𝑠 = Newtons (N)
  • ● Gravitational field strength (g) is the amount of gravitational force (weight) acting on an object, per unit of its mass. units = N/kg 𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ =𝑤𝑒𝑖𝑔ℎ𝑡/𝑚𝑎𝑠𝑠. 𝑔 = 𝑊/𝑚
    • ○ Both gravitational field strength and acceleration of free fall are represented by ‘g’ because they are equivalents (9.8N/kg and 9.8m/s2 respectively, on Earth). ○ As shown in the equation, weight is the effect of the gravitational field on a mass. Therefore, as the value of ‘g’ differs from planet to planet, an object’s weight differs from planet to planet too. An object’s mass, however, remains the same.
• 1.2 Motion
  • ● Speed is defined as the distance travelled per unit time: units = m/s. 𝑠𝑝𝑒𝑒𝑑 =𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑/𝑡𝑖me ● Velocity is the speed in a given direction: units = m/s. 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡/ 𝑡𝑖𝑚𝑒. 𝑣 =𝑠/𝑡 ● If speed is changing, an object is accelerating or decelerating. ○ In this case, the object’s speed is calculated as an average: units = m/s. 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑠𝑝𝑒𝑒𝑑 =𝑡𝑜𝑡𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑/𝑡𝑜𝑡𝑎𝑙 𝑡𝑖𝑚𝑒 ○ Acceleration is the rate of change of velocity: (units = m/s2). 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 /𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑡𝑖𝑚𝑒. 𝑎 =∆𝑣 /∆𝑡 ○ Deceleration is a negative acceleration.
  • The gradient is the velocity
    • ○ The object is at rest when the gradient is horizontal. ○ The object is moving at a constant speed/velocity when the gradient is straight. ○ The object is accelerating when the line is curved and the gradient is increasing.
    • ○ The object is decelerating when the line is curved and gradient is decreasing.
  • ● The gradient is acceleration. The object is at rest when the speed/ velocity is zero. The object is moving at a constant speed when the line is horizontal. The object is accelerating when the line has a positive gradient. The object is decelerating when the line has a negative gradient. The object is moving with constant acceleration when the line is straight. The object is moving with changing acceleration when the line is curved. To calculate the acceleration: 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 = 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 =𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑦/𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑥
  • ● The area under the graph gives the distance travelled. ○ To calculate the area under the graph, split it into rectangles and triangles:
  • The motion of objects falling: Objects in free fall, near the Earth’s surface, all will fall with an approximately constant acceleration of free fall known as ‘g’. g = approximately 9.8m/s2
• 1.1 Physical quantities & measurement techniques
  • Measurement techniques: -A ruler (rule) is used to measure the length of an object between 1mm and 1m. -A measuring cylinder is used to measure the volume of a liquid or an object that can sink. Placing the object into a measuring cylinder full of water causes the water level to rise. This rise is equal to the volume of the object. -Clocks and timers (both analogue or digital) are used to measure time intervals.
  • Physical quantities: ● A scalar quantity has magnitude only. ● A vector quantity has magnitude and direction