Physics revision- topic 1

Physics revision- topic 1

1.1Define weight as the force on an object due to a gravitational field: When an object comes in the presence of a larger mass such as a planet the larger mass exerts a force on that object Weight force is a gravitational force exerted due to the gravitational field surrounding the object, thus weight is the force on an object due to a gravitational field. The mass does not change anywhere, it is the amount of matter. The local value for weight varies due to: the variation in thickness of earths lithosphere Formula:W or F= Weight force (N)W = mg m = Mass (kg) g = Acceleration due to gravity (ms-2)

1.2 Define gravitational potential energy as the work done to move an object from a very large distance away to a point in a gravitational field: Gravitational Potential Energy is the work done to move an object from infinity to a point within a gravitational field. At infinity the gravitational potential energy is zero and decreases as a mass gets closer to the centre of a gravitational field Formula:

Ep = - G m1m2rEp = Gravitational Potential Energy (J)G = Gravitational constant (6.67x10-11)m1 = Mass of Planet (kg)m2 = Mass of Object (kg)r = Distance between the two masses (m)

The negative sign occurs because the Gravitational Potential Energy of a mass as it approaches infinite distance from the Gravitational field is always negative.

1.3Explain that a change in gravitational potential energy is related to work done: For Gravitational Potential Energy to change the object must move closer or further away from the centre of the gravitational field Work must be done on the object to make this change in distance, thus a change in Gravitational Potential energy is related to work done. For an object of mass m at a height h above the earths surface, the gravitational potential energy Ep is given by: Ep mgh

2.1 Describe the trajectory of an object undergoing projectile motion within the Earth's gravitational field in terms of horizontal and vertical components: Galileo showed the vertical and horizontal motion of a projectile could be treated independently To find the actual position and velocity the horizontal and vertical components are added together.

Projectiles: Any object that is thrown, dropped or otherwise launched into the air The object follows a parabolic path which, without factoring in resistance, is symmetrical During flight the projectile experiences the force of gravity and acceleration due to Gravity

The Trajectory: The path that a projectile follows during its flight Can be broken down into two separate and independent motions: Vertical Motion - affected by constant acceleration due to gravity Horizontal Motion - experiences no acceleration

Vertical Motion:uy = u sin The object, subjected to acceleration due to gravity, rises up, stops momentarily then falls down When it hits the ground it is travelling at the same speed at which it leftEquations: vy = uy + aytvy2 = uy2 + 2ay yy = uyt + ayt2

Horizontal Motion: ux = u cos When pushed horizontally, ideally, it experiences no acceleration in its direction of motion. Thus a projectile travels sideways in uniform motionEquations:vx = uxx = uxt

3.1Explain the concept of escape velocity in terms of the gravitational constant and the mass and radius of the planet: Definition: The minimum velocity required by an object to escape the gravitational pull of the Earth or other planet Formula:

G = Gravitational constant (6.67x10-11) M = Mass of planet (kg) r = Radius of planet (m)Explanation: The above equation shows that Escape velocity depends only on the Gravitational Constant, Mass and Radius of the planet, not on any properties of the projectile. An object projected at this velocity would never come back down During the rise the objects Kinetic Energy is converted into Gravitational Potential Energy

Outline Newton's concept of escape velocity: Explanation: Newton predicted that an object projected horizontally from a high mountain would undergo projectile motion as shown in the above diagram If the velocity of the projectile was increased enough, a speed would be reached where the object would fall around the Earth, i.e. orbit it. If the speed exceeded the escape velocity the projectile would spiral away from the Earth and stretch into an elliptical shape. Even faster velocities lead to the projectile following a parabolic or hyperbolic path away from the earth escaping it.

4.1Describe Galileo's analysis of projectile motion:Galileo showed that: All masses fall at the same rate, regardless of weight If Air Resistance is ignored, acceleration due to gravity is the same for all objects regardless of their mass All projectiles move in a parabolic shape Horizontal and vertical motion are separate; by using an inclined plane

5.1 Identify why the term g forces is used to explain the forces acting on an astronaut during launch: Definition: G force is a measure of acceleration force using the Earths gravitational acceleration as the unit. A positive g force is one that is directed from the feet to the head (upwards), whereas a negative g force is in the other direction (downwards). The sensation of feeling more weight than normal is a positive g force, whereas feeling less weight is due to negative g forces. To help astronauts withstand extremely large g forces during lift-offs, they lie down and also special cushions are utilised. Fighter plane pilots wear g suits to reduce the effect of the g forces when manoeuvring. The forces experienced by Astronauts during launch are these g-forces. G forces =

6.1 Compare qualitatively low Earth and geo-stationary orbits:Geo-stationary OrbitsLow Earth Orbits

Orbital radii altitude of approximately 35800km (higher than low Earth orbits)Lower orbital radii altitude

Orbital velocity of approximately 11000kmh-1 (slower than geostationary orbits)Higher orbital velocity of approximately 28000km/h

The satellite appears to be stationary in the sky when viewed from the surface of the EarthDo not have a fixed position

Their periods are the same as that of the Earth.Easy to track since each satelliteMay orbit the Earth many times per dayHarder to track as they are constantly moving

Does not experience orbital decayExperiences orbital decay

Have a limited view of the Earths surfaceAble to view the Earths entire surface over several orbits

Situated above the equatorCan be made to pass above any point on earth

Examples: Communications satellites, Weather satellites, Specialist TelescopesExamples: Studying weather patterns and geomapping.

6.2Account for the Orbital Decay of satellites in low Earth orbit: Atmospheric drag caused by Earth's Atmosphere Decays the orbit of a satellite by slowing it down causing it to lose altitude and velocity thus slowly spiralling towards the ground. Amount depends on density of Atmosphere and size of satellite These satellites orbit at lower altitudes where the atmosphere is denser, thus these satellites experience higher levels of atmospheric drag than other satellites in higher orbits.

Present Information and use available evidence to discuss the factors affecting the strength of the gravitational force. The Earths lithosphere varies in structure, thickness and density. Continental crust is thickest under mountain ranges. The Earths globe is flattened at the poles. The spinning Earth also affects the value of g. At the equator, the spin effect is greatest resulting in a lowering of the value of g. As you travel from the equator to the poles, the spin effect on g shrinks to zero.

6.3Define the term orbital velocity and the quantitative and qualitative relationship between orbital velocity, gravitational constant, mass of the central body, mass of the satellite and the radius of the orbit using Kepler's Law of Periods: Definition of orbital velocity: The instantaneous speed and direction of an object in uniform circular motion along its path. In one revolution the object moves:

V = orbital velocity (ms-1) G = 6.67x10-11M = mass of the planet (kg) R = radius of the planet (m)Formula: orbital velocity

G M RV =

r = the radius of the orbit (m)T = the period of the orbit (s)G = 6.67x10-11M = mass of the planet (kg)Keplers Third Law of Periods: The ratio r3/t2 was the same for every planet.

r = radius of planet, T= period or orbit of planet.From Keplers law of periods, we get the relationship between the period of an orbit and the velocity along that circular orbit:

7.1Discuss issues associated with safe re-entry into the Earth's atmosphere and landing on the Earth's surface:Extreme Heat: Spacecraft has significant kinetic energy and gravitational potential energy. This must be lost. The Spacecraft experiences friction with molecules of the atmosphere as it re-enters, this generates huge amounts of heat. The Spacecraft's Kinetic energy is converted into heat. This is minimised by: Blunt shape for re-entry, produces a Shockwave as it moves through the air, this shockwave absorbs most of the heat Ablation - Covering with ceramic material which is vaporised (ablated) during re entry Taking longer to re-enter the atmosphere, thereby lengthening the time needed to convert heat to energy.G Forces: G forces- units of gravitational acceleration. A force of 5g is equivalent to acceleration five times the acceleration due to gravity. Greater angles of re-entry result in greater g-forces experienced by the astronaut. Humans cannot tolerate g-forces greater than 8g but 3g is the maximum advised. If the acceleration is in the direction of the persons head they may experience a black out. Astronauts lie down so that blood is not forced away from the brain, fully supporting the body. The body is supported in as many places as possible

Ionisation Blackout: As heat builds up around the spacecraft, atoms in the air ionize forming a layer around the spacecraft This layer causes a period of no communication with the spacecraft known as Ionization Blackout approximately 4-16 minutes

Reaching the Surface: Parachutes released in last portion of spaceships descent to slow it down before splashing into the ocean Space shuttle has wings which allow the pilot to control its descent, underbelly provides blunt surface for protection.

7.2Identify that there is an optimum angle for safe re-entry for a manned spacecraft into the Earth's atmosphere and the consequences of failing to achieve this angle:

The Re-entry Angle: Optimum angle between 5.2o 7.2o Angle too shallow spacecraft will bounce off due to the compression of the atmosphere beneath Angle too steep spacecraft will burn up on re-entry, due to too much heat being created

8.1Analyse the forces involved in uniform circular motion for a range of objects, including satellites orbiting the Earth: Centripetal Force: A centripetal force is required for objects to move in uniform circular motion This force is always directed to the centre, perpendicular to the velocity of the moving object Causes the object to continually change direction to follow a circular path

m = Mass (kg)v = velocity (ms-1)r = radius of motion (m)

Formula:

mv2 rFc =

Examples: Satellite orbiting the Earth - Centripetal Force comes from Gravitational attraction between Earth & the satellite Car driving around the corner - Centripetal Force provided by friction between the tyres and the road Moon revolving around earth- moon to earth gravitational pull

9.1Discuss the effect of the Earth's orbital motion and its rotational motion on the launch of a rocket: Rotational Motion: Rockets launched from the equator, towards the east - takes advantage of Earth's Rotational Velocity Leaves at its own velocity plus the velocity of the Earth's rotation

Orbital Motion:To reach outer planets: Launched in the direction of Earth's orbit, leaves at Earth's orbital velocity (30 kms-1) plus its own velocity. Moves in an elliptical orbit allowing it to reach outer planets.

To reach inner planets: Launched in opposite direction, leaves at own velocity minus the Earth's Moves in elliptical orbit closer to the sun allowing it to reach inner planets

10.1Present information on the contribution of Goddard to the development of space exploration: Measured the fuel values for various rocket fuels, such as liquid hydrogen and oxygen. Launched the worlds first multi-staged liquid-fuel-powered instrument rocket. Launched the first liquid-fuelled supersonic rocket. Developed pumps for liquid fuels, as well as rocket engines that have automatic cooling systems. Proved that a rocket will work in a vacuum, that it needs no air to push against (i.e. space). Proposed and explained how the use of rocket propulsion could be used to reach high altitudes.

11.1Analyse the changing acceleration of a rocket during launch in terms of the Law of Conservation of Momentum and the forces experienced by astronauts:

Rocket engines differ in that both fuel and oxygen supply must be carried. Modern rockets use both solid and fuel propellants.

Law of Conservation of Momentum:The total momentum of a closed system remains unchanged The momentum of the gases shooting out of the rear of the rocket is equal but opposite to the momentum of the rocket itself Backward momentum of gases equal in magnitude to forward momentum of rocket Since change in momentum = impulse (Force x time), at any one second interval the backward force on the gases equals the forward force on the rocket As the rocket moves, its ` decreases significantly, thus its velocity increases significantly (acceleration)

Forces Experienced by Astronauts: Two forces acting on an astronaut during launch, upward thrust and downward weight force. If the thrust remains constant, the rate of acceleration of the rocket will increase as it ascends Rockets mass also decreases as fuel burns and weight decreases at higher altitudes. The acceleration of rockets will increase steadily and the astronaut will experience building G forces A rockets acceleration at the point when they are initially launching is described by newtons second law: The trend is that acceleration reaches a climax just before all the fuel is exhausted; this is when G forces are at the highest. At this point, rockets experiences weightlessness for a few seconds and the second stage will start burning causing maximum thrust again but at a lower level. This repeats itself.

A = acceleration T= thrust mg =force (mass x gravity) m = mass The apparent weight of the person is the value of the contact force applied to them. Therefore, the apparent weight of the person is given by m(g + a) which is greater than their true weight mg.

12.1Describe a gravitational field in the region surrounding a massive object in terms of its effects on other masses in it: Gravitational field- a field in which any mass will experience a gravitation field. Any mass, regardless of its size, will have a gravitational field around it. When a mass is placed within another masses gravitational field it will experience a force towards the massive object This force of attraction is dependent on the mass of the objects and the distance between them. The strength of the gravitational field around a big object is proportional to the square of the distance from the centre of the object.

12.2 discuss the importance of Newton's Law of Universal Gravitation in understanding and calculating the motion of satellites: Formula

v = orbital velocity (m/s)G = Universal Gravitation Constant (6.67x10-11)me = Mass of earth (5.97 X 1024)(kg)re = radius of earth (6.38 X 106) (m) h= altitude in orbit, in m

Description: It is shown that the orbital velocity is dependent on the mass and radius of the earth as well as the altitude from earth.

Motion of Satellites: Occurs when centripetal force required to keep satellite moving in circular orbit equals gravitational attraction When the gravitational attraction is greater than the centripetal force, the satellite spirals inwards When the gravitational attraction is less than the centripetal force, the satellite spirals outwards Satellite orbiting closer to central planet has shorter orbital period and higher orbital velocity

12.2.1 Define Newton's Law of Universal Gravitation

F = Force of attraction (N)G = Universal Gravitation Constant (6.67x10-11)m1 & m2 = Masses of the two objects (kg)r = distance between the two objects (m)

Newton said that objects attract every other object in the universe. He proved this by devising the formula above. As shown in the formula above the attractive force's strength is determined by: the mass of the two objects, the distance between their centres,. Newtons Law of Universal Gravitation allows us to calculate gravitational force, orbital velocity and orbital periods of satellites

13.1Identify that a slingshot effect can be provided by planets for space probes The Slingshot Effect: Involves moving a probe in a hyperbolic orbit to gain velocity from the Gravitational Field Probe approaches planet from in front, swung around by planet's gravity Planet loses kinetic energy while the probe gains some. This speed gain is enough to throw the spacecraft back out again. away from the planet. By controlling the approach, the outcome can be manipulated.

For space probes: Sometimes employed by deep space probes to save fuel

As seen above, the space probe swings around the planet, changing direction and increasing speed. Vf > Vi

identify data sources, gather, analyse and present information on the contribution of one of the following to the development of space exploration: Tsiolkovsky, Oberth, Goddard, Esnault-Pelterie, ONeill or von Braun. Considered the theoretical father of rocketry, he theorised many aspects of space travel and rocket propulsion before others, and played an important role in the development of Russian space programs Inuenced by science fiction, he began to introduce real technical problems into his writings, and his dream was for humanity to become a space civilisation He demonstrated the reaction principle by experimenting with a cask filled with compressed gas; he discovered that the movement of the cask could be regulated by alternating the pressure of the gas released from it He outlined how a reaction thrust motor could demonstrate Newton's Third Law to allow humans to escape the bounds of Earth His drafted design for the first rocket involved an explosive mixture of liquid oxygen and liquid hydrogen, which produces condensed and heated gases; these gases a.re cooled and rareed with the resulting exhaust providing the thrust He also speculated on a multi-stage approach to spaceflight; as each individual stage consumed its fuel, it would be discarded to keep the overall weight to a minimum; he recognised that it would require a tremendous amount of fuel for the rocket to reach escape velocity . He wrote over 50 scientific papers, and although he did not create any rockets himself, his fundamental principles remain basic to contemporary astronautics

14.1Outline the features of the aether model for the transmission of light:The Aether Model: 19th Century - like all waveforms light needed a medium through which to travel Called this medium the Luminiferous Aether.

Properties: Filled all of space, stationary in space Perfectly Transparent Filled all matter Low Density Great Elasticity

15.1Describe, evaluate the Michelson-Morley attempt to measure the relative velocity of the Earth through the aether:Analogy: When you drive a car through the air, wind is generated. The direction of the wind is in the opposite direction to the motion of the car.

The Experiment: Designed to detect Aether using light and interference, with an interferometer (observer) on a turntable Single beam of light split in half by half silvered mirror One ray travelling across the Aether wind, another travelling through it, same distance & return Because of the Aether wind produced by the moving Earth, light would be slowed down heading into it When beams meet, interference pattern produced, should change as table is rotated through 90 No such change, thus no difference in speed Null result no Aether wind was detected. If the aether wind exists, the rays would travel at different velocities;whentheapparatusisrotated,theinterferencepatternshouldbeseentoshift,and therelativevelocityoftheaetherwindcanbedeterminedbyusingthisshift.

15.2Discuss the role of the Michelson-Morley experiments in making determinations about competing theories:

It provides experimental support for the theory of relativity

At the Time of the Experiment: Did not sway scientific belief at the time of the Experiments Null result seen as indication that the model needed improvement Various modifications offered, each resulting in predictions failing to be proven

Einstein's Relativity: Einstein proposed theory of relativity with its own predictions and it disproved the aether. If it were not to be violated then, light must travel at 3 108 ms 1 Important in helping others decide between the competing theories, along with comparative success of relativity experiments

Inertial Frames of Reference An inertial frame of reference is one that is either stationary or moving with a constant velocity. You cannotperformanymechanicalexperimentorobservationthatwouldrevealtoyouwhetheryouweremovingwithuniformvelocityorstandingstill. A non-inertial frame of reference is one that is undergoing acceleration.

16.1 Outline the nature of inertial frames of reference: An inertial Frame of Reference is one in which Newtons First Law is obeyed. These are at rest or moving with a constant velocity. They involve no acceleration.

16.2Discuss the principle of relativity: Einstein principle of Relativity: All steady motion is relative and cannot be detected without reference to an outside point. The speed of light is constant regardless of the observers frame of reference. The Aether is not needed to explain light, and in fact, does not exist. Principle applies only to inertial frames of reference Within an inertial frame of reference you cannot perform any experiment to detect your motion without referring to another frame of reference.

Forexample,ifyouheldupastringwithasmallobjecttiedtotheend, the object would hangso that the string was vertical. However,ifthe vehiclein which you were travelling in accelerated, the object would swing backwards so that the string is no longer vertical.

16.3Describe the significance of Einstein's assumption of the constancy of the speed of light & identify that if c is constant then space and time become relative:

The Speed of Light: The speed of light has to be constant for the Principle of relativity to hold true If the speed of light were not constant, differing speeds of light in different frames of reference could be used in determining our motion Thus violating the Principle of Relativity. Since then, thedistanceandtimewitnessedbybothobserversmustbedifferent.

Relative Space & Time: Classic physics - space (position, displacement, velocity etc.) are relative to an observer, time is absolute passing identically for everybody Theory of Relativity - for speed of light to be constant time and space must both be relative Time passes differently for different observers depending on their relative motion Showed that there is no absolute frame of reference, all are equivalent

17.1Explain qualitatively and quantitatively the consequence of special relativity in relation to:Relativity of Simultaneity: Two events judged by one observer to be simultaneous, not generally judged to be simultaneous by other observer in different frame of reference in relative motion Whether or not two events are seen to be simultaneous depends on position relative to the two events

Equivalence between Mass & Energy: Mass can be converted to energy and energy can be converted to mass under extraordinary circumstances. Equation; E = mc2 expresses equivalence between mass and energy, when an objects mass is at rest, there will be rest energy. rest energy- the energy equivalent of a stationary objects mass measured withintheobjectsrestframe.

Length Contraction: Length of an object measured within its rest frame is called it's proper length (Lo) Observers in other frames of reference in relative motion always measure this length (Lv) to be shorter

Time Dilation: Time taken for event to occur within it's rest frame is called proper time (to) Observers in different reference frames in relative motion always judge the time taken (tv) to be longer. Thereisalightclockonatrainsothatthelightbeamswillrunthelengthofthetrain. The observer on the train will see the light go up and down but, the observer outside will see a longer journey.

Mass Dilation: Mass of a moving object increases as it's velocity increases - mass dilation The rest mass (mo) of the object compared to the relativistic mass (mv) of the particle is much smaller.

18.1Discuss the concept that length standards are defined in terms of time in contrast to the original metre standard.

Length by Original Standards: When the SI units were set up the meter was defined as the distance between two lines scribed on a single bar of platinum-iridium alloy

Length in terms of Time: One meter is the distance travelled by light in a vacuum in one 299792458th of a second

19.1Discuss the implications of mass increase, time dilation and length contraction for space travel:Time Dilation: Allows travel into the future at high speeds, but not back to the past. Astronauts travelling in a relativistic spacecraft will age slower than people back on earth, which means they can comparatively live longer during space travel and people on earth will pass away before they return. (Twin paradox)

Length Contraction: As a space craft speeds up, the apparent distance to objects ahead decreases. This means trips on a relativistic spacecraft will appear to cover less distance to observers in the spacecraft. Could possibly allow travel to distant stars etc.

Mass Dilation: As the speed of a spacecraft increases to the speed of light, its mass will increase up to infinity and hence restricting the velocity it is able to achieve. Travelling at a constant velocity (constant thrust), mass increases causing acceleration to decrease as the thrust becomes less and less effective requiring more fuel.

20.1Analyse and interpret some of Einstein's thought experiments involving mirrors and trains and discuss the relationship between thought and reality:Einstein had two main thought experiments: Looking at himself in a mirror on a train moving at the speed of light Bouncing light from the roof to the floor and back in a moving train

Mirror Thought Experiment: Einstein wondered whether he would be able to see his face normally in a mirror he held in front of his face if the train was travelling at the speed of light. He decided that he would be able to, because he was in an inertial frame and should have no way to determine he was moving at c. Although if this was correct, a stationary observer would see light travelling away from Einsteins face at c, but as the train was also moving at c, the observer would see light travel twice the distance in the same amount of time. Einsteins interpretation of this was that the time observed for light to travel that distance had changed increased (time dilation), so that a stationary observer would see light travelling at c.

Light Bouncing Thought Experiment: Inside the moving train, the light is seen to travel straight up and down from the roof to floor and back again. From a stationary observer however, the light is seen to travel a much longer path, but in the same amount of time, which would result in a changed speed of light (going against Einsteins theory) Again his interpretation was that time had to increase (dilate) so that c remains constant.

20.2Analyse information to discuss the relationship between theory and the evidence supporting it, using Einsteins predictions based on relativity that were made many years before evidence was available to support it:Time Dilation: GPS reset clocks to maintain Earths time Muons half life of 2.2s; observed half-life of 400s

Length Contraction: Muons travel 100km in the atmosphere; observed as 577m

Mass Dilation: Particle accelerators showed that particles cannot go faster than the speed of light.

Energy and mass equivalence (E=mc2): Nuclear reactors use very little mass to create lots of energy. The products of a nuclear reaction have less mass after the reaction and the energy yields are proportional.

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