Presentation on theme: "Ch 7: Universal Gravitation"— Presentation transcript:
1 Ch 7: Universal Gravitation Barry LathamHonors PhysicsBloom High SchoolCh 7: Universal Gravitation
2 7.1: Kepler’s Laws of Motion Based on years of observations, Kepler devised a series of laws known to us as his “Laws of Planetary Motion.”1st Law: The path of each planet about the sun is an ellipse with the sun at the focus.2nd Law: Each planet moves so that an imaginary line drawn from the sun to the planet sweeps out equal areas in equal periods of time.3rd Law: The ratio of the squares of the periods T of any two planets revolving about the sun is equal to the ratio of the cubes of their mean distances s to the sun: (T1/T2)2=(s1/s2)3.
4 Problem Solving As always, picture/data/formula/algebra! Keep the unknown as a numeratorRearrange the formula BEFORE you plug the values in.Watch parenthesis, squares, cubes and their roots.Recall that: x½ = 2√(x) x1/3 = 3√(x)x2/3 = 3√(x2) x3/2 = 2√(x3)
5 Distance to Mars?According to Kepler, Mars’ “year” is 687 Earth-days. Using what we know about the distance and period of the earth, we can find Mars’ distance. The mean distance of the Earth to the Sun is 1.50x1011 m.Picture/Data/Formula/Algebra(T1/T2)2=(s1/s2)3T1=Mars=687 days s1=Mars’ orbital radiusT2=Earth= days s2=Earth orbital radius= 1.50x1011 m(687 days/ days)2=(s1/1.50x1011 m)3(1.5x1011 m)(687 days/ days)2/3=s1s1=2.28x1011 m
6 Newton’s Law of Universal Gravitation Every particle in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. This force acts along the line joining the two particles.F=Gm1m2/r2
7 Universal Gravitation Equation F=Gm1m2/r2F= the force of gravity (in Newton’s)m1 & m2= the two individual masses that we are measuringr= the distance between the masses, measured in a straight line and using their center of massG= the universal gravitation constant6.67x10-11 N m2/kg2F=(N m2/kg2)(kg)(kg)/(m)2F=N (big surprise)
8 The Romance of Gravity Find the force of attraction between two people. A 74 kg person is sitting 1.2 m away from a 84 kg person at a lab bench. Estimate the force attracting them together.Picture/Data/Formula/AlgebraF=Gm1m2/r2G= 6.67x10-11 N m2/kg2m1=74 kgm2=84 kgr=1.2 mF=G(74 kg)(84 kg)/(1.2 m)2F= 2.879x10-7 N (equal to a 0.03 mg object)
9 Less Romantic Gravity, part 1 As you get far away from Earth, does your weight decrease?What is the weight of a 2000kg satellite when it is in the lab at NASA?Fg=mgFg=(2000 kg)(9.80 m/s2)Fg=19,600 N
10 Less Romantic Gravity, part 2 What is the force of gravity acting on a kg satellite when it orbits two Earth radii from the Earth’s center (rEarth = 6.38x106 m).F=Gm1m2/r2F=G(2000 kg)(5.97x1024 kg)/(2x6.38x106 m)2Fg=4,891 NThis is 1/4th of what it was in the lab!
11 Universal Gravitation & Kepler’s 3rd Law If we take Newton’s law of universal gravitation,F=Gm1m2/r2And Kepler’s 3rd Law (with centripetal acceleration included)F=m4p2r/T2m=mass of central bodyT=period of orbit (time of one revolution)Set them equal (they are both F) and solve for TT=√(r3(4p2)/(Gm))This represents the time it takes for an object to go around a central body one time.
12 Cavendish’s DeviceUsing a device shown to the right, Cavendish was able to accurately measure the force of gravity.By twisting the string slightly, the scale displayed the force of attraction between the sphere on the rod and sphere A.
13 Geophysical Applications When discussing objects on the earth’s surface, m1 becomes mE and r is the radius of the earth (rE).Original formula F=Gm1m2/r2substituted m2 g for F m2g=GmEm2/rE2cancel m2 g=GmE/rE2So the acceleration due to gravity of an object is dependant upon the mass of the object and the radius of the object (in this case, the earth)In lieu of Earth, we can use the data from any object!
14 What’s the mass of Earth? Unless you have a HUGE scale, we need another way to determine it.g=GmE/rE2 but solve for mEmE=g rE2/GmE=(9.80 m/s2)(6.38x106 m)2/(6.67x10-11 N m2/kg2)mE=5.98x1024 kg
15 Is “g” really 9.8? Find g at sea level on Earth. g=GmE/rE2 g=(6.67x10-11 N m2/kg2)(5.98x1024 kg)/(6.38x106 m)2g= m/s2So, yes, it’s basically 9.8.
16 Is “g” really 9.8?Gravity at the top of Mt. Everest. Mt. Everest has a height of 8,850 m above sea level.g=GmE/rE2g=(6.67x10-11 N m2/kg2)(5.98x1024 kg)/(6.38x106 m + 8,850 m)2g= m/s2So, yes, it’s basically still 9.8.
17 Is g 9.8 everywhere on Earth? Explain the losses or gains in gravity.HW p q. 6-10
18 7.2: SatellitesArtificial satellites are put into orbit with three different final velocities.Circular orbit (space shuttles)27,000 km/hElliptical orbit30,000 km/hIf an object needs to leave earth’s orbit, such as solar probes, and extra-solar objects40,000 km/h
19 Satellite Orbital Velocity Geostationary satellite (communications & weather)Stays above the same point on earth
20 Satellite Equation Derivation To remain above the same point, the satellite must have a period (T) of one day (one cycle per day).Apply Newton’s Second law and a new formula (centripetal acceleration)Assume the orbit is circular (which it pretty much is)The only force on the satellite is the force of universal gravitation. Plug our new gravity equation and centripetal acceleration into F=ma.F=ma F=Gm1m2/r2 ac=v2/rGmsatmE/r2=(msat)(v2/r)
21 Speed (v) of a Satellite Orbiting a Central Body in Circular Orbit Since the satellite revolves around the earth at the same rate the earth turns, v=2pr/TT is the period of the movement, 1 day (86,400 s)GmsatmE/r2=msatv2/r cancel msat and rGmE/r=v2 solve for vv=√(GmE/r) mE is the mass of the Earthbut can represent the central body√((6.67x10-11)(5.98x1024)/(4.23x107))=3070 m/sWe find the same result if we use, v=2pr/T.v=2p (4.23x107)/(86,400)=3076 m/s
22 Period (T) of a Satellite Orbiting a Central Body in Circular Orbit We saw this on Slide 11!T=√(r3(4p2)/(Gm))This represents the time it takes for an object to go around a central body one time.If the mass (m) is Earth, then m=mE
23 Orbital height (r) of a Satellite Orbiting a Central Body in Circular Orbit Using any number of equations containing r, we can find the orbital heightRemember that height is from the center of the central object to the center of the satelliteF=Gm1m2/r2 Newton’s Universal Gravitationr= √( Gm1m2/F)F=m4p2r/T2 Kepler’s Third Lawr=FT2/(m4p2)T= 2p√(r3/(Gm)) Newton’s version of Kepler’s 3rd Lawr=v=√(GmE/r) v of satellite in circular orbitr=GmE/v2
24 Inertial MassAccording to F=ma, the mass of an object is the relationship between the Force applied and the acceleration of the object.Because every object with mass has inertia, we can rearrange and use subscripts.minertia=Fnet/aMore mass means less reaction to the same forceInertial mass is independent of location or a reference bodyIt’s how they measure the astronauts mass in space!Lab 7-2
25 Gravitational MassUnlike Inertial Mass, gravitational mass is dependant on a reference material (sometime a whole planet!)Using Newton’s Universal Gravitation formula, we can rearrange to find the mass of an objectFgrav=GmgravmE/r2mgrav=r2Fgrav/(GmE)To measure this, we use a triple beam or digital balance in the presence of gravity
26 Relativity & GravityAccording to Einstein, gravity is not a force, but an effect of space itselfMass causes space to curve, so objects are pulled toward the “dimple” in space-time (even light)Larger objects make larger “dimples” hence larger gravitational pull
27 Earth’s gravitational field distorting space-time
28 Blackholes, definedBlack hole- a concentration of mass so dense that nothing can escape the pull of it’s gravity, not even lightFormed by the severe gravitational pull overcoming the thermonuclear reaction pushing the material outBlack hole formationSize? Black holes can be measured by mass or areaMass to 1036 kg (10-1,000,000 times as massive as our Sun)Volume- based on Schwartzchild radius or radius of the horizonA black hole with a mass the same as the Sun would be 3 km in radiusThe Sun has a radius of 700,000 km now!A 10-solar-mass black hole would be 30km and so onEvent Horizon- the point of no return, the Schwartzchild radius
29 Black Holes evaporate?Black Holes will not eventually suck up everything in the universeThey emit “Hawking radiation”At the Event Horizon, a particle-antiparticle pair can appear from within the black holeOne particle gets sucked back in, the other escapesMomentum & Energy are conservedThe emitted particle is detectedThis happens so frequently, it is observed as a matter/energy stream
33 End of Unit Review, 1What would make you weigh more: an earth twice as massive with the same diameter or one having half its diameter with the same mass?Solution: Find out using F=Gm1m2/r2Fg1=GmEmyou/rE2 normally, what is your Fg?Fg2=G(2mE)myou/rE2 now, what is your Fg?Fg3=GmEmyou/(½rE)2 now, what is your Fg?How do they all compare?
34 End of Unit Review, 2Would one need a longer rocket to take off from the moon vs. earth?What is needed to escape the Earth? The Moon?Gravity holds us in place, so which has less gravity?Solution: Find out using g=GmE/rE2For earth, g= m/s2For the Moon, g=GmM/rM2g=G(7.349x1022 kg)/(1.738x106 m)2g=1.62 m/s2
35 End of Unit Review, 3Based on our prior work, what would your weight be on the Moon?FgE=m(9. 80 m/s2)FgM=m(1.62 m/s2)FgE=(84.1 kg)(9. 80 m/s2)=824 NFgM=(84.1 kg)(1.62 m/s2)=136 N
36 End of Unit Review, 4If an astronaut was to break the connection rope between him and the space shuttle, will gravity quickly pull him back to the shuttle?F=Gm1m2/r1-22g=Gm/r2What happens to the acceleration of an asteroid as it approaches a planet?If g based on distance?