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2/25/2014 Gravity? Is gravity real or does the Earth just suck? Distribution of matter Simulated image that shows the distribution of matter in the . Universe. Image credit: MPG Distribution of Galaxies Survey 1 2/25/2014 Gravity Review Gravity is a FORCE 1. What happened when you decreased the distance between objects in the simulation? 2. What happened when you changed the mass of the objects in the simulation? 3. What two factors affect the amount of attraction between two objects? 4. Do you think that the force of gravity increases with the distance from the Earth’s surface? • Gravity is a natural force of attraction by ANY and ALL objects that have mass, which is directly proportional to the mass and inversely proportional to the distance between them. • When gravity pulls something towards the Earth - we call this force the weight. 8 The Law of Universal Gravitation: • All objects in the universe attract each other through gravitational forces. The size of the force depends on the masses of the objects and the distance between these objects The Equation 12 ∝ • • • • • 2 Rules for Law of Universal Gravitation: 1. The Gravitational Force INCREASES as the mass of the object INCREASES 2. The Gravitational force DECREASES as distance between the objects INCREASE Mass & Distance Affect Gravity Directly Proportional to Mass Inversely Proportional to Distance F = gravitational force ∝ = proportional m1 = the mass of one object m2 is the mass of the other object d is the distance between the two objects. 2 2/25/2014 Example • ∝ • ∝ Example • If m1 is doubled to 4kg, • If m1 = 2 kg, m2 = 2 kg, m2 = 2 kg, and R = 2 m and R = 2 m • then F is proportional to • then F is proportional to 4 x 2 / 22 = 8/4 = 2 2 x 2 / 22 = 4/4 = 1. • 2 times higher (since gravity is directly proportional to mass). • ∝ • Using the original masses, if R is doubled to 4, then F is proportional to 2 x 2 /42 = 4/16 = ¼ • Is this relationship similar to what they saw in the simulations in the lab? • 1/4 the original force (since gravity is inversely proportional to the distance squared). Force of Gravity Demo! Force of Gravity Demo! 1. Does the rubber band pull with the same force on both balls? 2. What will happen to the balls when I release the rubber band? 3. Observe both balls during the demo. What happened? 4. Why didn’t the steel ball move as much as the tennis ball? The rubber band is like the gravitational force acting on objects. 1. Is the gravitational force felt by both balls the same? 2. Are the effects of the gravitational force the same on both balls? Did this happen during your simulations? Post-Lab Questions The Law of Universal Gravitation: • Use your lab to answer the questions on the Post-Lab Questions Worksheet • All objects in the universe attract each other through gravitational forces. The size of the force depends on the masses of the objects and the distance between these objects 3 2/25/2014 Gravity and mass Gravitational force is Gravitational force is SMALL between objects that have a small mass LARGE between objects that have a large mass Misconceptions about Gravity • Mass and weight are the same quantity of measurement • Gravity is related to movement, closeness to earth, or magnetic fields • The Moon has no gravity (It does) • Astronauts are weightless because there is no gravity in space • Planets with thin atmospheres have little gravity • Planets distant from the sun have less gravity • Gravity is stronger between the most distant objects Gravity down under More Misconceptions about Gravity • • • • • People do not fall off the Earth • • • • Weightlessness means no gravity. Heavier objects fall faster than lighter ones. Gravity is a very strong force. There is no gravity in space, on the moon, or in a vacuum Gravity is caused by Earth’s spin, air pressure, push from above. Gravity is weaker under water. Gravity is stronger the higher you are in a building or mountain that’s why it is easier to fall. Gravity will be stronger the longer and object falls. 21 Two objects dropped at the same time… On the Moon – Apollo 15 A heavy geological hammer and a falcon feather were dropped from the same height. They hit the lunar surface at the same time. Will hit the ground at the same time! 23 astronaut David Scott NASA pictures 4 2/25/2014 More Misconceptions about Gravity • Shape affects gravity • Gravity is weaker the higher you go. That’s why things fall faster the farther they fall. • Astronauts are weightless because they are beyond Earth’s gravity. • Gravity is the attraction of the larger body. The Earth and me are not attracted by other objects pulling up. • Gravity causes objects to fall down through the center of the Earth and out the other side if there was a hole in through the Earth. • Gravity pulls objects towards the south pole and if the Earth would not stop it they would go off into space away from the south pole. • Gases are not affected by gravity. No Gravity? • • • • Would an apple fall? Would things stay on earth? What would happen? Would the universe be the way it is today? 26 Launching a satellite Weightless? • or are they? The cannon ball must move very quickly to orbit the earth. The Moon orbits the Earth (NASA pictures) 27 28 apparently weightless GRAVITY (pictures or animation only –not a demonstration) • Things with mass are attracted to one another • Things fall • Things have weight • Things stay in place 29 30 5 2/25/2014 Force of Gravity Thinking about Gravity 1. Do you feel the gravitational pull from the Earth? 2. Does the Earth experience the gravitational pull from you? 3. Is the gravitational force between you and the Earth the same? (Think about the rubber band demo) 4. Does the Earth and you experience the same effects of the gravitational force? Why? • Can you think of a weigh to measure the strength of gravity at the surface of the Earth? Strength of Gravity on Earth Strength of Gravity on Earth • Rearranging the gravity equation for Earth we get: Strength of gravity = = 9.8m/s2 • G = the gravitational constant Gravity on Other Planets Force of Gravity on Other Planets Planet • Would you weigh the same on other planets or moons? Why or why not? Strength of Gravity (Earth = 1) Mercury 0.38 Venus 0.91 Earth 1.00 Mars 0.38 Jupiter 2.54 Saturn 1.06 Uranus 0.92 Neptune 1.18 Pluto 0.06 To determine your weight on another planet: Multiply your weight by the strength of gravity on another planet 6 2/25/2014 On the Moon Very heavy boots- Why? Answers? • No gravity on the Moon • Weightless on the Moon • To stop them floating away • They weigh very much less than on Earth • To stop huge bounces • The Moon’s gravity is very much less than the Earth’s gravity NASA pictures 37 The Earth 38 The Earth The Moon The Moon 39 40 Final Thoughts… The Universe: Gravity Video! • The force of gravity is proportional to the mass, and is inversely proportional to the distance squared. • So, as mass increases, gravity increases. As distance increases, gravity decreases (proportional to the square of the distance). • COMPLETE your post-lab questions and TURN IN! • Answer the questions on the back of your gravity notes 7 2/25/2014 Gravity • Sir Isaac Newton (1642-1727) developed a law of gravitation which said all objects in the universe attract each other through gravitational force • Universal Gravitation Equation • F = G m1m2 d2 • Equation shows grav. Force ↑ as both masses ↑ • Equation shows grav. Force ↓ as the distance b/w masses ↑ 8