1. Scientists create both scientific theories and scientific laws as they make observations and conduct experiments about the natural world. Which of the following statements most accurately compares the difference between scientific theories and scientific laws? Scientific laws are based on evidence, while scientific theories are not. Scientific theories involve only biology, while laws involve all types of science. Scientific theories involve mathematical equations, while scientific laws are based on observations. Scientific theories are ideas that explain natural events, while scientific laws more reliably predict natural events. 2. There are many important laws and theories in science. Which of the following is a law of science? A living thing is made of at least one cell. An atom is made of electrons orbiting a nucleus. Gravity will pull two objects toward each other. Natural selection will change species to fit their environments. 3. Astronauts need sophisticated spacesuits to protect them from the harsh conditions of space. These spacesuits are very heavy for astronauts to wear on Earth. For an astronaut on the Moon, however, the suit would seem lighter. Which of the following statements explains why the spacesuit would feel lighter on the Moon? There is no gravity on the Moon. Earth’s gravity can’t be felt from the Moon. The force of the Moon’s gravity is less than Earth’s The force of the Sun’s gravitational attraction varies throughout the Solar System.

Benchmark C - voice level 0 H – raise hand and wait for teacher (no restroom passes) A – guided notes M – remain in seat unless given permission P – independent Unit 3 Lesson 2 Gravity and the Solar System Benchmark SC.8.E.5.4 Explore the Law of Universal Gravitation by explaining the role that gravity plays in the formation of planets, stars, and solar systems and in determining their motions. Essential Question:  What is gravity’s role in the formation of Solar Systems and in determining the motion of objects as they revolve around the Sun?  How does changing the mass of an object in space change the gravitational attraction? 2

What is gravity? Gravity Gravity is a force of attraction between objects that is due to their masses and the distances between them. Every object in the universe pulls on every other object. 3

What is gravity? Objects with greater masses have a greater force of attraction than objects with lesser masses have. Objects that are close together have a greater force of attraction than objects that are far apart have. Gravity is the weakest force in nature, yet it is one of the most important forces in the universe. 4

What is gravity? Gravity accounts for the formation of planets, stars, and galaxies. Gravity also keeps smaller bodies in orbit around larger bodies. An orbit is the path that a body follows as it travels around another body in space. 5

What is the law of universal gravitation? Using Kepler’s laws, Sir Isaac Newton became the first scientist to mathematically describe how the force of gravity behaves. He reasoned that gravity is the force that accounts for both the fall of an apple from a tree and the movement of the moon around Earth. In 1687, Newton formulated the law of universal gravitation. 6

What is the law of universal gravitation? The law of universal gravitation states that all objects in the universe attract each other through gravitational force. The strength of this force depends on the product of the masses of the objects. Gravitational force is also inversely proportional to the square of the distance between the objects. 7

How does gravity affect planetary motion? If a ball is attached to a string and is swung around, it moves in a circular path. The inward force that causes an object to move in a circular path is called centripetal force. If the string breaks, the ball will move off in a straight line. When the string is intact, the centripetal force prevents the ball from flying off. 8

How does gravity affect planetary motion? When planets orbit the sun, a force similar to centripetal force prevents them from moving out of their orbits and into a straight line. The sun’s gravity is the force that keeps the planets moving in orbit around the sun. 9

All of the planets in our Solar System orbit the Sun All of the planets in our Solar System orbit the Sun. Why do planets stay in orbit, rather than drift off into space?   The planets need the warmth of the Sun to maintain their stability. The magnetic field of the planets and the Sun keep them attracted to one another. The gravitational pull between the planets and Sun keep them in orbit. The gravitational pull of the outer planets keeps the inner planets in their orbit.

Diagram A because the planets are closer. 2. The planets in diagram A are 10,000 Km away from each other. The planets in diagram B are 50,000 Km away from each other. Which planets would have a greater gravitational attraction between them?   Diagram A because the planets are closer. Diagram A because they both have different masses. Diagram B because the planets are farther apart. Diagram B because they both have different masses. Diagram A Diagram B

3. Mars only has about 11% of Earth’s mass. The diagram shows the size of Earth (left) compared to the size of Mars (right). 3. Mars only has about 11% of Earth’s mass. What can you conclude about the gravity of these two planets? Earth has less gravity because it has more atmosphere. Earth has more gravity because it has more mass. Mars has more gravity because it has less mass. Mars has less gravity because it has more atmosphere.