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Newton’s Laws of Motion The First Law “An object at rest tends to stay at rest, and an object in motion tends to stay in motion (straight line, constant velocity) unless acted on by outside force.” In the Solar System that outside force is usually gravity. Why we need seat belts… If you break the Seat Belt Law, you might be obeying Newton’s First Law! The Moon tries to “obey” the First Law, but the Earth’s gravitational pull forces it into an elliptical orbit. Concept of Escape Velocity • If a cannon is fired from a high mountain, it will eventually be brought down to Earth. • Given more velocity it will encircle the Earth. If more velocity is given… • The orbit will increase its altitude, becoming elliptical. • When a speed of around 18 mps is reached, the cannonball escapes. Newton’s Cannon Animation The Second Law “The force exerted on a body is equal to the mass multiplied by the acceleration.” F = ma In other words, the greater the force the more the object accelerates. Falling Bodies Unchecked, the force of gravitation causes objects to accelerate toward the center of the Earth. Without air resistance, they fall at the same rate. With air resistance, less massive objects slow down. The Third Law “For every action there is an equal and opposite reaction.” You push me and I push you back! Jet Propulsion Binary Star Animation Newton’s Law of Universal Gravitation F=G•M•m 2 r This means that objects with high mass are strongly attracted to each other. The Tides The farther apart the two objects are, the weaker the force of gravity. SUN MERCURY NEPTUNE This explains why planets closer to the Sun move faster than those farther away. It also helps to explain Kepler’s Third Law: D3 = P2