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Newton’s Laws of Motion Newton’s 1st Law Newton’s 2nd Law and Friction Newton’s 3rd Law and Law of Universal Gravitation

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Newton’s 1st Law Law of Inertia Everything in motion stays in motion, and everything at rest stays at rest unless an outside force acts upon it. Everything in motion and at rest has inertia.

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Mass Mass determines the amount of inertia possessed by an object. Mass is the amount of matter in an object.

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Mass is NOT Volume Volume is the amount of space an object takes up. EXAMPLE: A pillow will have a volume larger than a car battery but less mass.

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Mass is NOT weight Weight is the force of gravity acting on a body Objects in space will still have mass but will not have weight. Metric Units:mass - kilogram weight - Newton

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On the surface of the earth, mass and weight are proportional. 1 kg = 9.8 Newtons

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Calculating the weight of an object Since weight depends on the mass of an object AND the gravitational pull of the earth, we can say that Weight = mass x gravity Or Weight = mg

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The weight of an object is measured in Newtons. Because weight depends on gravity, it is also a force.

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What is the weight of a 500 gram rock? 1. Convert to standard units 500 grams =.5 kg 2. Remember the constant for gravity is 9.8 m/s 2.

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3. Plug into the equation and solve. Weight = mg Weight = (.5 kg)(9.8 m/s 2 ) Weight = 4.9 N (Newtons)

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Questions 1. Your empty hand is not harmed if it bangs against the wall, but it is harmed if you are carrying a heavy load. Why? 2. Does a person diet to lose mass or to lose weight? 3. Can the force of gravity on a 1 kg mass ever be greater than on a 2 kg mass? Explain how. 4. A car at a junk yard is compressed until its volume is less than 1 cubic meter. Has its mass changed? Has its weight changed? Has its volume changed? Explain. 5. If you jump up in a bus that is moving at a constant velocity, will you land farther back in the bus? Explain. Return to Home Page

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Answers 1. Your empty hand has little inertia therefore it cannot exert a very large force on the wall (F=mg). When you’re holding a heavy weight your hand’s inertia is increased by the mass of the weight.

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2. You lose weight as a result of losing mass. You lose fat which has mass and if in a gravity field also has weight.

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3. The pull of gravity or weight depends on “g”. If g is small enough (say on the moon), a 1 kg mass on Earth (9.8N) will weigh more than a 2 kg mass on the moon (3.2N).

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3. Continued F=mg for the moon would be (2kg)(1.6m/s 2 ) which = 3.2N

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4. Mass is the same, same amount of stuff-mass is a measure of the amount of stuff or inertia an object has. Weight is the same since same amount of mass and same force of gravity.

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4. Continued Volume is different since the car is taking up less space.

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5. NO. You and the bus are moving together at a constant speed. You will appear to move straight up and down to an observer in the bus, but will appear to move in a curved projectile path to an observer on the ground.

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Newtons 2nd Law of Motion Law of Acceleration What causes an object to accelerate? -or change its state of motion? NET FORCE

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What is a Net Force? A net force is the combination of all forces acting on an object Applied ForcesNet Force 5 N 10 N 15 N 10 N 5 N

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Equilibrium Applied ForcesNet Force 5 N 0 N If the net force is zero, the motion of the object will not change.

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This is a condition called EQUILIBRIUM. If an object is in equilibrium, we have to go back to the Law of Inertia.

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Acceleration Acceleration is directly related to the net force. Acceleration ~ Net Force The larger the net force is acting on an object, the greater the acceleration of the object.

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Acceleration Acceleration is inversely proportional to the mass of an object Acceleration ~ Try to push a 250 pound crate. Now try to push a 25 pound crate. Which could you move faster? 1mass

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Newton’s 2nd Law of Motion The acceleration of an object is directly proportional to the net force acting on it, is in the same direction as the net force, and inversely proportional to the mass of the object. a = FmFm OR F = ma

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F m a

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Friction Friction is a force that affects motion by slowing an object down. It is always opposite to the direction of motion and affects how much an object can accelerate.

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Friction is present because of irregularities in the surfaces of objects that are in contact with one another. Fluids like water and air also exert a friction force on a moving object.

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Friction Direction of motion FRICTION BETWEEN TIRES AND ROAD Return to Home Page

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Newton’s 3rd Law of Motion Action / Reaction Law of Interaction Newton realized that force is not a thing in itself, but part of an interaction between one thing and another.

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Forces always occur in pairs One force is called the action force and the other the reaction force. Action: Object A exerts a force on Object B.

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Reaction: Object B exerts an equal and opposite force on Object A.

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Newton’s 3rd Law For each action force, there is an equal and opposite reaction force. Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

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Action / Reaction Forces Action: Tire pushes road Reaction: Road pushes tire Action: Rocket pushes gas Reaction: Gas pushes rocket Action: Earth pulls ball Reaction: Ball pulls Earth

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Action / Reaction Forces Consider the apple at rest on the table. If we call the gravitational force exerted on the apple action, what is the reaction force? Are there any other action/reaction forces present?

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Action / Reaction Forces If a cement truck and a car have a head-on collision, which vehicle will experience the greater impact force? A. The cement truck B. The car C. Both the same D. ….it depends on other factors

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Newton’s Law of Universal Gravitation Every object in the universe is physically attracted to every other object. Examples:Earth and Sun Earth and Moon You and the earth

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Newton’s Law of Universal Gravitation The amount of gravitational pull between two objects depends upon two things: The amount of mass each body has How far apart the two bodies are from each other

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Newton’s Law of Universal Gravitation Gm 1 m 2 d2d2 F = Where: F = Force of gravitational pull between the two objects m 1 and m 2 are the masses of the two bodies d is the distance between their centers of mass G is the universal gravitational constant (6.67 x 10 -11 Nm 2 /kg 2 )

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Can you attract another person gravitationally? A 50-kg person and a 75-kg person are sitting on a bench.5 m apart. Calculate the gravitational force each exerts on the other. Which is so small, it is unnoticed except with very delicate instruments. Return to Home Page

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Forces and Motion Book Chapter 12

Forces and Motion Book Chapter 12

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