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Section 4: Action and Reaction Objectives: state Newton’s third law of motion define and calculate momentum and state the law of conservation of momentum

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I. Newton’s 3 rd Law of Motion A.For every action, there is an equal and opposite reaction. i. Forces act in pairs A hammer exerts a force on a nail, pushing it into a piece of wood. At the same time, the nail exerts a force back on the hammer, causing its motion to come to a sudden stop.

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FYI: Equal but opposite

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FYI:

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FYI: Action-Reaction Example When a squid pushes water out, the expelled water pushes back and forces the squid to move ahead. The force the squid exerts on the water is the action force.

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FYI: If a toy fire engine and a real fire engine were rolling towards a concrete wall at 10 mps, which one would cause more damage to the wall? Why?

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II. Momentum Of course the real fire engine would cause more damage because it has more mass, therefore it has more momentum. A. Momentum = Mass X Velocity

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B. The more momentum an object has, the harder it is to stop or to change its direction. FYI: Think about an empty shopping cart and a shopping cart full of canned goods. Which one is harder to stop or turn? Why?

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FYI: Momentum is Conserved FYI: In the absence of friction, momentum is conserved when two train cars collide. Look at the mass and speed of each car before colliding. What is the momentum of each car and the total momentum? What happens to the mass and speed of each car after colliding? What about the momentum?

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III. Conservation of Momentum A.The total amount of momentum of a group of objects does not change unless outside forces act on the objects. i. Momentum lost by one object is picked up or gained by the other object. The amount of momentum before the train cars collision is the same as the amount of momentum after the train cars collide.

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What is the mass and speed of each car before colliding? What is the total momentum before colliding? What happens to the mass, speed and momentum of the cars after the cars collide? Example: Red Train is moving, blue is not moving

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What is the mass and speed of each car before colliding? What is the total momentum before colliding? What happens to the mass, speed and momentum of the cars after the cars collide? Example: Same thing, but trains get connected

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Problems Which has more momentum, a 250-kg dolphin swimming at 6 m/s, or a 450-kg manatee swimming at 2 m/s? Why? (Explain using the formula.)

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Which has more momentum: a 3-kg sledgehammer swung at 1.5 m/s or a 4-kg sledgehammer swung at 0.9 m/s? Explain using your formula.

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A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum? (Explain completely.)

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Answer momentum = mass x velocity 0.045 kg x 16 m/s = 0.72 kgm/s 0.14 kg x 7 m/s = 0.98 kgm/s The baseball has greater momentum.

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What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s?

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Answer 0.018 Kg X 15 m/s = 0.27 kgm/s

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