3.1 Impulse and Change in Momentum
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Impulse-Change in Momentum 3.1 Change in Momentum (2 days) Mr. Richter Agenda Warm-Up Problem Solving Practice Intro to Change in Momentum Momentum and Impulse Quiz Friday Notes: Impulse-Change in Momentum Calculating Impulse Conservation of Momentum Objects Pushing Off of Each Other (Recoil) Objectives: We Will Be Able To… Understand the relationship between the time interval for which a force is applied and the change in an object’s momentum. Solve problems with changes in momentum. Understand the relationship between Newton’s Third Law and the Law of Conservation of Momentum. Calculate speeds of object’s pushing off of each other (recoil problems). Warm-Up: If you were falling from a second floor window, would you be safer if you fell onto a concrete sidewalk or a mattress? Why? (Do not just say the mattress is softer.) Discuss at your table, and we will discuss as a class in a few minutes. Impulse – The Change in Momentum Impulse – The Change in Momentum To change the momentum of an object, assuming that it doesn’t lose any mass, you must change its speed. It will either get faster or slower. What is the difference between a car that gradually comes to a stop, and a car that slams into a wall? Hint, the change in momentum is the same because the change in speed is the same. Impulse – The Change in Momentum The difference between gradually coming to a stop and slamming into a wall is how long it takes. Gradually coming to a stop: it takes a long time a small force is applied Slamming into a wall takes very little time a large (dangerous) force is applied Impulse – The Change in Momentum Impulse is the change in an object’s momentum. Impulse depends on how much force is applied, and for how long. This determines by how much the speed of an object changes. Units: kgm/s (same as momentum) Practice Problems 1. A 1200-kg car traveling at 15 m/s comes to a stop. a. What is its starting momentum? b. What is its final momentum? c. What is its change in momentum? 2. A 140-N force is applied to a 10-kg bowling ball for 3 seconds. What is the bowling ball’s change in momentum? Homework: Impulse-Change in Momentum Worksheet #1-4 Due Thursday Separate Sheet of Paper! Warm Up: What do we mean when we say that something is conserved? Write 1-2 sentences in your notebook, then discuss at your table. The Law of Conservation of Momentum The Law of Conservation of Momentum What does the word “conservation” mean? You probably said “save” “don’t waste” maybe “preserve” In physics, conservation means that nothing is lost or gained. Nothing is created or destroyed. THE LAW OF CONSERVATION OF MOMENTUM: when we say that momentum is conserved, we mean: The Law of Conservation of Momentum In other words: the total momentum that exists in a system does not change. Momentum just transfers from one object to another. The Law of Conservation of Momentum Where does this law come from? When two objects interact with each other: they interact for the same amount of time N3L says that the forces on the objects must be equal and opposite If the forces are equal and opposite, then the momentums are equal and opposite! The man and the ball have equal momentum in opposite directions. Recoil Recoil When two objects push off of each other from a stationary position, this is called recoil. A skateboarder throwing a ball. A person jumping off the Earth. A rifle shooting a bullet. Conservation of Momentum in Recoil: m1v1 = -m2 v2 Momentum of = Opposite of Momentum Object 1 of Object 2 Recoil Practice An 80-kg astronaut floating (stationary) in space throws a 5kg moon rock at 8 m/s forward. What is the astronaut’s speed after he throws the rock? -0.5 m/s Wrap-Up: Did we meet our objectives? Understand the relationship between the time interval for which a force is applied and the change in an object’s momentum. Solve problems with changes in momentum. Understand the relationship between Newton’s Third Law and the conservation of momentum. Calculate speeds of object’s pushing off of each other (recoil problems). Homework p. 81 -82 #6-9 Study for your quiz!
