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1 Linear Momentum

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2 What we will cover. Momentum and Its Relation to Force Collisions and Impulse Conservation of Momentum Conservation of Energy and Momentum in Collisions Elastic & Inelastic Collisions in One Dimension

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Mr. BaldwinPHYSICS IMPULSE & MOMENTUM31-Oct-13 Aim: Why is a follow through so important? Do Now: If a toddler and a sumo wrestler are both running toward you, which one would be easier to stop? Why? Homework: ESSAY is Due tomorrow

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Let’s start with everyday language. What do you say when a sports team is on a roll? They may not have the lead, but they sure may have ___________ MOMENTUM A team that has momentum is hard to stop. An object with a lot of momentum is also hard to stop

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Momentum is defined as the product between an object’s mass and its velocity. It is a vector & symbolized by the symbol p. CHECK: Can you derive the units for momentum? The unit is: kg∙m/s The rate of change of momentum is equal to the net force: This is nothing but Newton’s second law. 5 Momentum and Its Relation to Force

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Let’s practice A 1200 kg car drives West at 25 m/s for 3 hours. What is the car’s momentum? Identify the variables: –m = 1200 kg –v = 25m/s, West –t = 3 hrs p = mv = 1200 kg x 25 m/s= 30,000 kg∙m/s, West

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How hard is it to stop a moving object? To stop an object, we have to apply a force over a period of time. This is called Impulse The definition of impulse J: the change in momentum of an object OR force times time of contact. CHECK: What is the unit of Impulse? The unit is: N∙s

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8 Collisions and Impulse During a collision, objects are deformed due to the large forces involved.

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Why does an egg break or not break? An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why? In both cases, m and Δv are the same, so Δp is the same. If Δt goes up, what happens to F, the force? Right! Force goes down. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops. CHECK: Why are dashboards padded?

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Practice Problem A 57 gram tennis ball falls on a tile floor. The ball changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball? Identify the variables: m = 57 g = 0.057 kg Δv = +1.2m/s – (-1.2m/s) = 2.4 m/s t = 0.02 s using FΔt= mΔv F x (0.02 s) = (0.057 kg)(2.4 m/s) F= 6.8 N

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Sample Regents Question

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Mr. BaldwinPHYSICS IMPULSE & MOMENTUM1-Nov-13 Aim: What happens when objects collide? Do Now: What would happen if a 2 tonne truck rear ends a 1 tonne pick up? How would the wreckage move afterwards? Homework: DO the first five problems on Momentum and Impulse worksheets.

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14 Conservation of Momentum During a collision, measurements show that the total momentum does not change. The total momentum in any collision is conserved.

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15 Conservation of Momentum More formally, the law of conservation of momentum states: The total momentum of an isolated system of objects remains constant.

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Mr. BaldwinPHYSICS IMPULSE & MOMENTUM4-Nov-13 Aim: How does a rocket (space shuttle) take off? Do Now: QUIZ Suppose you are an astronaut that became stranded in space when your tether line breaks. You begin to float away from your space station with only a wrench in hand. Explain how would you save your life? How would you return to the space station? (Use the terminology you have learned.) Homework: Design an Egg Drop Experiment

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PHY 205 BALDWIN17

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18 Conservation of Momentum Momentum conservation works for a rocket. WHY?

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19 Conservation of Momentum Momentum conservation works for a rocket. WHY? What about Newton’s 3 rd law? Does it work here?

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What are the equations associated with momentum and impulse? PHY 205 BALDWIN20

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A 0.2 kg baseball is travelling at 40 m/s. After being hit by a bat, the ball's velocity is 50 m/s in the opposite direction. Find a) the change in momentum b) the impulse, J c) the average force, F, exerted by the bat if the ball and bat are in contact for 0.002 s. d) In which direction is the force acting? 21

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A 1000 kg car travelling 22 m/s (about 50 mi/hr) hits a concrete bridge support and comes to a stop in.5 s. a) Find the average force acting on the car b) if the bridge support had been cushioned so that the stopping time was increased to 3 s, what would have been the average force. c) In which direction is the force acting and why? 22 CHECK…YOUR TURN!!!

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Mr. BaldwinPHYSICS Conservation of Momentum7-Nov-13 Aim: How is momentum conserved in elastic and inelastic collisions? Do Now: QUIZ (10 minutes) A 2000 kg car is travelling at 20 m/s. The driver hits the gas and increases his speed to 50 m/s. Find a) the change in momentum of the car b) the impulse, J, imparted to the car c) the average force, F, exerted by the engine if his foot was on the gas for 5.0 s d) In which direction is the force acting? Can you explain? Homework: Design an Egg Drop Experiment

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24 What can you tell about your impulse when you jump off from a high place? Why do you bend your knees? The impulse tells us that we can achieve the SAME change in momentum with a large force acting for a short time OR a small force acting for a longer time. This is why you should bend your knees when you land after jumping; it increases time; it reduces force.

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25 Why do airbags work? Why does landing on a pillow hurts less than landing on concrete. What can you tell about your impulse when you impact your airbags? How is this similar to when you bend your knees after landing from a jump? IT’S ALL ABOUT THE FORCE!

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Impulse/Impact Decreasing momentum in a short time Car hitting concrete wall-- great damage by quick change Karate master-- great momentum of hand causes breaking of bricks

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Impulse

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28 Can you tell me what a collision is? Collisions are the interactions of two or more objects. Momentum is conserved in ALL collisions.

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29 Elastic Collisions Collisions in which kinetic energy is conserved. Momentum is perfectly transferred from one object to the next.

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30 Inelastic Collisions With inelastic collisions, some of the initial kinetic energy is lost to thermal, sound or potential energy. A completely inelastic collision is one where the objects stick together afterwards, so there is only one final velocity.

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CHECK: CAN YOU GIVE ME SOME EXAMPLES OF ELASTIC & INELASTIC COLLISIONS? OK…How would you classify these types? Two lumps of clay colliding Rubber ball on hard surface Two billiard balls colliding Two-Car collision Two bumper cars collision Wad of gum tossed onto a rotating cd Meteor impacting the Earth 31

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You are to design and build a device that will allow a chicken egg to survive the impact of dropping from various heights. In the process you will make a hypothesis about what you think will happen to the egg by explaining its inertia, motion, gravity, momentum and impulse. BE CREATIVE!!! 32 Design an Experiment & Build a Device http://weirdsciencekids.com/EggDropExperiment.html

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