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Momentum and Impulse Physics 11, Unit 3 This section: see pages 195 to 205 in text

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Think about… What do you already know about the following. Think about them as we study this unit… Work Energy Power Efficiency Momentum Impulse

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Think about… Why is energy important? Where does energy come from? Where does energy go? How do we capture energy? Why is this an important part of our everyday lives??? How does energy impact kinematics (motion) and dynamics (forces)?

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First … What is momentum? The product of mass and velocity. What is the formula for momentum? p = mv (don’t use P!!! Power!) What are the units for momentum? kg m/s

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An example to think about… Give an example involving momentum. Think of two cars of equal mass. If one car is moving faster, it will take more force to stop the faster car in the same time interval. If you have a car and a truck, the truck has more mass so has more momentum – it will take longer for it to stop than the car. Is momentum a vector or scalar? Vector (same direction as the motion or velocity).

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Example A cement truck full of cement has a mass of 42000kg. It travels north at 18 m/s. A) Calculate the truck's momentum. B) How fast must a 750 kg Honda Civic travel in order to have the same momentum?

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Momentum Momentum can also be defined starting from Newton’s Second Law The rate of change of momentum can also be used to determine the force

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Example A cyclist is travelling at 32km/h and the bike and rider have a mass of 85kg. What is their momentum?

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Try these: Page 197, question 29

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Relating this to Force According to Newton's first law, if no net force acts on an object, its velocity is constant. Its mass will not change. Therefore, in this situation, momentum is constant. Momentum is conserved. Newton's second law describes how the velocity of a body changes if a net force acts on it.

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We have looked at motion and forces already…However… When an object is accelerated, typically the force will only be applied for a given time This holds true for things like: Explosions Collisions Recoil So instead of considering Newton’s Second Law (F = ma) as we have previously discussed it, we will rearrange the equation so we can look at momentum and impulse

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Impluse - Derivation Using the definition of acceleration (change in velocity divided by change in time) we can rewrite Newton’s Second Law Rearranging gives impulse

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Impulse Force applied over a given time It is a VECTOR and the direction is the same direction as the force Its symbol is j Formula: j = F∆t Units: Ns

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But what is impulse? http://www.youtube.com/watch?v=yUpi V2I_IRI http://www.youtube.com/watch?v=yUpi V2I_IRI Why are buses safer than other vehicles??? Up to 13 minutes (rest is next unit…types of energy)

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Example A tennis ball is struck by a racquet with a force of 750N; if the time of contact was 0.023s, what impulse was delivered to the ball?

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Try these… Page 200, questions 30, 31, 32

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Problem A baseball of mass 0.145kg is pitched toward a batter with an initial velocity of 35m/s. If the batter hits the ball in the opposite direction at 45m/s, determine the force that is applied by the bat on the ball if the contact time was 0.013s.

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Did you get an answer of 890 N [away from the batter]

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Impulse-Momentum Connection Collisions and explosions happen so quickly that it is often impossible to calculate anything more than an average force. This is because the force changes so quickly. By examining the momentum before and after the interaction between 2 objects, we can determine impulse.

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Impulse-Momentum Theorem Impulse is the difference of the final momentum and initial momentum of an object involved in an interaction.

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Impulse-Momentum Theorem Impulse and momentum can be related in order to solve dynamics problems in one step as opposed to two or more

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More Practice http://www.physicsclassroom.com/class/mo mentum/u4l2e.cfm http://www.physicsclassroom.com/class/mo mentum/u4l2e.cfm Think of a dump truck and a car. If they are stopped in the same time interval, it will take a ______ (more or less) force to stop the truck than the car. Why? Think of two cars of equal mass. If one car is moving faster,it will take _____ (more or less) force to stop the faster car in the same time interval.

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A cement truck full of cement has a mass of 42000 kg. It travels north at 18 m/s. Calculate the truck's momentum. How fast must a 750 kg Honda Civic travel in order to have the same momentum?

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A golf club strikes a golf ball. The club and ball remain in contact for 0.60 ms. The 45 g ball leaves the club with a speed of 70. m/s. Calculate the average force of the club on the ball.

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Example from before… A baseball of mass.145kg is pitched toward a batter with an initial velocity of 35m/s. If the batter hits the ball in the opposite direction at 45m/s, determine the force that is applied by the bat on the ball if the contact time was 0.013s.

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Example

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Try These Page 203 33-35 Page 209 37-46

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