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Physics 11 Advanced Mr. Jean April 2 nd, 2012

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The plan: Video clip of the day Forces & Acceleration Applied 2d forces

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The rocket: A 450g rocket experiences 16N of thrust force in the upwards direction. The force continues for 5 seconds. –A) Draw the force diagram for this situation. –B) What is the rockets acceleration? –C) What is the rockets velocity after the 5 seconds? –D) How fast is the rocket traveling 10 seconds after the thrust concludes?

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Force Diagram:

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Acceleration:

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Velocity after 5 seconds:

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Velocity 10 seconds after thrust:

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Second Law Part #2 http://www.brightstorm.com/science/physic s/newtons-laws-of-motion/law-of-force- and-acceleration-newtons-second-law-of- motion/http://www.brightstorm.com/science/physic s/newtons-laws-of-motion/law-of-force- and-acceleration-newtons-second-law-of- motion/

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Forces in two-dimensions: A 45kg crate is being pulled eastwards horizontally across the floor. The coefficient of friction is 0.2. You pull the crate with a force of 115 N at an angle of 15 degrees. –A) What is the acceleration? –B) How long does it take the crate to be moving at 1.4m/s?

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2-D Applied Forces: Step #1: Draw a free body diagram Step #2: List all of the forces / motions you know. Step #3: List all of the forces / motions you need to know. Step #4: Key assumptions. Step #5: Determine net forces in vertical direction. Step #6: Determine net forces in horizontal direction. Step #7: Solve for acceleration.

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Draw the “Force Diagram”:

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Write down the known variables:

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Begin to solve the problem: Find normal force: NOTE: We must account for upwards pull by rope. (F vertical will reduce F n ’s load) F n =

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F f = is opposing the horizontal acceleration Once we find F f we can then compare the F net to determine the horizontal motion. Find Force of Friction:

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V = 1.4m/s when…

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Problems to work on: P. 171 –Question #11 & 12 –Solutions in 20 minutes

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Solutions:

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Situations 1 of 4 1) I am driving on a level surface when I jam my brakes in my 2000kg car. My car is not stopped, but it is coming to a stop. The surface and tires have a coefficient of friction of 0.5.

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Situations 2 of 4 2) A 2kg brick is being pushed across a level surface at a constant velocity of 4 m/s [W].

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Situations 3 of 4 3) A 4kg object is thrown off a cliff on the moon at the angle of 45 degrees from the horizontal.

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Situations 4 of 4 4) A 50kg person is accelerating upwards on an elevator at 1.5m/s 2.

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P. 171 Mini-Lab:

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