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Published byBasil Jordan Logan Modified over 8 years ago
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Work Energy Kinetic Energy Potential Energy Mechanical Energy Conservation of Mechanical Energy
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The capacity to do work The energy transferred to an object equals the work done on the object E T = W equals the work done on the object E T = W
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Energy is present in many forms Heat (thermal energy) Kinetic Energy = “Motion Energy” Potential Energy = “Stored Energy” Mechanical energy – Sum of Potential and Kinetic Nuclear Sound Electromagnetic energy Chemical energy
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The parallel component does work The (F cos ) component does work
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NO work done if there: 1.Is no movement 2.The force if perpendicular to the displacement
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In which photo(s) is WORK being done? In which photo(s) is WORK being done? no yes
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If a 10 Newton force displaces a 20 kg block 40 meters calculate the work done on the block by the force. The Normal force (F N ) and weight (mg) do no work in this problem, WHY? F = 10N
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Work = Force Distance The component of force parallel to the displacement DOES WORK The perpendicular component DOES NO WORK - Between the force and displacement Scalar Measured in Joules (J)
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Positive Work - Force and displacement in same direction. Negative Work - Force opposite the displacement. Zero Work - Force is perpendicular to the displacement. FdFd FdFd F d
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If the Force is in the Direction of Motion: In this case, = 0 o, so: If the Force is in the Opposite Direction of Motion: In this case, = 180 o, so:
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If the Force is Perpendicular to the Direction of Motion: In this case, = 90 o, so: If the Object Being Pushed Doesn't Move: In this case, x = 0, so:
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A 10 N force acts 30 0 above the horizontal and displaces an object 5 meters horizontally how much work is done? F= 10 N 30 0 W = F d cos = (10N)(5m) cos 30 0 = 43.3 J
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How much work does gravity do on a 70 kg person who falls 100 meters in the free fall ride?
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Kinetic energy (KE) is the energy of a moving object. Energy associated with motion and mass.
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A 500 kg car is traveling at 10 m/s, 10 sec. later it is traveling 30 m/s. Calculate the following: 1. The initial kinetic energy 2. The final kinetic energy 3. The change in KE
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Work Energy Theorem: The change in kinetic energy equals the work done.
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The animation shows a block of mass and initial speed v sliding across a floor that is not frictionless. A kinetic friction force f k stops block during displacement d. Thus we can relate work done by friction to the change E in the system's energy
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For hammer: Moving hammer can do work on nail! For nail:
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Matt’s little red wagon with a mass of 4.6 kg moves in a straight line on a frictionless horizontal surface. It has an initial speed of 10 m/s and is pulled by Matt 4.0 m with a force of 18N in the direction of the initial velocity. Use the work-energy relation (W Net = KE) to calculate the wagons: a.Change in Kinetic Energy b.Final speed.
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In a test of old sports car, it’s found that engines provided around 1,000 N of force. If the typical mass is 400 kg and they accelerate from rest, how fast will they be going 100 m down the road?
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How much work is done holding a box in place on an incline? How much work is done pushing a 15 kg box up a 30° incline at a constant speed for 3 m
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Force vs. Distance Area under curve equals the work done
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Force vs. Distance Area under curve equals the work done
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Area above the curve – work is positive Area below the curve – work is negative
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Work done by a variable force equals Area under the curve
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Gravitational Potential Energy (PE) : Gravitational Potential Energy (PE) : The energy an object has due to its height above a reference point. The potential energy change is independent of the path between the initial and final points. link
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PE=mgh Can gains Potential Energy equal to mgh
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One serving of Bagel Crisps contains 543 kJ. How many pull- ups are needed to burn it off? M = 60 kg∆h =.5 m ∆PE = mg ∆H 543000J = (60kg)(9.81N/kg)(.5m)(n) 1845 = n But the human body is only abour 20% efficient so, n is only 369!
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Who does more work in lifting the respective equal masses to the top of the incline at a constant speed?
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1. His initial potential energy with respect to the ground 2. His potential energy 1 seconds after being released 3. His change in potential energy 4. Where did its energy go when it hits the ground? Rufus the 5 kg cat falls 10 meters from above the surface of the earth. Calculate:
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Conservative Force : The work done is independent of the path taken. Only depends on the initial and final position. Ex: Gravity Non Conservative Force : The work done depends on the path taken. Ex: Friction
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Don’t disregard the Conservation Laws
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