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Physics 3.3

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Work WWWWork is defined as Force in the direction of motion x the distance moved. WWWWork is also defined as the change in total energy. WWWWork is given in N·m, or Joules. WWWWork is a scalar quantity. Why? LLLLift a weight, put it back where it came from, displacement is zero, but work was done.

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??? A constant force of 1900 newtons is required to keep and automobile having a mass of 1.0 E3 Kg moving at a constant speed of 20. meters per second. The work done in moving the automobile a distance of 2.0 E3 meters is: W = Fd = ∆E T

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Power Power is the rate at which work is done Power is a scalar quantity. Power equal work divided by time. P = W / t Units of power are the watt (j / s).

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??? A 3.0 kilogram block is initially at rest on a frictionless, horizontal surface. The block is moved 8.0 meters in 2.0 seconds by the application of a 12 – newton horizontal force. What is the average power developed while moving the block? P = W / t = Fd / t = Fv

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??? A motor having a power rating of 500. watts is used to lift an object weighing 100. newtons. How much time does the motor take to lift the object a vertical distance of 10.0 meters? P = Fd / t

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Potential and Gravitational Potential Energy: The energy an object has because of its height. PE = mass x acceleration due to gravity x height ∆PE = mg∆h. Units of gravitational potential energy is joules or kg∙m² / s².

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??? An object weighing 15 newtons is lifted from the ground to a height of 0.22 meter. The increase in the object’s gravitational potential energy is approximately: ∆PE = mg∆h.

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Hooke’s Law Force The force (F s ) placed on a spring is equal to the spring constant k times the change in distance from the rest position ∆x. F s = k ∆x

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??? A 10-newton force is required to hold a stretched spring 0.20 meters from its rest position. What is the spring constant? F s = k ∆x

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Potential Energy of a Spring A compressed or stretched out spring stores potential energy and can be calculated 2 ways: 1.The area of a F s vs ∆x graph. 2.PE s = ½kx²

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??? A 10.-newton force is required to hold a stretched spring 0.20 meter from its rest position. What is the potential energy stored in the stretched spring? PE s = ½kx²

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Kinetic Energy The energy associated with motion. The unit of kinetic energy is joules or kg∙m² / s². kg∙m² / s². KE = ½mv².

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??? The kinetic energy of a 980-kilogram race car traveling at 90. meters per second is approximately: KE = ½mv².

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??? An Object moving at a constant speed of 25 meters per second possesses 450 joules of kinetic energy. What is the object’s mass? KE = ½mv².

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??? If the speed of a car is doubled, the kinetic energy of the car is: KE = ½mv².

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Work and Energy Work changes into potential of kinetic energy. When there is friction, work also changes into internal energy.

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??? An average force of 20. newtons is used to pull back the string of a bow and arrow 0.60- meters. As the arrow leaves the bow, its kinetic energy is: W = Fd = ∆E T

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Conservation of Energy: Energy cannot be created or destroyed. Ideal mechanical system –Total energy equals potential and kinetic energies. There is no friction or air resistance. Non-ideal mechanical system –Total energy equals potential, kinetic, and internal energies. There is friction and/or air resistance.

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A Pendulum and a Mass on a Spring Both can be used to show the relationship between KE and PE. During an oscillation, the rest position represents 0 PE and max KE. The two points where there is a change in direction, and furthest from the rest position is where KE is zero, and PE is maximum.

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