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## gravitational potential energy & kinetic energy

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**consider a wrecking ball that is raised and then dropped to**destroy an old building as it is raised up it gains gravitational potential energy (Eg) due to the force of gravity pulling down on it once it is released, the energy is gradually transformed into kinetic energy (Ek) as its speed increases on contact, this energy is transferred to the building to do work real world**the amount of Eg depends upon how high the object is above**the surface this is known as our reference level (Δh) it also depends on the weight of the object (Fg = mg) therefore:Eg = mgh where Eg = gravit. pot. energy (J) m = mass (kg) g = gravit. constant (9.8 N/kg) h = height (m) the equation**A 0.45 kg book is resting on a desktop 0.64 m high.**Calculate the book’s gravitational potential energy relative to (a) the desktop and (b) the floor. (a) 0.0 J (b) 2.8 J practice**when an object is moving, it has kinetic energy (Ek)**this depends upon two factors: the object’s mass and its speed it is given by the following formula: Ek = ½ mv2 where Ek = kinetic energy (J) m = mass (kg) v = speed (m/s) kinetic energy**Calculate the kinetic energy in each of the following:**(a) During a shot put, a 7.2 kg shot leaves an athlete’s hand at a speed of 12 m/s (b) A 140 kg ostrich is running at 14 m/s (a) 518 J (b) 13 720 J practice**the sum of gravitational potential energy and kinetic energy**is called mechanical energy since energy transfers between the two we get the following formula: Emech = Eg + Ek at its maximum height, the wrecking ball has all Eg and as it hits the building it has all Ek along the way, as Eg decreases, Ek increases but the mechanical energy of the system remains constant mechanical energy**a roller coaster is raised up and then released to travel**around the track pile driver hammer is raised up and then dropped to do work on the pile damming a river to produce hydroelectric energy – as the water falls it turns turbines other examples**#4, 5 page 142**#9, 10 page 143 #2 - 6 page 146 classwork