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 (E g ) due to the force of gravity pulling down on it once it is released, the energy is gradually transformed into kinetic energy (E k ) as its speed increases on contact, this energy is transferred to the building to do work

the amount of E g 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 (F g = mg) therefore: E g = mgh whereE g = gravit. pot. energy (J) m = mass (kg) g = gravit. constant (9.8 N/kg) h = height (m)

A 0.45 kg book is resting on a desktop 0.64 m high. Calculate the books gravitational potential energy relative to (a) the desktop and (b) the floor. (a) 0.0 J (b) 2.8 J

when an object is moving, it has kinetic energy (E k ) this depends upon two factors: the objects mass and its speed it is given by the following formula: E k = ½ mv 2 whereE k = kinetic energy (J) m = mass (kg) v = speed (m/s)

Calculate the kinetic energy in each of the following: (a) During a shot put, a 7.2 kg shot leaves an athletes hand at a speed of 12 m/s (b) A 140 kg ostrich is running at 14 m/s (a) 518 J (b) J

the sum of gravitational potential energy and kinetic energy is called mechanical energy since energy transfers between the two we get the following formula: E mech = E g + E k at its maximum height, the wrecking ball has all E g and as it hits the building it has all E k along the way, as E g decreases, E k increases but the mechanical energy of the system remains constant

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

#4, 5 page 142 #9, 10 page 143 #2 - 6 page 146