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

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)

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

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

whereEk = 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 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

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

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

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