8.3 Moving AboutFocus 3: Energy and Work

Outcomes26. Define the law of conservation of energy

27. Identify that a moving object possesses kinetic energy and that work done on that object can increase that energy

28. Solve problems and analyse information to determine the kinetic energy of a vehicle and the work done using the formulae Ek = ½mv2 and W = Fs

29. Describe the energy transformations that occur in collisions

30. Analyse information to trace the energy transfers and transformation in collisions leading to irreversible distortions

Two Useful Formulae

•Ek = ½mv2

•W = Fs

▫Ek = kinetic energy in Joules▫m = mass in kilograms▫v = velocity in ms-1▫W = work in Joules▫F = force in Newtons▫s = displacement in m

Work can be done on an object in order to increase its kinetic energy . When this happens, F s = 1/2mv2

Note – “s” for displacement rather than “r”

Law of Conservation of Energy

•Energy cannot be created or destroyed, only changed in form.

•Energy can be …▫transferred from one object to another.

e.g.: a hot cup of coffee warming your hands.

▫Transformed from one form to another e.g.: chemical potential energy in a battery

changing into light energy in a light globe.

Types of Collision•Elastic collision: all the kinetic energy from

the first vehicle is either still “in” the first vehicle or has been transferred to the second vehicle.

•Inelastic collision: some of the kinetic energy from the first vehicle is transformed into other forms of energy.▫Examples of the other forms of energy include

heat, sound, light and elastic potential energy in the crumpled part of the car.

Energy transfers and transformations in collisions leading to irreversible distortions

kinetic energy

sound energy

heat energy

elastic potential energy

“crash, bang!!”

in the “crumple zone”

sparks, friction

If two cars collide, some of the kinetic energy of the first car will be transferred to the second car