Energy Mechanical

Facts There are two types of mechanical energy Potential energy and kinetic energy. Potential energy is the energy a body has due to its position PE = mgh Kinetic energy is the amount of energy a particle possesses because of its motion KE = ½mv2 Both are measured in joules J

How to answer the questions Draw a clear forces diagram Label zero potential energy Think what is happening to the energies

And now for the examples A mass of 100kg accelerates from 2ms-1 to 4ms-1. Find it’s gain in kinetic energy Draw the diagram a 4ms-1 2ms-1 100kg before 100kg after KEafter = ½mv² = ½ x 100 x 4² = 800 J KEbefore = ½mv² = ½ x 100 x 2² = 200 J KEgain = KE after – KE before = 800 – 200 KEgain = 600J

Summarise the information to see what you’ve got An apple of mass 0.1 kg falls from a height of 2m. Taking g = 10ms-2 , find the loss of potential energy Draw a diagram 0.1g 2m Zero PE level Summarise the information to see what you’ve got PE = ? m = 0.1 kg g = 10 ms-2 h = 2m PE = mgh PE = 0.1 x 10 x 2 PE = 2 J

Summarise the information to see what you’ve got A roller coaster train, of mass 8 tonnes, is winched up a slope inclined at 20º to the horizontal. The slope is 80 m long. Determine the increase in potential energy of the roller coaster train from the bottom of the slope. 80 m h 8000g 20º Zero PE level 8000g Summarise the information to see what you’ve got PE = ? m = 8000kg g = 9.8 ms-2 h = ? We need to find h before we can find PE Using trigonometry we have h = 80 x sin 20º h = 27.36 m

h PE = ? m = 8000 kg g = 9.8 ms-1 h = 27.36 m PE = mgh 20º Zero PE level 8000g PE = ? m = 8000 kg g = 9.8 ms-1 h = 27.36 m PE = mgh PE = 8000 x 9.8 x 27.36 PE = 2 145 150.3 PE = 2 150 000J (3 s.f.) PE = 2150 kJ (3 s.f.)

Your turn Blue book pg 254, 255 Mechanics 2 pg 78 question 1, 3, 6, 7, 8, 9, 10