Mechanical Energy Eureka : http://www.youtube.com/watch? v=7K4V0NvUxRg Gravitational Potential Energy The energy possessed by an object because of its position relative to a lower position
The potential of an object to be pulled down by gravity A GPE Machine Eg = mgh Eg gravitational potential energy (J) m mass (kg)
g gravitational field strength (N/kg, m/ s2) h height above reference level (m) Compare with Work for lifting something We always have to indicate a reference level Ex. The gravitational potential energy is 30J relative to the
Earths surface Example 1 A 58 kg skydiver stands in the door of a plane preparing to jump. If the earth is 2.8 km below the skydiver, what is the skydivers potential energy relative to the earth?
Example 2 A 10.0kg rock is on top of a house 5.00m high on the edge of a cliff 20.0m high. What is the gravitational potential energy of the rock relative to a) the roof of the house? b) the floor of the house? c) the bottom of the cliff?
Example 2 Contd a) The h=o is at the top of the house. E g mgh (10.0kg)(9.81N / kg)(0m) 0J relative to the top of the house.
b) The h=0 is the floor of the house. E g mgh (10.0kg)(9.81N / kh)(5.00m) 490J relative to the floor of the house. c) The h=o is at the bottom of the cliff.
E g mgh (10.0kg)(9.81N / kg)(25.0m) 2450J 2.45 10 3 J relative to the bottom of the cliff. Kinetic Energy the energy possessed by
an object due to its motion Calculating Kinetic Energy: Ek = mv2 Ek Kinetic Energy (J) m mass (kg) v Speed (m/s)
Example 1 What is the kinetic energy of a 60 g tennis ball that has a speed of 3.6 m/s? Example 2 What is the kinetic energy of a 6.0kg curling stone sliding at 4.0m/s?
Given: m=6.0kg, v=4.0m/s 1 2 E k mv 2 1 (6.0kg)(4.0m /s) 2 2 48J
Example 3 What is the speed of a 5.44kg shotput if its kinetic energy is 68J? Given: m=5.44kg, Ek=68J 1 E k mv 2 2 2E v2 k
m 2(68J) 5.44kg m2 25 2 s v 5.0m /s
Potential and Kinetic Energy Snowboarding http://www.youtube.com/watch?v=58YvhpxXaN0 Mechanical Energy The sum of the gravitational potential energy and the kinetic energy Think of TOTAL energy of an
object due to its motion/position Mechanical energy will always stay the same unless WORK is done Ex. When a parachutist jumps from a plane, she initially has a large GPE and no kinetic energy. As she falls, she speeds up as her GPE converts to kinetic energy
Her Mechanical Energy will always be the same Calculating Mechanical Energy Em = Eg + Ek Ek Kinetic Energy (J) Eg Gravitational Potential Energy (J) Em Mechanical Energy (J)
Example 1 A boy has a stationary 5.1kg ball in his hand 3.2m off the ground. a) Calculate the Ek, Ep and Em of the ball He then drops the ball, b) Calculate the speed when it drops 1.2 m using the five constant acceleration equations.
c) Using the value from b) calculate Ek, Ep and E of the ball at this point Applications of Mechanical Energy South African vultures eats bones if they are too big the bird will drop them from a greater height so they
break Applications of Mechanical Energy A roller coaster on a track the height of the first hill has to be high enough giving a high potential energy this will be converted into kinetic energy which will
make it go really fast Top Thrill Dragster Cedar Point, Ohio 128 m tall, 193 km/h on the up and down Energy Skate Park: Basics
Complete the PHeT Intro to Energy Lab (instructions on class site) http:// phet.colorado.edu/en/simulation/energy-ska te-park-basics Practice Problems Practice Problems posted on class site.
Extra Help Everything you need to know from this lesson: http://www.youtube.com/watch?v=iYEWIuQBVyg Other Eureka Videos: http://youtu.be/BGmUVoX5s58 http://youtu.be/Tji6PDBck_8
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