Mechanical Energy Eureka :

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

E g = mgh Eg – gravitational potential energy (J) m – mass (kg) g – gravitational field strength (N/kg, m/s 2 ) 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 Earth’s 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 skydiver’s 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 Cont’d

Kinetic Energy the energy possessed by an object due to its motion

Calculating Kinetic Energy: E k = ½ mv 2 E k – 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

Example 3 What is the speed of a 5.44kg shotput if its kinetic energy is 68J? Given: m=5.44kg, Ek=68J

Potential and Kinetic Energy Snowboarding

Mechanical Energy The sum of the gravitational potential energy and the kinetic energy Think of TOTAL energy of an object due to it’s 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 E m = E g + E k E k – Kinetic Energy (J) E g – Gravitational Potential Energy (J) E m – Mechanical Energy (J)

Example 1 A boy has a stationary 5.1kg ball in his hand 3.2m off the ground. a)Calculate the E k, E p and E m 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 E k, E p and E m 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) skate-park-basics

Practice Problems Practice Problems posted on class site.

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