2 Energy and Work are closely related Work is a transfer of EnergySI unit for work is Joules (J)Energy is the capacity to do WorkWe recognize it by the changes it causesWe use energy to carry a backpack upstairs.(work)
3 Kinds of Energy 1. Potential Energy(PE) the energy that an object has because of its position, shape or condition.
4 Potential Energy of Position PE depends on an objects mass, height, and the acceleration due to gravity.PE= mgh
5 Kinds of Potential Energy Potential Energy of Position (Gravitational PE)Figure 11, page 392: the apple has potential energy that results from the gravitational attraction between the apple and earth
6 Kinds of Potential Energy Elastic Potential EnergyThe PE of an object that is stretched or compressedEx- stretching a rubber band or bouncing a basketball
7 Calculations with PE= mgh Ex. calculations- page 393Units for PE: Joules (J)A joule is a kg-m2 / s2
8 Practice ProblemsCalculate the gravitational potential energy in the following systems:A car with a mass of 120 kg at the top of a 42 m hillA 65 kg climber on top of Mt Everest (8800 m high)A 0.52 kg bird flying at an altitude of 550 m
9 A car with a mass of 120 kg at the top of a 42 m hill PE=mgh=(1200kg)(9.8m/s2)(42m)=4.9x105J
10 a 65 kg climber on top of Mount Everest (8800m high) PE=(65kg)(9.8m/s2)(8800m)=5.6x106J
11 a 0.52kg bird flying at an altitude of 550m PE=(0.52kg)(9.8m/s2)(550m)=2.8x103J
12 Another ProblemA science student holds a 55g egg out a window. Just before the student releases the egg, the egg has 8.0 J of gravitational potential energy with respect to the ground. How far is the student’s arm from the ground (in meters)?
16 KE= ½ mv 2 Kinetic Energy (KE) The energy of a moving object due to the object’s motionThe KE of any moving object depends on its mass and velocitySee math skills pg 395**Note: if you double the mass, it doubles the KE. If you double the velocity, it will QUADRUPLE the KEKE= ½ mv 2
17 Practice ProblemCalculate the kinetic energy in joules of a 1500kg car moving at the following speeds:i. 29m/sii. 18m/siii. 42km/sA 35kg child has 190J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?
18 AnswersCalculate the kinetic energy in joules of a 1500kg car moving at the following speeds:i. 29m/s KE=1/2mv2=(1/2)(1500kg)(29m/s)2=6.3x105Jii. 18m/sKE=(1/2)(1500kg)(18m/s)2=2.4x105Jii. 42m/sKE=(1/2)(1500kg)(42m/s)2=
19 AnswerA 35kg child has 190J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?