2Energy 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)
3Kinds of Energy 1. Potential Energy(PE) the energy that an object has because of its position, shape or condition.
4Potential Energy of Position PE depends on an objects mass, height, and the acceleration due to gravity.PE= mgh
5Kinds 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
6Kinds of Potential Energy Elastic Potential EnergyThe PE of an object that is stretched or compressedEx- stretching a rubber band or bouncing a basketball
7Calculations with PE= mgh Ex. calculations- page 393Units for PE: Joules (J)A joule is a kg-m2 / s2
8Practice 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
9A car with a mass of 120 kg at the top of a 42 m hill PE=mgh=(1200kg)(9.8m/s2)(42m)=4.9x105J
10a 65 kg climber on top of Mount Everest (8800m high) PE=(65kg)(9.8m/s2)(8800m)=5.6x106J
11a 0.52kg bird flying at an altitude of 550m PE=(0.52kg)(9.8m/s2)(550m)=2.8x103J
12Another 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)?
16KE= 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
17Practice 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?
18AnswersCalculate 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=
19AnswerA 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?