Kinetic and Potential Energy. Kinds of Energy. Potential Energy

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April Bennett
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1 Energy and Work are closely related Kinetic and Potential Energy Energyis the capacity to do Work We recognize it by the changesit causes Workis a transfer of Energy SI unit for work is Joules (J) We use energy to carry a backpack upstairs. (work) Kinds of Energy 1. Potential Energy(PE) the energy that is stored as a result of its position, shape, or condition. Potential Energy The energy of position The amount of energy contained in an object at rest Gravitational Potential Energy Potential Energy of Position (Gravitational PE) The apple has potential energy that results from the gravitational attraction between the apple and earth Potential Energy of Position PE depends on an objects: mass(in kilograms) height(in meters) acceleration due to gravity (9.8 m/s 2 ) PE= mgh 1

More height= more potential More mass = more potential If you know an object s height and mass, you can calculate it s potential Calculations with PE= mgh m = mass in kilograms g = acceleration due

2 More height= more potential More mass = more potential If you know an object s height and mass, you can calculate it s potential Calculations with PE= mgh m = mass in kilograms g = acceleration due to gravity 9.8 m/s 2 h = height in meters PGE=weight x height Weight = Mass* gravity Units for weight: Newton(N) A Newton is a kg-m/s 2 Units for Energy: Joules(J) A joule is a kg-m 2 / s 2 Practice Problems Calculate the gravitational potential energy in the following systems: A car with a mass of 120 kg at the top of a 42 m hill A 65 kg climber on top of Mt Everest (8800 m high) A 0.52 kg bird flying at an altitude of 550 m 2

3 Which Equation to use? Look at your UNITS! If the mass is given, the unit is kg. Use mgh If the weight is given the unit is N. Use weight x height A car with a mass of 120 kg at the top of a 42 m hill PE=mgh=(1200kg)(9.8m/s 2 )(42m)= J =4.9x10 5 J a 65 kg climber on top of Mount Everest (8800m high) a 0.52kg bird flying at an altitude of 550m PE=(65kg)(9.8m/s 2 )(8800m)= J = 5.6x10 6 J PE=(0.52kg)(9.8m/s 2 )(550m)= 2800 J = 2.8x10 3 J Kinetic Energy = the energy an object possesses because of its motion VELOCITY Velocity = distance time Distance = meters Time = seconds The amount of Kinetic energy is dependent on the mass of the object in motion and it s velocity. 3

4 If you know an object s mass and velocity, you can calculate it s kinetic Kinetic Energy (KE) The energy of a moving object due to the object s motion The KE of any moving object depends on its mass and velocity KE= ½ mv 2 **Note: if you doublethe mass, it doublesthe KE. If you doublethe velocity, it will QUADRUPLEthe KE Practice Problems Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds: i. 29m/s ii. 18m/s iii. 42km/s Steps for ½ mv 2 What is the kinetic energy of a 1500kg car moving at a speed of 29 meters a second? Find your mass Divide it by / 2 = 750 Find your velocity Square it 29*29= 841 Multiply your two answers 750*841 = J Answers Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds: i. 29m/s KE=1/2mv 2 =(1/2)(1500kg)(29m/s) 2 = 6.3x10 5 J = J ii. 18m/s KE=(1/2)(1500kg)(18m/s) 2 = 2.4x10 5 J = J iii. 42m/s KE=(1/2)(1500kg)(42m/s) 2 = 1.323x10 6 J = J Potential or Kinetic Energy? Moving car Tree branch Bent car fender Balloon filled with air Balloon squirting around room Person inside a moving car 4

Conversion of Potential to Kinetic Energy What type of energy does the space shuttle have at lift off? In this picture both kinds of energy are evident. Can you point them out?

5 Conversion of Potential to Kinetic Energy What type of energy does the space shuttle have at lift off? In this picture both kinds of energy are evident. Can you point them out? The water at the top has potential energy When water falls to a lower level, the potential energy is converted to kinetic Example Day 1 Complete Station 1 of the Kinetic and Potential Energy Lab Day 2 Pick THREE stations to complete on the Kinetic and Potential Energy Lab At least ONE station must be from # 2-5 At least ONE station must be from # 6-8 5

6 Potential Energy Stations #2: The 200 g pendulum Kinetic, Potential, Kinetic, WOW! #3: The 500 g pendulum Kinetic and Potential Energy Transformed #4: Sketching the pendulum (at front of classroom) Kinetic Energy Stations #6: What Variables Effect Kinetic Energy #7: What is the Effect of Mass on Kinetic Energy? #8: Virtual Roller Coaster #5: Mass and Potential Energy Isolating other variables Another Problem A 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? v = 2 KE / m v = 2(190 J ) / 35 m / s v = 3.3 m / s Another Problem A 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)? Answer h= PE/mg = 8.0J/(0.055kg)(9.8m/s 2 )=15m 6

7 Another Problem A diver has 3400J of gravitational potential energy after stepping up onto a diving platform that is 6.0m above the water. What is the diver s mass in kilograms? Answer m=pe/gh=(3400j)/(9.8m/s 2 )(6.0m)=58kg 7

Potential / Kinetic Energy Remedial Exercise

Potential / Kinetic Energy Remedial Exercise This Conceptual Physics exercise will help you in understanding the Law of Conservation of Energy, and its application to mechanical collisions. Exercise Roles: