the ability to do work SI units are Joules (J) Energy the ability to do work SI units are Joules (J)
2 Categories Kinetic Energy (KE) Potential Energy (PE) Mechanical (ME) Kinetic Energy (KE) Potential Energy (PE) energy of motion - stored energy due to KE = ½ m v2 position gravitational elastic stored energy due to height - stored energy due PE = mgh to elongation PE = ½ k x2
2 Categories Non-Mechanical Energy -energy of particles too small to be seen -Nuclear -Electrical -Chemical -Thermal -Light
Identify the type(s) of ME in each situation A dog running while on the ground. KE An airplane flying while high above the ground. KE, PE A book held 4m above a desktop. PE A loaded dart gun with a compressed spring mechanism.
Total Mechanical Energy Add the kinetic and potential energy
Example: A 2kg ball is thrown with a speed of 4 m/s at a height of 5 m above the ground. How much mechanical energy does the ball have? m = 2kg v = 4 m/s ME = ? h = 5 m (Since the ball has a speed, it has KE) (Since the ball has height, it has PE) ME = KE + PE = ½ m v2 + mgh = ½ (2) (4)2 + (2) (9.81) (5) = 16 + 98.1 = 114.1 J
Energy Transformations
Energy cannot be created or destroyed. Energy can only change form. Energy Conservation Energy cannot be created or destroyed. Energy can only change form.
A skier: Total Energy Remains Constant
Potential and Kinetic Transformations -A ball is dropped. It starts with all potential energy. -As the ball falls it loses potential energy because its height is decreasing. -That potential energy is turned into kinetic energy because as it falls its speed increases. -At the bottom all of its energy is kinetic. -If it started with 5000J of PE at the top then how much KE would it have at the bottom?
Ball Tossed Upward KE PE KE This ball is tossed upward, reaches maximum height and falls back down into his hand. We’ll say that in his hand the ball has no height. What type of energy does the ball have when it leaves his hand? What type of energy does the ball have at maximum height? What type of energy does the ball have as it returns to his hand? KE PE KE
Roller Coaster: Fill in the Blanks
Pendulum: Fill in the Blanks
KEinitial + PEinitial + Work = KEfinal + PEfinal Work – Energy Theorem When energy is conserved: MEinitial = MEfinal KEinitial + PEinitial = KEfinal + PEfinal Sometimes work is done on an object which can increase or decrease its mechanical energy. + work increases energy - work decreases energy KEinitial + PEinitial + Work = KEfinal + PEfinal
Work – Energy Theorem The work must be done by an external force to cause a change in energy. Gravity and the spring force are not external forces. Friction is an external force and does negative work. It converts mechanical energy into heat. http://phet.colorado.edu/en/simulation/energy-skate-park
An 850 kg roller-coaster is moving at 4m/s at Point A An 850 kg roller-coaster is moving at 4m/s at Point A. There is no friction between Points A and C. a) How fast is the coaster moving at Point B? Point C? b) What force will stop the coaster at Point D if the brakes are applied at point C?