Conservation of Energy

Conservation of Energy Watch video: http://www.youtube.com/watch?v=BVxEEn3w688

Work = Force|| x Distance • Distance is the magnitude of the displacement • Only the component of force parallel to the displacement “does” work http://onlinephys.com/work3.html http://www.physics4kids.com/files/motion_work.html

Power • measured in watts (W) • One watt of power is expended when one joule of work is done in one second • Walk a mile or run a mile • For the same person • how does power compare? • how does work compare? fitsugar.com

What is horsepower? • 1 horsepower = 746 Watts • A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute.

Potential (PE) VS Kinetic Energy (KE) • PE: stored chemical energy or energy of position • elastic, gravitational, and chemical • KE: energy of motion

Gravitational Potential Energy • PE = Weight ´ height • PE = m g h • Question: • How much potential energy does a 10kg mass have relative to the ground if it is 5 meter above the ground?

Kinetic Energy • The energy of motion. • Kinetic Energy = ½ mass ´ velocity2 Ek = ½ mv²

Mechanical Energy (ME) • Energy possessed by an object due to its motion or its stored energy of position. • ME can either be potential energy or kinetic energy. • All forms of energy are measured in joules (J). • ME= KE + PE http://bishopcook09.wikis.birmingham.k12.mi.us/Potential+and+Kinetic+energy%3B+Chinese+Dragons

Object Falling from Rest http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2

Conservation of Energy • Energy cannot be created nor destroyed; it may be transformed from one form into another, but the total amount of energy never changes. • Why worry about conserving energy? http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html http://water.me.vccs.edu/courses/env211/lesson2_print.htm

Energy Transformation of a Pendulum http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html

Ball Bounce • GPE: Gravitational Potential Energy • KE: Kinetic Energy • EPE: Elastic Potential Energy http://www.gcsescience.com/pen30-energy-ball-bounce.htm

m= 10kg In this case, the weight does positive work d = 2 m Work = mgd = (100N)(2m) Work = 200 Nm =200J m= 10kg mg = 100N

FA m= 10kg In this case, the weight does negative work mg = 100N d = 2 m Fa Work = -mgd = -(100N)(2m) Work = -200 Nm = -200J m= 10kg mg = 100N

Work/Energy Relationship • If you want to move something, you have to do work. • The work done is equal (ideally) to the change in kinetic energy. • what is ideal? is this actual? • W = DKE http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html

Kinetic and Potential Energy Conversions • Sketch the diagrams and fill in the blanks: http://onlinephys.com/work3.html

Work-Energy Relationship • W = ΔKE • What happens to your stopping distance when you… • Double your speed? • Triple your speed? http://www.thegrid.co.uk/index_files/Mickel-crash-brands.jpg

Forces in Car Crashes • http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html#cc3

Seatbelt Physics http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html#cc1

Bungee Jumping • You wish to bungee jump off of a platform. Assuming that there is no air resistance and the spring constant of the 40 m long bungee cord is 100 N/m. How high should your platform be? • Determine all the forces acting ON the body. • Draw a free body diagram. • Apply Newton's second law. • Think about elastic potential energy. • Solve. http://yugawad.wordpress.com/page/2/

Elasticity of an object • tendency of the object to return to its equilibrium (natural ) shape natural shape when Fnet =0 • Restoring Force • the force required to return object to its equilibrium shape is called the restoring force – directed opposite to the deformation of the object. Relative “bounciness” of various types of balls http://www.exploratorium.edu/baseball/bouncing_balls.html

A 2kg mass is placed on a frictionless track at point A and releases from rest. (assume the gravitational potential energy of the system is zero at point E. • Calculate: • the total mechanical energy of the mass. Use g = 10m/s/shint: focus on point A • the PE at point B • Using prior calculations, calculate the KE at point B • Using prior calculations, calculate the speed of the mass at point B. • If you want to calculate the speed at point C, do you have to repeat the computations again? • Calculate the speed at E • Calculate the speed at Fhttp://onlinephys.com/work3.html

In the above diagram, a toy car having a mass of 400 kg starts from rest at point A and travels 3.60 meters along a uniform track until coming to rest at point B. (cannot go higher) A) How can you determine that the track is not frictionless? (there is friction between the car and the road) B) Suppose the PE is 0 at ground level. Calculate the PE at point A and at point B: PEA = ________________ and PEB = ____________________ C) Determine the energy loss between these two points . (Find PEA - PEB) D) the energy lost has been transferred into thermal energy because of the work of the force of friction. E) If the car travels a distance of 3.60 meters along the track, calculate the magnitude of the frictional force acting on the car. Hint: energy lost = work done by friction http://onlinephys.com/work3.html

Conservation of Energy