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## What do you think of when you hear these words: Work… Energy… Power…

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**What do you think of when you hear these words:**Work… Energy… Power…**Energy**• Universe is made up of matter and energy. • Energy is the mover of matter. • Energy has several forms: • Kinetic • Potential • Electrical • Heat • etc. http://tiki.oneworld.net/energy/energy.html**Energy**• The Universe is made up of matter and energy. • Energy is the “mover” of matter. • There are many forms of energy. • Conversions from one form of energy to another continually occur. • Energy cannot be created or destroyed.**Work = Force|| x Distance**In this case, the distance is the magnitude of the displacement. Only the component of force parallel to the displacement does work**Work**• Force applied in the direction of motion • Work = Force||´ Distance • W = F||d • The unit for work is the Newton-meter which is also called a Joule. • 1 joule is equal to a force of 1 N exerted over a distance of 1 m**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**Power**• measured in watts (W) • One watt of power is expended when one joule of work is done in one second. http://www.cartoonstock.com/newscartoons/cartoonists/pto/lowres/pton114l.jpg**Power**• Power is equal to the amount of work done per unit time. • The unit for power is the Joule/second which is also called a Watt.**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.**Mechanical Energy**• Energy possessed by an object due to its motion or its stored energy of position. • It can either be potential energy or kinetic energy. • All forms of energy are measured in joules (J). • Em = Ek + Ep**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. • So why worry about conserving energy? http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html**Conservation of Mechanical Energy**• Ek (before) + Ep (before) = Ek (after) + Ep (after) • When no other type of energy is present. • Why does a dropped ball not return to its original elevation? http://www.science-projects.com/Drop/DropBalls.htm**Energy Transformation of a Pendulum**http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html**Potential Energy**• (PE) stored chemical energy or energy of position. • An object’s ability (potential) to do work by virtue of its position. • Types: elastic, gravitational and chemical. • Examples: • Rubber bands • Springs • Bows • Batteries • Gravity?**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?**Potential Energy**• The same work is done on each block. What mattersis the final elevation, not thepath followed http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html**Kinetic Energy**• The energy of motion. • Kinetic Energy = ½ mass ´ velocity2 Ek = ½ mv²**How much work does gravity do on the falling mass?**PE = mgh mg h h Work = mgh mg**How much energy does the mass have at the bottom of its**fall, just before it hits the ground? PE = mgh mg h h Kinetic energy mg v**PE = mgh**If potential energy at top of path is 100 J, how much kinetic energy does the ball have just before it hits the ground and comes to a stop? mg h KE = ½ mv2 mg v**Check for Understanding**Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s. If the roller coaster car in the last problem were moving with twice the speed, then what would be its new kinetic energy?**More Energy Transfer**• High speed winds are used to do work on the blades of a turbine at the so-called wind farm. • Mech. Energy from the air gives the air particles the ability to apply a force to the blades. • As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.**Energy Transfer...**• This diagram shows that the potential energy of the boy is changing as he swings. Explain what is happening, in other words how is the energy changing?**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**Work-Energy Relationship**• 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**Joule to Joule**http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/cs.html**http://www.cartoonstock.com/newscartoons/cartoonists/jko/lowres/jkon533l.jpg**http://www.cartoonstock.com/newscartoons/cartoonists/jko/lowres/jkon533l.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**The formula for KE is _____?**• Kinetic Energy is dependent upon ___ & ____ • If an object’s velocity doubles, its kinetic energy _____. If velocity triples, it will have _____ times the kinetic energy. • If mass doubles, kinetic energy _____. If mass triples, kinetic energy _____. • …so what happens to KE if mass doubles and velocity doubles? What if mass is cut in half and velocity doubles?**Think about standing on your desk…if you jumped off, how**fast would you be moving just prior to reaching the floor? • How much work would you do on the floor? http://school.discoveryeducation.com/clipart/images/in-desk.gif**Object Falling from Rest**http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2**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.**Momentum**• Moving inertia • Symbol- p • Equation p = mv • Units: kg • m/s • VECTOR inertia In motion**A Thought Experiment:**• Suppose that you were captured by an evil physicist who gave you the following choice: • You must either: • Stand in front of a 1000 kg. truck moving at 1 m/s, or • Stand in front of a 1 kg. frozen meatball moving at 1000 m/s. • …think…**Truck:**• Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s • Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules • Meatball: • Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s • Meatball's kinetic energy = 0.5 mv2 = (0.5)(1 kg)(1000 m/s)2 = 500 000 Joules**Diagramming:**• A 120 kg lineman moving west at 2 m/s tackles an 80 kg football fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw a vector diagram in which the before- and after-collision momenta of each player is represented by a momentum vector. Label the magnitude of each momentum vector.**Impulse**• Force applied over a period of time creates impulse • Impulse equals the change in momentum**Total pbefore collision = Total pafter collision**pT before = pT after (p1 + p2) before = (p1 + p2) after (m1v1 + m2v2 )before = (m1v1 + m2v2)after