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Work and Energy

Work and Energy. No vectors!. Work. Work has a specific meaning in physics Work is a force applied over a distance. Understand that work is done ON something and that something then moves Equation for work is Work = Force X Distance W = FD. Example – A powerlifter.

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Work and Energy

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  1. Work and Energy No vectors!

  2. Work • Work has a specific meaning in physics • Work is a force applied over a distance. • Understand that work is done ON something and that something then moves • Equation for work is Work = Force X Distance • W = FD

  3. Example – A powerlifter • A force is exerted on a bar and that bar and • That bar is MOVED by the force • The bar then moves THROUGH A DISTANCE • How much work is he doing standing there holding the bar still? http://powerliftinghub.com/images/andy.jpg

  4. Work concepts • Work doesn’t care about direction (it’s not a vector) • Let’s say you apply 5 N to an object to move it 2 meters. What is the work done? • Now say you double the distance. How does the work done change? 5N 2m

  5. Work concepts questions • Now say you double the distance you apply a force. How does the work done change? • Let’s say that instead, you keep the same distance, but apply twice as much force. How does the work done change? • Let’s say you now go crazy psycho and apply twice the force over twice the distance. How does the work done change?

  6. For those who were paying attention during the momentum chapter… • What is the difference between work and impulse? • Work is force applied over a distance • Impulse is a force applied during a period of time

  7. Work’s Units • The units for work are called Joules (rhymes with ‘cools’). • If work = force X distance, and force is in Newtons and distance is in meters, then… • 1 Joule = 1 Newton•meter

  8. Power • Power is work done in a certain time interval • The units of power are • Watts (you probably have heard of this from the ratings on light bulbs) or • Kilowatts (as you may know from the electric meter on your house) • Horsepower (as you may know from your car engine)

  9. Relating the esoteric to the everyday… • Lifting a quarter pounder with cheese 1 meter in 1 second is roughly one Watt. • Lifting a 225 pound man 1 meter in one second is roughly one kilowatt.

  10. And now, it’s time to run up and down the stairs • Work done against gravity = weight X height change • Remember, weight is a force and height is a distance, so work done = mgh, where m=mass, g = 9.8m/s2 and h = height • Your power output is your work done against gravity divided by your time. • Let’s go!

  11. Mechanical Energy • Mechanical energy is the total amount of energy in a system. • Mechanical energy is the sum of 2 kinds of energy: • Potential energy • Kinetic energy • In the absence of friction and air resistance, mechanical energy is conserved

  12. Potential Energy • An object may store energy simply because of its position. • This stored energy is called potential energy, because it has the potential to do work. • Examples: a compressed spring, a stretched rubber band, etc. • If an object is raised to a height h above the floor, then it has gravitational potential energy.

  13. Gravitational Potential Energy • Gravitational PE = weight X height • Weight = mg, height = h • So PE = mgh • m = mass in kg, h = height in meters, g=9.8m/s2 • For gravitational PE, all you care about is the height above the ground or floor. It doesn’t matter how it got there or what path it takes when it falls.

  14. PE Questions • How much work is done on a 100N boulder that you carry horizontally across a 10m room? What is the boulder’s change in PE? • How much work is done on a 100N boulder that you lift vertically 1m? • What power is expended if you lift that boulder 1m in 1 sec? • What is the boulder’s gravitational PE in the lifted position?

  15. Kinetic Energy • Kinetic Energy is the energy of something in motion • KE = ½ mv2 • Work and energy are related (and have the same units) • Work done on an object = change in KE • So: Fd = ½ mv2 • Note that velocity is squared. How does the KE of an object change if you double the speed?

  16. Stopping distances • Friction is the force that causes a car to slow to a stop when you slam on the brakes. • Friction applied over a distance = work, and that work will decrease the KE of the car • Typical stopping distances: • Speed = 30 km/hr, skid = 10m • Speed = 60 km/hr, skid = 40m • Speed = 120 km/hr, skid = 160m

  17. Conservation of Energy • The Law of Conservation of Energy says: • “energy cannot be created or destroyed. It can be transformed from one form to another, but the total amount of energy never changes.” • That means that the total energy of a system stays the same • E = PE + KE, and this remains constant

  18. Conservation of Energy http://www.physicsclassroom.com/class/energy/u5l2b21.gif

  19. Conservation of Energy http://serc.carleton.edu/images/sp/library/uncertainty/diagram_conservation_energy_si.jpg

  20. Energy of a Pendulum • At position A, the pendulum has only PE • Between A and B, it has a mix of PE and KE (with PE decreasing and KE increasing) • At B, it has purely KE • Between B and C, it has a mix again (with KE decreasing and PE decreasing)

  21. Machines • A machine is something that multiplies force or changes the direction of a force. • In general an important concept with machines is that work input = work output • Remember that work = force X distance • So, (force X distance)input = (forceXdistance)output

  22. Levers http://www.sciencelearn.org.nz/var/sciencelearn/storage/images/contexts/sporting_edge/sci_media/mechanical_advantage/17442-5-eng-NZ/mechanical_advantage_full_size_landscape.jpg

  23. Levers Continued How does this work? Lever in action • The girl’s input force is labeled “effort” • The input distance = the length of the lever arm to the pivot point • The load moved is very heavy • The distance on the load side of the pivot is very short

  24. Levers continued • So the input work of the girl is her force X the lever arm distance • The output work is the load weight X the short distance of the lever • Let’s say the load was 200 N. Then let’s say that the lever arm is 10 m and the short side of the lever is 1m. What does the girl’s input force need to be to lift the boulder? • (F)(10m) = (200N)(1m), so F = (200N)(1m)/(10m) • F = 20 N

  25. Levers yet again • So, the advantage of the lever system is that you put in less force to move a heavy object. • The disadvantage is that the object doesn’t move very far. • But hey, what do you want?

  26. Mechanical advantage • The ratio of the output force to the input force is called the Mechanical advantage. • In the previous lever problem, the ratio was 200N/20 N = 10x • Let’s do some mechanical advantage demonstrations.

  27. Incline planes and pulleys • An incline plane is a way to get a heavy load moved vertically upwards. • The mechanical advantage of an incline plane is the ratio of length of incline to the vertical distance raised. • Example: a 6 m incline that has one end raised up 1m has a mechanical advantage of 6x. You only have to apply 1/6 of the weight of the load to push it up the ramp. • Systems of pulleys can also be used for mechanical advantage.

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