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Work, Power, and Energy

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Zaheda M

on 15 April 2013

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Transcript of Work, Power, and Energy

Work, Energy, and Power WORK CLAIM Energy is work If you push harder (with more force) you do more work. If you push longer (with more distance) you do more work. Work is effort (force) exerted over a certain distance When a force makes an object move in a direction parallel to that force, work is done. EVIDENCE W=Fd Work (in joules) Force (in newtons) distance (in meters) Energy is stored work Stores energy Energy can cause forces which can cause motion which can do work Work uses energy and creates it. You need to have energy to complete work. For example to do 100 joules of work you need 100 joules of energy. Energy can be thought of as the currency to do work. Since force and motion are in the same direction positive work is done on the ball. When Tigger stopped the ball by catching it, he exerted a forward force in a direction opposite from the ball's motion. He did negative work on the ball. Work is measured in units called joules, abbreviated J. 1 J= 1N x 1m 1 joule of work is done when a force of one newton is exerted over a distance of 1 meter. REASONING WORK CAN MAKE ENERGY ENERGY CAN DO WORK ENERGY IS WORK Because of the formula W=Fd we know that if you push harder you do more work, and if you push longer (with more distance) you do more work. We can see this in this example: You push a 500 newton car 5 meters. How much work did you do? W=500 N x 5m
W=2500 J W=1000 N x 5m
W=5000 J If we increase the force to 1000 Newtons what will happen? The work increased because we pushed the car harder (the force increased.) In the same way if we pushed the car longer (with more distance) the work would have increased as well! To do work a force must be in the direction of motion. In the evidence I gave an example of throwing and catching a ball. When you throw a ball a force is in the direction of motion, while when you catch a ball it is not. To do work a force must be in the DIRECTION OF THE MOTION 1 N 1 N 1 N 1 m Half of this force does work (the half that pushes parallel to the motion). All of this force does work (it is all parallel to the motion) None of this force does work (none of it is parallel to the motion) POWER CLAIM Power is a measure of how quickly work is done In more formal language, it is the rate at which work is performed. The basic unit of power is the watt, abbreviated w. A machine that works faster is more powerful A more powerful lightbulb gives off the same amount of light (same work) it just does it faster. 60 watts 90 watts same amount of work (Joules) EVIDENCE P= W/ t power in watts Work in joules time in seconds The basic unit of power is the watt, which is defined as one joule of work per second. REASONING EXAMPLE:

If someone wanted to carry 100 lbs of weights upstairs in 10 seconds, how much power would they use? P=W/T
P=20J/10s= 2w The person used two watts of power. What would happen if the person decided to go faster and run up the stairs in five seconds instead of 10? P=20J/5s
P=4w Work= 20 J
Time= 5s The person is still doing the same joules of work because the weights are moving the same distance. The power increases as the person is running faster. The faster you go the more your power increases! POTENTIAL
ENERGY CLAIM Potential energy is stored energy that an object has because of its position or condition.

An object gets potential energy from height, mass, and gravity.

An object with potential energy has the potential to do work. This potential is only released if the object falls. The energy is then transformed into kinetic energy or transformed into work.

The more mass and height you have the more potential energy! Evidence Ep= mgh Potential Energy in Joules mass (in kilograms) height (in meters) acceleration due to gravity (9.8 m/s2) EXAMPLE When you ride up a ski lift your potential energy increases as you climb higher. On the top of the mountain you have a LOT of potential energy even though you are not moving at all The heavier the person is the more potential energy . Also the more height there is the more potential energy REASONING We now know that potential energy is the energy of work and that an object gets potential energy from height, mass, and gravity. We saw in the example of the ski lift that the higher the people went the more their potential energy increased. We also know that people with heavier masses have higher potential energies. 10 kg 20 kg Less Ep More Ep 5m 10 kg 10 kg More Ep Less Ep 5m 3m Kinetic Energy CLAIM Kinetic energy is the energy of motion. If something isn't moving it doesn't have kinetic energy.

An object gets kinetic energy from its mass and velocity.

A fast moving object with a lot of mass will have a lot of kinetic energy.

Kinetic energy can be transferred from one object to the other when objects collide. EVIDENCE Ek= (1/2) mv^2 Kinetic energy (in joules) mass (in kilograms) velocity (m/s) EXAMPLE Lets say we had two snow tractors. Tractor number one is moving at 30 km/h and tractor number two is moving at 40 km/h. Tractor number two has the greater speed and that means more kinetic energy When you go bowling a lot of times the ball only hits one pin. The rest of the pins still go down. The kinetic energy transferred from the ball to the pin that was struck, and to all the other pins in a domino effect. REASONING From the evidence we saw that the tractor with more speed (velocity) had greater kinetic energy than the truck with lesser speed. We also saw from the formula that the same thing would happen if the other tractor had more mass.

We also know that kinetic energy can be transferred from one object to another as we saw in the bowling scenario when the ball transferred kinetic energy to one pin, and that pin transferred the kinetic energy to all the rest of the pins causing them to all fall down. You can see kinetic energy in action when you are riding a bicycle uphill. Kinetic energy helps you overcome gravity as the faster you are moving the easier it is to get up the hill. Conservation of Energy CLAIM Law of Conservation of Energy Energy is never created nor destroyed, just transformed into other forms of energy. EVIDENCE If energy can only be transformed, then, for any object being thrown into the air or dropped:
Ep=Ek OR
mgh=(1/2) mv^2
The potential energy at the top equals the kinetic energy at the bottom. EXAMPLES:

A student lifts his 2.0 kg pet rock 2.8 m straight up. He then lets it drop to the ground. Use the Law of Conservation of Energy to calculate how fast the rock will be moving (a) half way down and (b) just before it hits the ground. Use Ep = Ek a) 5.2 m/s b) 7.4 m/s A 4 kg ball is thrown into the air. It reaches a height of 1.8 meters. How fast was it going when thrown into the air?H = 1.8 m; M = 4 kg; G = 9.8 m/s2 ( use g = 10); V = ?The law of conservation of energy says that the Ep at the top = Ek at the bottom.Ep = Ek = mgh = (½)v2 = 2gh = v2 = 2(10)(1.8) = 2(18) = v2 = 36V = 6 m/s2 REASONING We now know for sure that energy is never created or destroyed but it is just transformed into other forms of energy. We also know that Ep=Ek.

There are many examples of this in real life. A car engine burns gasoline, converting the chemical energy in gasoline into mechanical energy. Solar cells change radiant energy into electrical energy. Energy changes form, but the total amount of energy in the universe stays the same. Works CitedBattery. Digital image. Androidspin.com. Web. 14 Apr. 2013. <http://androidspin.com/tag/battery-life/>."BrainPOP | Potential Energy." BrainPOP | Potential Energy. Web. 14 Apr. 2013. <http://www.brainpop.com/science/energy/potentialenergy/>.Candace Flynn. Digital image. Phineasundferb.wikia.com. Web. 14 Apr. 2013. <http://phineasundferb.wikia.com/wiki/Candace_Flynn>.Cars. Digital image. Coolchaser.com. Web. 14 Apr. 2013. <http://www.coolchaser.com/graphics/14957>.Cartoon Girl. Digital image. Drawingcoach.com. Web. 14 Apr. 2013. <http://www.drawingcoach.com/cartoon-girl.html>.Cartoon Lifter. Digital image. Tierschutz-burglengenfeld.de. Web. 14 Apr. 2013. <http://tierschutz-burglengenfeld.de/images/15/cartoon-lifter>.Cute Cartoon. Digital image. Drawsketch.about.com. Web. 14 Apr. 2013. <http://drawsketch.about.com/od/cartooning/ss/cutecartoon.htm>.Cute Cartoon Girl. Digital image. 123rf.com. Web. 14 Apr. 2013. <http://www.123rf.com/photo_13143059_cute-little-cartoon-girl-standing-on-stack-of-books--high-quality-3d-illustration.html>.Digital image. Lindathorlakson.wordpress.com. Web. 14 Apr. 2013. <http://lindathorlakson.wordpress.com/>.Girl Rollerblading. Digital image. Primoclipart.com. Web. 14 Apr. 2013. <http://www.primoclipart.com/view-clipart/girl-rollerblading-clip-art>."Kinetic Energy." BrainPOP. Web. 14 Apr. 2013. <http://www.brainpop.com/science/energy/kineticenergy/>.Money. Digital image. Tuscaloosada.com. Web. 14 Apr. 2013. <http://www.tuscaloosada.com/office-divisions/restitution-recovery-unit/money/>.Winnie the Pooh and Rabbit. Digital image. Disneyshirts.blogspot.ae. Web. 14 Apr. 2013. <http://disneyshirts.blogspot.ae/2012/08/winnie-pooh-and-rabbit-throwing.html>.Woman Running. Digital image. Illustrationsource.com. Web. 14 Apr. 2013. <https://www.illustrationsource.com/stock/image/2236/a-woman-running-up-the-steps/?&results_per_page=1&detail=TRUE&page=2>."Work." BrainPOP. Web. 14 Apr. 2013. <http://www.brainpop.com/science/motionsforcesandtime/work/>."Work, Energy, and Power." HyperPhysics.edu. Web. <http://hyperphysics.phy-astr.gsu.edu/hbase/work.html>. During a physics lab, Jack and Jill ran up a hill. Jack is twice as massive as Jill; yet Jill ascends the same distance in half the time. Who did the most work? ______________ Who delivered the most power? ______________ Explain your answers.

Jack does more work than Jill. Jack must apply twice the force to lift his twice-as-massive body up the same flight of stairs. Yet, Jill is just as "power-full" as Jack. Jill does one-half the work yet does it one-half the time. The reduction in work done is compensated for by the reduction in time.

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