Physics Energy
Work
the product of the force on an object and the distance through which the object is moved: the quantity force × distance We do work when we lift a load against Earth's gravity. The heavier the load or the higher we lift it, the more work we do. The application of force and the movement of something by that force If the force is constant and the motion takes place in a straight line in the direction of the force, the work done on an object by a net force is the product of the force and the distance through which the object is moved. W=Fd
A boulder at the top of a vertical cliff has a potential energy of 100 MJ relative to the ground below. It rolls off the cliff. When it is halfway to the ground its kinetic energy is
the same as its potential energy at that point.
Raising an auto in a service station requires work. Raising it in half the time requires
twice the power.
Raising an auto in a service station requires work. Raising it twice as high requires
twice the work
What is the theoretical mechanical advantage for each of the three lever systems shown?
1; 2; 0.5
When 100 J are put into a device that puts out 40 J, the efficiency of the device is
40%.
Why does a small, lightweight car generally have better fuel economy than a big, heavy car? How does a streamlined design improve fuel economy?
A car with less mass has less KE for a given v; streamlining reduces drag
Lever
A lever is a simple machine made of a bar that turns about a fixed point. If the heat from friction is small enough to neglect, the work input will be equal to the work output. work input = work output Since work equals force times distance, we can say (force × distance)input = (force × distance)output
Machines
A machine is a device used to multiply forces or simply to change the direction of forces. The concept that underlies every machine is the conservation of energy. A machine cannot put out more energy than is put into it.
Pulleys
A pulley is basically a kind of lever that can be used to change the direction of a force. Properly used, a pulley or system of pulleys can multiply forces. A pulley can change the direction of a force. A pulley multiplies force. Two pulleys can change the direction and multiply force. The mechanical advantage for simple pulley systems is the same as the number of strands of rope that actually support the load. The mechanical advantage of this simple system is 2. Although three strands of rope are shown, only two strands actually support the load. The upper pulley serves only to change the direction of the force.
Elastic Potential Energy
A stretched or compressed spring has a potential for doing work. When a bow is drawn back, energy is stored in the bow. The bow can do work on the arrow. A stretched rubber band has potential energy because of its position. These types of potential energy are elastic potential energy.
Work against another force
An archer stretches her bowstring, doing work against the elastic forces of the bow. When the ram of a pile driver is raised, work is required to raise the ram against the force of gravity. When you do push-ups, you do work against your own weight.
Inclined Planes
An inclined plane is a machine. Sliding a load up an incline requires less force than lifting it vertically.
Most earth satellites follow an oval-shaped (elliptical) path rather than a circular path around Earth. The PE increases when the satellite moves father from Earth. According to the law of energy conservation, does a satellite have its greatest speed when it is closest to or farthest from Earth?
Closest; when PE is least, KE is greatest
Unit of Measurement of Work
Combines a unit of force, N, with a unit of distance, m. The unit of work is the newton-meter (N•m), also called the joule. One joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m (lifting an apple over your head).
An astronaut in full space gear climbs a vertical ladder on the earth. Later, the astronaut makes the same climb on the moon. In which location does the gravitational potential energy of the astronaut change more? Explain.
Earth; greater g means greater PE
Energy
Enables an object to do work Like work, energy is measured in Joules
Kinetic Energy
Energy in motion If an object is moving, then it is capable of doing work. It has energy of motion, or kinetic energy (KE). The kinetic energy of an object depends on the mass of the object as well as its speed. It is equal to half the mass multiplied by the square of the speed. KE=1/2 mv^2 Fd=1/2 mv^2
Potential Energy
Energy that is stored and held in readiness is called potential energy (PE) because in the stored state it has the potential for doing work.
State two reasons why a rock projected with a slingshot will go faster if the rubber is stretched an extra distance.
Extra stretching involves grater force and greater distance. This means greater PE of the stretched rubber and thus greater KE of the rock when released.
If a mouse and an elephant both run with the same kinetic energy, can you say which is running faster? Explain in terms of the equation for KE.
From KE = 1⁄2 mv2, a small m must mean a large v if the KE is to be the same. The mouse must be running must faster than the elephant to have the same KE.
Fuel Cells
Hydrogen is the least polluting of all fuels. Because it takes energy to make hydrogen—to extract it from water and carbon compounds—it is not a source of energy. If you make the electrolysis process run backward, you have a fuel cell. In a fuel cell, hydrogen and oxygen gas are compressed at electrodes to produce water and electric current.
Solar Power
Sunlight is directly transformed into electricity by photovoltaic cells. We use the energy in sunlight to generate electricity indirectly as well: sunlight evaporates water, which later falls as rain; rainwater flows into rivers and into generator turbines as it returns to the sea. Solar shingles look like traditional asphalt shingles but they are hooked into a home's electrical system.
Nuclear and Geothermal Energy
The most concentrated form of usable energy is stored in uranium and plutonium, which are nuclear fuels. Earth's interior is kept hot by producing a form of nuclear power, radioactivity, which has been with us since the Earth was formed. A byproduct of radioactivity in Earth's interior is geothermal energy. Geothermal energy is held in underground reservoirs of hot water. In these places, heated water near Earth's surface is tapped to provide steam for running turbogenerators.
You lift a 100-N boulder 1 m. a. How much work is done on the boulder? b. What power is expended if you lift the boulder in a time of 2 s? c. What is the gravitational potential energy of the boulder in the lifted position?
a. W = Fd = 100 N·m = 100 J b. Power = 100 J / 2 s = 50 W c. Relative to its starting position, the boulder's PE is 100 J. Relative to some other reference level, its PE would be some other value.
An energy supply is needed for the operation of a(n)
automobile. living cell. machine. all of these
After you place a book on a high shelf, we say the book has increased
gravitational potential energy.
The main sources of energy on Earth are
solar and nuclear.
Work done to change the Speed
Bringing an automobile up to speed or in slowing it down
Watt
In honor of James Watt, the 18th century developer of the steam engine The unit of power is the joule per second, also known as the watt. One watt (W) of power is expended when one joule of work is done in one second. One kilowatt (kW) equals 1000 watts. One megawatt (MW) equals one million watts. .75 kW= 1 hp (horsepower)
When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it's moving 3 times as fast?
Nine times farther. The car has nine times as much kinetic energy when it travels three times as fast: .5m(3v)^2=.5m(9v)^2=(9(.5)mv^2)
You tell your friend that no machine can possibly put out more energy than is put into it, and your friend states that a nuclear reactor puts out more energy than is put into it. What do you say?
No, the energy from a nuclear reactor comes from the fuel. The reactor cannot give more energy than the fuel provides.
Suppose you are at the edge of a cliff and throw one ball down to the ground below and another up at the same speed. The upward thrown ball rises then falls to the ground below. How do the speeds of the balls compare when striking the ground? Neglect air resistance and use the conservation of energy to arrive at your answer.
Same; each starts with the same (PE + KE) and when they reach the ground all of the energy is the same KE
Mechanical Advantage
The child pushes down 10 N and lifts an 80-N load. The ratio of output force to input force for a machine is called the mechanical advantage. The mechanical advantage is (80 N)/(10 N), or 8. Neglecting friction, the mechanical advantage can also be determined by the ratio of input distance to output distance.
Efficiency
The efficiency of a machine is the ratio of useful energy output to total energy input—the percentage of the work input that is converted to work output. EFFICIENCY=USEFUL WORK OUTPUT/TOTAL WORK INPUT To convert efficiency to percent, you multiply by 100%. An ideal machine would have 100% efficiency. No real machine can be 100% efficient. Wasted energy is dissipated as heat. Efficiency can be expressed as the ratio of actual mechanical advantage to theoretical mechanical advantage. efficiency=actual mechanical advantage/ theoretical mechanical advantage Efficiency will always be a fraction less than 1 Transforming 100% of thermal energy into mechanical energy is not possible. Some heat must flow from the engine. Friction adds more to the energy loss. Even the best-designed gasoline-powered automobile engines are unlikely to be more than 35% efficient.
Mechanical Energy
The energy due to the position of something, or the movement of something The two forms of mechanical energy are kinetic energy and potential energy.
If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate?
The forklift that delivers twice the power will lift twice the load in the same time, or the same load in half the time.
A child on a sled (total weight 500 N) is pulled up a 10-m slope that elevates her a vertical distance of 1 m. What is the theoretical mechanical advantage of the slope?
The ideal, or theoretical, mechanical advantage is input distance / output distance = 10 m / 1 m = 10
Conservation of Energy
The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes. Everywhere along the path of the pendulum bob, the sum of PE and KE is the same. Because of the work done against friction, this energy will eventually be transformed into heat. The water behind a dam has potential energy that is used to power a generating plant below the dam. The generating plant transforms the energy of falling water into electrical energy. Electrical energy travels through wires to homes where it is used for lighting, heating, cooking, and operating electric toothbrushes.
Fulcrum
The pivot point, or fulcrum, of the lever can be relatively close to the load. Then a small input force exerted through a large distance will produce a large output force over a short distance. In this way, a lever can multiply forces. However, no machine can multiply work or energy. In the lever, the work (force × distance) done at one end is equal to the work done on the load at the other end.
Power
The rate at which work is done. When carrying a load up some stairs, you do the same amount of work whether you walk or run up the stairs. Power= WORK DONE/TIME INTERVAL
Potential Energy Equation
The upward force required while moving at constant velocity is equal to the weight, mg, of the object, so the work done in lifting it through a height h is the product mgh. gravitational potential energy = weight × height PE = mgh
Ways to set up a lever
There are three common ways to set up a lever: A type 1 lever has the fulcrum between the force and the load, or between input and output. A type 2 lever has the load between the fulcrum and the input force. A type 3 lever has the fulcrum at one end and the load at the other. For a type 1 lever, push down on one end and you lift a load at the other. The directions of input and output are opposite. For a type 2 lever, you lift the end of the lever. Since the input and output forces are on the same side of the fulcrum, the forces have the same direction. For a type 3 lever, the input force is applied between the fulcrum and the load. The input and output forces are on the same side of the fulcrum and have the same direction.
Work-Energy Theorem
To increase the kinetic energy of an object, work must be done on the object. If an object is moving, work is required to bring it to rest. The change in kinetic energy is equal to the net work done. The work-energy theorem describes the relationship between work and energy. We abbreviate "change in" with the delta symbol, ∆ Work = ∆KE Work equals the change in kinetic energy. The work in this equation is the net work—that is, the work based on the net force.
Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?
W = Fd = 60 N × 4 m = 240 J
The energy we require to live comes from the chemically stored potential energy in food, which is transformed into other energy forms during the digestion process. What happens to a person whose combined work and heat output is less than the energy consumed? What happens when the person's work and heat output is greater than the energy consumed? Can an undernourished person perform extra work without extra food? Defend your answers.
Weight (actually mass) is gained; weight is not lost; not without losing more weight
Sources of Energy
Wind, caused by unequal warming of Earth's surface, is another form of solar power. The energy of wind can be used to turn generator turbines within specially equipped windmills. Harnessing the wind is very practical when the energy it produces is stored for future use, such as in the form of hydrogen.
Gravitational Potential Energy
Work is required to elevate objects against Earth's gravity. The potential energy due to elevated positions is gravitational potential energy. Water in an elevated reservoir and the raised ram of a pile driver have gravitational potential energy. The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity to lift it.
Does an object with momentum always have energy? Does an object with energy always have momentum? Explain.
Yes; no, it may only have PE.
Does using an automobiles air conditioner while driving increase fuel consumption? What about driving with the lights on? What about playing the car radio when parked with the engine off? Explain in terms of the conservation of energy.
Yes; yes; yes; the energy consumed comes from the fuel
