Lesson 15Work and Energy Eleanor Roosevelt High School Chin-Sung Lin
Energy • A central concept underlying all sciences • However, concept of energy is very difficult to define • Concept of energy is unknown to Isaac Newton. Its existence was still debated in the 1850s • A quantity that is often understood as the capacity to do work on a physical system
Energy • We observe only the effects of energy when something is happening • When energy is being transferred from one place to another • When energy is being transformed from one form to another
Forms of Energy • Thermal energy • Chemical energy • Electrical energy • Radiant energy • Nuclear energy • Magnetic energy • Elastic energy • Sound Energy • Mechanical energy • Luminous energy
Work • The amount of energy transferred by a force acting through a distance • Work = net force x distance W = Fnetxd • Unit of work: newton-meter (Nm) = joule (J)
Work • A scalar quantity • Independent of time • Unit of work: newton-meter (Nm) = joule (J) • 1 J = 1 N x 1 m = 1 kgm2/s2 • 1 KJ = 1,000 J • 1 MJ = 1,000,000 J
Work Two ways to do work • Force acts to change motion • Force acts against another force
Work Example • An 80 N force has been applied to a block and move it 20 m along the direction of the force. How much work has been done to the block?
Work Example • An 80 N force has been applied to a block and move it 20 m along the direction of the force. How much work has been done to the block? W = F d = 80 N x 20 m = 1600 J
Work Exercise • Calculate the work done when a 20-N force pushes a cart 3.5 m ?
Work Exercise • How much work is required to lift a 360 kilogram piano to a window whose height is 10 meters from the ground?
Work Exercise • A box rests on a horizontal, frictionless surface. A girl pushes on the box with a force of 18 N to the right and a boy pushes on the box with a force of 12 N to the left. The box moves 4.0 m to the right. Find the work done by (a) the girl, (b) the boy, and (c) the net force.
Work vs. Impulse • impulse = net force x time J = F t impulse changes the momentum of a system unit of impulse: newton-second (Ns) • work = net force x distanceW = F d work changes the energy of a system unit of work: newton-meter (Nm) = joule (J)
Work and Force with an Angle • W = F cos θ d d
Work Exercise • If 100 N force has 30o angle pulling on 15 kg block for 5 m. What’s the work?
Work Exercise • If 100 N force has 30o angle pulling on 15 kg block for 5 m. What’s the work? W = F dcos W = 100 N (5 m) cos 30o = 433 J
Positive or Negative Work • Man does positive work lifting box • Man does negative work lowering box • Gravity does positive work when box lowers • Gravity does negative work when box is raised
Power • Power – The rate at which work is done • Power = (Work Done) / (Time Interval) P = W/t • Unit of power: joule per second , or Watt (W) • 1 watt (W) = 1 J / 1 second • 1 kilowatt (kW) = 1,000 watts • 1 megawatt (MW) = 1,000,000 watts
Power • When W is in the equation, W means work; when W is in the unit, W means Watt • In metric system of units, automobiles are rated in kilowatts; in the US, we rate engines in units of horsepower (hp). • 1 hp = 0.75 kW or 134 hp = 100 kW
Power Example • Calculate the power expended when a 500 N barbell is lifted 2.2 m in 2 s
Power Example • Calculate the power expended when a 500 N barbell is lifted 2.2 m in 2 s W = Fd/t = 500 N x 2.2 m / 2 s = 1,100 J / 2 s = 550 watts
Power Exercise • An escalator is used to move 20 passengers every minute from the first floor of a department store to the second. The second floor is located 5-meters above the first floor. The average passenger's mass is 60 kg. Determine the power requirement of the escalator in order to move this number of passengers in this amount of time
Power • Power = Force x Average Velocity P = W/t = Fd/t = Fv
Power Example • A 500N force is applied to an object. If the object travels with a constant velocity of 20 meters per second, calculate the power expended on the object
Power Example • A 500N force is applied to an object. If the object travels with a constant velocity of 20 meters per second, calculate the power expended on the object P = Fv = (500N)(20m/s) = 10,000 W = 10 kW
Power Exercise • An elevator must lift 1000 kg a distance of 100 m at a velocity of 4 m/s. What is the average power the elevator exerts during this trip?
Mechanical Energy • A compression of atoms in the material of an object, a physical separation of attracting bodies, or a rearrangement of electric charges in the molecules of a substance • Energy enables an object to do work • Unit of energy: energy is measured in joules
Mechanical Energy • Two most common forms of mechanical energy: potential energy (PE) and kinetic energy (KE) • Mechanical energy can be in the form of either one, or the sum of the two • Potential energy— the energy due to the position of something, or • Kinetic energy— the energy due to the movement of something
Potential Energy • Definition: An object may store energy by virtue of its position • The energy that is held in readiness is called potential energy (PE), because in the stored state it has the potential for doing work
Potential Energy • Examples of potential energy: • A stretched/compressed spring • A drawn bow • A stretched rubber band • The chemical energy in fuels (at submicroscopic level) • Any substance that can do work through chemical action possesses potential energy (such as fossil fuels, electric batteries and food)
Gravitational Potential Energy • Work is required to elevate objects against Earth’s gravity • The potential energy due to elevated positions is called gravitational potential energy (GPE) • The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in lifting it
Gravitational Potential Energy • The upward force required while moving at constant velocity is equal to the weight (mg) of the object • Gravitational Potential Energy = Weight x Height GPE = Fgh = mghor PE = mgh • The gravitation potential energy depends only on mg and h, does not depend on the path • Unit of gravitational potential energy: kg m2/s2 =joule (J)
Potential Energy Example • The floor of the basement of a house is 3.0 m below ground level, and the floor of the attic is 5.0 m above ground level. What is the change in potential energy if an 2.0 kg object in the attic is brought to the basement?
Potential Energy Example • The floor of the basement of a house is 3.0 m below ground level, and the floor of the attic is 5.0 m above ground level. What is the change in potential energy if an 2.0 kg object in the attic is brought to the basement? PE = mgh = (2.0 kg)(9.8 m/s2)(5.0 m + 3.0 m) = 156.8 J
Potential Energy Exercise • A box has a mass of 5.8kg. The box is lifted from the garage floor and placed on a shelf. If the box gains 145J of Potential Energy (PE), how high is the shelf?
Potential Energy Exercise • A man climbs on to a wall that is 3.6m high and gains 2268J of potential energy. What is the mass of the man?
Spring Force • Hooke’s Law: Hooke's Law says that the spring restoration force (FS) due to a spring is proportional to the length that the spring is stretched or compressed, and acts in the opposite direction • FS = kx • k is spring constant, and • x change in spring length from the equilibrium position • Unit of spring force: newton (N)
Hooke’s Law • The more force that was put on materials the more they extended • With some materials they also extended in a regular way e.g. if the force was doubled so did the extension • This was true as long as their elastic limit was not exceeded
Elastic limit • The material no longer shows elastic behavior (i.e. does not return to original size when stretching force is removed) • The material is permanently deformed i.e., is larger or longer than originally
Spring Constant • kis called the spring constant and is a measure of the stiffness of the spring or material • Unit of of k: N/m(newtons per meter) • The higher the k the stiffer the spring, it needs a large force to for a given extension
Spring Force Example • A linear spring has a constant of 377.16 N/m. How much force is necessary to stretch it 0.39 m?
Spring Force Example • A linear spring has a constant of 377.16 N/m. How much force is necessary to stretch it 0.39 m? Fs = kx = (377.16 N/m)(0.39 m) = 147.09 N
Spring Force Exercise • It requires a force of 594.55 N to stretch a certain linear spring 0.15 m. What is the constant for this spring?
Spring Potential Energy • Spring Potential Energy: A spring may store energy due to compression or stretching and is called spring potential energy (PES) • The potential energy stored in spring is equal to the work done to the spring: • PES = FSx = (1/2FS)x = (1/2kx)x = 1/2kx2 PES = 1/2kx2 • Unit of Spring Potential Energy: N m=joule (J)
Spring Potential Energy Example • Vincent's mountain bike has a spring with a constant of 64 N/m in the front-wheel suspension, and it compressed 0.17m when she hit a bump. How much energy does the front spring now store?
Spring Potential Energy Example • Vincent's mountain bike has a spring with a constant of 64 N/m in the front-wheel suspension, and it compressed 0.17m when she hit a bump. How much energy does the front spring now store? PEs = (1/2) kx2 = (1/2)(64 N/m)(0.17 m)2 = 0.92 J