F=ma. Exam 1. Today. Announcements: The average on the first exam was 31/40 Exam extra credit is due by 8:00 am Friday February 20th.

Size: px

Start display at page:

Download "F=ma. Exam 1. Today. Announcements: The average on the first exam was 31/40 Exam extra credit is due by 8:00 am Friday February 20th."

Wilfrid Stephen Roberts
5 years ago
Views:

1 Today Exam 1 Announcements: The average on the first exam was 31/40 Exam extra credit is due by 8:00 am Friday February 0th. F=ma Electric Force Work, Energy and Power Number Score ISP09s7 Lecture ISP09s7 Lecture Newton s Second Law of Force A new Force! F=ma Force is equal to mass times acceleration. For a given force, the amount of acceleration is inversely proportional to the mass. Force causes acceleration. If you observe acceleration, there must be a force acting. ISP09s7 Lecture Charge is a property of matter. It is measured in Coulombs C. Like charges repel, unlike charges attract. Coulomb s Law of Electric Force kq Q 1 F = k = 8.99E 9 N! m r 1 ISP09s7 Lecture C - + r 1

2 Why? Coulomb s law looks like Newton s Law of gravity. Why? Why does charge come in two types and mass only came in one type? Why do we always get r? I hate squares. Why is k = 8.99E9 Nm /C so much bigger than G = Nm /kg? Two possible answers: (1) I can t tell you until you are older. () I don t know. ISP09s7 Lecture Energy Energy is the ability to do work Energy comes in two forms Kinetic (KE) energy of motion Potential (PE) energy of position There are many variants on these type main types, e.g. chemical, nuclear, thermal, ISP09s7 Lecture Energy and Power Work: Using a Force to Move Something Energy is the ability to do work: Work = force x distance = F d Energy comes in two forms Kinetic (KE) energy of motion Potential (PE) energy of position 1 KE = mv m - mass v - velocity Gravitational GPE = m (gh); g = 9.81 m/s on Earth, h height Power (measured in W = J/s) is the rate of change (or use) of energy ISP09s7 Lecture In common English, work refers to any kind of effort you put into performing a task, whether physical or mental. In physics, work is done whenever an object is pushed or pulled through a distance; there must be both force and motion. work = force! distance = Fd ISP09s7 Lecture 10-8-

Work and Energy: A simple Example Slowly lift your book some height h and then lower it. You did work on the book while lifting it; the book did work on you while you lowered it.

3 Work and Energy: A simple Example Slowly lift your book some height h and then lower it. You did work on the book while lifting it; the book did work on you while you lowered it. The raised book has an increased ability to do work, and does this work as you lower it. What if you lift your book some height h and then drop it? Can it still do work? Work and Energy: A simple Example Work requires motion It accelerates downward, acquiring kinetic energy as it loses height. You could have the dropped book do work by driving a thumbtack into the floor. ISP09s7 Lecture ISP09s7 Lecture Quantitative Look at Energy An object s energy is defined as the amount of work it can do. Therefore, the gravitational potential energy (GPE) of an object is its weight multiplied by its height, GPE = mgh The kinetic energy (KE) of an object in motion can be derived from Newton s laws Quantitative Look at Energy GPE when you let it drop is the same as its KE just before it hits the floor. KE = 1/ * MV ISP09s7 Lecture ISP09s7 Lecture 10-1-

Quantitative Look at Energy If you add the gravitational energy and the kinetic energy at any point in the book s fall, you will find that the sum stays the same. Energy is conserved.

4 Quantitative Look at Energy If you add the gravitational energy and the kinetic energy at any point in the book s fall, you will find that the sum stays the same. Energy is conserved. An Aside on Semantics A system is a distinct collection of objects and/or fields that are interacting in some way. Any system having the ability to do work is said to have energy. A system with energy need not do work, but a system that does work must have had energy. ISP09s7 Lecture ISP09s7 Lecture Some Forms of Energy Kinetic energy: energy of motion Gravitational energy: energy associated with a raised Object (aka potential energy ) Elastic energy: energy of a stretched or deformed object Thermal energy: energy in the form of heat due to the random microscopic motion of atoms and molecules Forms of Energy Electromagnetic energy energy associated with electric and magnetic fields Radiant energy energy of electromagnetic waves such as light, infrared, and X-rays Chemical energy energy involved in chemical reactions Nuclear energy energy involved in nuclear reactions ISP09s7 Lecture ISP09s7 Lecture

5 The Law of Conservation of Energy Experiments have found that energy is always conserved, although it may change its form. The total energy of all the participants in any process remains unchanged throughout that process. That is, energy cannot be created or destroyed. Energy can be transformed (changed from one form to another), and it can be transferred (moved from one place to another), but the total amount always stays the same. ISP09s7 Lecture The Work-Energy Principle Another way of stating the conservation of energy is what the book calls The work-energy principle: Work is an energy transfer. Work reduces the energy of the system doing the work and increases the energy of the system on which work is done, both by an amount equal to the work done. ISP09s7 Lecture Transformations of Energy What happens after it hits the floor? Transformations of Energy One way to visualize energy transformations is through the use of an energy flow diagram. The one below is for the dropped book. Where is all that energy now? It s gone into heat both your book and the floor are now slightly warmer. ISP09s7 Lecture ISP09s7 Lecture 10-0-

6 Some Example Problems Examples: A mass of 1.0 kg is raised 1.0 m. How much work was done? W =!GPE = mg!h = 1.0 kg x 9.81m/s^ x 1.0 m = 9.81 J A 90.0 kg ISP09 professor walks up two flights of stairs. How much did his/her potential energy increase? DATA 1 flight of stairs = 3.00 m!gpe = 90.0 kg x 9.81m/s x flights x (3 m/flight) = 5.9 kj ISP09s7 Lecture Conservation of Energy In nature certain quantities are conserved. Energy is one of these quantities. Charge is another. Example: Ball on a hill A 1.00 kg ball is rolled toward a hill with an initial speed of 5.00 m/s. If the ball roles without friction, how high, h, will the ball go? 1 m KE = mv PE = mgh ; g = 9.80 s 1 v ( 5 m / s) mv = mgh " h = = = 1.8 m g m! 9.80 s ISP09s7 Lecture Work Work = Force x distance Work is a scalar and is measured in Joules, J Bill pushes on a wall with 10 N for 33 s. If the wall does not move, how much work is done on the wall? Work = 10.0 N x 0.0 m = 0.0 N How does that make sense? Work has a strict definition. If the kinetic or potential energy of the wall did not change, no work was done on the wall. Work changes energy from one form to another. ISP09s7 Lecture Power Power is the rate of change of energy Power = (change in energy)/(change in time) Power is a scalar and is measured in watts. Light bulbs are measured in watts Sun (a big light bulb) !10 6 W ISP09s7 Lecture 10-4-

7 Information Horsepower 746 W = 1 horsepower In fourteen hundred and ninety-two Columbus sailed the ocean blue. And if you divide by two You get watts in a horsepower too Food energy is measured in kcal 1 food cal = J 1 Calorie = 1 kcal (what we call calories are actually kilocalories) ISP09s7 Lecture Example Problem How many kcal are burned by doing 1500 J of work? DATA: The human body is 10% efficient in converting food energy to work. cal = energy' 1500 J ' 1cal J 1cal J & ' $ % & 1 # ' $! % efficiency " 1 #! = 3590.cal = 3.59 kcal 0.1" ISP09s7 Lecture 10-6-

Today. Exam 1. The Electric Force Work, Energy and Power. Comments on exam extra credit. What do these pictures have in common?

Today. Exam 1. The Electric Force Work, Energy and Power. Comments on exam extra credit. What do these pictures have in common? Today Exam 1 Announcements: The average on the first exam was 31/40 Exam extra credit is due by :00 pm Thursday February 18th. (It opens on LONCAPA today) The Electric Force Work, Energy and Power Number