Bell ringer October 10, 2014 –If a person lifts a box 2 meters high to place it on a shelf, have they done any work? –IF they hold the box over their head without moving it, are they doing work at that moment?

P. Sci. PS Standard 6 Chapters 4 & 5 Energy & Work

Work When a force causes an object to move – work is done.

Work cont. Work = Force x distance Or W = F x d

If the object does not move then no work is done. W = F x d If d = 0 any number times 0 is 0 so no work.

The SI unit for work is joules (J) 1 J = 1kg x m 2 /s 2 = 1 Nm F = N d = m So W = Nm

Solving problems: 1. A book weighing 1.0 Newtons is lifted 2 meters. How much work is done? 2. A force of 15 Newtons is used to push a box a distance of 3 meters. How much work is done? 3. It takes 100 newtons to lift a rock. Work done was 1500 Joules; how far was the rock lifted? F= 1.0 N and d= 2m W= F x d = 1.0 N x 2m = 2. J F= 15 N and d= 3 m W = F x d = 15N x 3m = 45 J F = 100 N and work = 1500 J solve for distance D = work/ force= 1500J/ 100 N= 15 m

Monday, October 13 Bell ringer: A baby has a mass of 5 kg. What is its weight in Newtons? If the baby’s mother lifts the baby 2 meters high off the floor to its crib, how much work does the mother do?

Power Power is the rate at which work is done Power= work/time It is measured in watts (W) or kilowatts (kW) Example: Lifting an elevator 18 m takes 100 kJ. If doing so takes 20 seconds, what is the average power of the elevator during the process? Work= 100 kJ Time = 20 seconds Power = work/ time P= 100kJ/ 20s = 5 kW

Why do we use machines? To increase force A car jack (Changes a small force over a large distance into large force over a small distance) To increase distance Oars in a row boat (you exert a large force over a short distance and other end move a large distance) To change direction Pulling back on oars pushes boat forward

PROJECT You will begin working on your project today about Machines, Work and Energy. Go to my web site and open the Unit 3: Work and Energy page You will find the directions for the project there Project is DUE Monday, Oct. 20 at beginning of class.

Bell Ringer October 14 Today you will take a short quiz Then, you will continue reading your book and complete some assessments on your reading After the pledge & moment of silence, please begin reading. I will direct you in a few minutes on the assessments.

Bell ringer Oct. 15 –John moves a rock 5 meters by applying a 10N force. It takes him 20 seconds to move the rock. Find the power. D= 5 m and F = 10N and t= 20 seconds Power= work/time P = (F x d)/ t P = (10N x 5 m)/ 20 s P= 50 J/ 20 s= 2.5 Watts

Energy The ability to cause change

Energy and Work Whenever work is done, energy is transformed or transferred to another system. Energy is the ability to do work.

Remember – work is done only when an object moves. But - energy can be present in an object or a system when nothing is happening. However – it can only be observed when it is transferred from one object or system to another.

SI Unit of Energy Because the amount of energy transferred is measured by how much work is done – energy and work are expressed in the same unit. Joules

Potential Energy Potential Energy is energy that is Stored. You can’t see it but you know it’s there AKA – Energy of Position

Types of Potential Energy Gravitational Potential Energy –Energy stored due to position (objects that are above Earth’s surface). Chemical Potential Energy – Energy stored in chemical bonds such as food or fuel. Elastic Potential Energy – energy stored by something that can stretch or compress such as a rubber band or spring.

Kinetic Energy Energy in motion.

Note: Kinetic energy depends more on speed than on mass.

Bell Ringer Oct. 16/17 What is the difference between kinetic and potential energy? You are watching a video today called “Roller Coaster Physics.” As you watch, you will answer some assessment question.

Bell Ringer Oct. 20 What kind of energy does a roller coaster car have at the top of a hill? What kind of energy does it have half- way down that hill travelling at 45 miles per hour?

Mechanical Energy The sum of the Potential and Kinetic energy in a system. KE + PE = ME * The total energy in the system

Law of Conservation of Energy Energy cannot be created nor destroyed it can only be changed. Energy can be transferred to another object/system or to another form (potential to Kinetic)

Other forms of Energy In almost every system, there are hidden forms of energy that are related to the motion and arrangement of atoms that make up the objects in the system.

Electrical energy: results from the flow of charged particles or electrons. Light energy: (electromagnetic energy) can travel through space unlike sound energy. Thermal (heat) Energy: energy given off as heat. (friction)

Solar energy: Sun gives energy to living organisms. Nuclear Energy: The sun gets energy from nuclear reactions like nuclear fusion. Sound Energy: related to sound waves