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

OrW = 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 m2/s2 = 1 Nm

F = N d = mSo 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= 2mW= F x d = 1.0 N x 2m = 2. J

F= 15 N and d= 3 mW = F x d = 15N x 3m = 45 J

F = 100 N and work = 1500 J solve for distanceD = 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 forceA car jack (Changes a small force over a large

distance into large force over a small distance)

To increase distanceOars in a row boat (you exert a large force over

a short distance and other end move a large distance)

To change directionPulling 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 RingerOctober 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 RingerOct. 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 RingerOct. 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