CHAPTER 8 ENERGY

Energy What is energy? You can see its effects, but it can be difficult to understand. First, let’s look at a closely related concept: Work

8.1 Work WORK = the product of the force and the distance through which the object is moved.

To calculate WORK: WORK = force X distance or WORK = Fd

Two things affect work: 1. the application of force to an object and 2. movement of that object by the force applied

Units for WORK: Units are measured in newtons X meters Or “newton meters”

1 newton meter = 1 JOULE Work is measured in JOULES.

8.2 Power Power involves TIME The rate at which work is done.

POWER Power = work / time Or P = w/t Work/time = Joule/second

POWER Joule/second = watts Power is measured in WATTS

Watts are named after James Watt – inventor of the steam engine.

8.3 Mechanical Energy Energy = that property of an object or a system which enables it to do work Energy is measured in JOULES.

MECHANICAL ENERGY is the energy due to the position or the movement of something Mechanical energy may be either kinetic or potential.

8.4 Potential Energy (PE) Potential energy is energy stored and held in readiness that has the potential to do work.

For example: A ball sitting atop a hill – has energy called “gravitational potential energy”

To calculate gravitational potential energy: Multiply weight X height PE = (mass X g) X (height) or PE = mgh

8.5 Kinetic Energy Kinetic energy is the energy of motion

To calculate KE: KE = ½ mv 2 Where KE = kinetic energy m = mass v = velocity

Also… KE = Fd Where KE = kinetic energy F = net force d = distance

Therefore: Fd = ½ mv 2

8.5 Kinetic Energy Work-Energy Theorem: the theorem that states that whenever work is done, energy changes.

8.6 Conservation of Energy How is energy transformed? 10 J of PE 8J of KE 2 J heat

10 J of PE 8J of KE 2 J heat Some of the energy is lost as heat; the rest is transformed into KE.

Law of Conservation of Energy Energy cannot be created nor destroyed; it can only be transformed. Total energy remains constant.

PE transformed to KE

At the top of the hill, the cart has only PE. Towards the middle of the hill, the cart has equal amounts of PE and KE. At the bottom of the hill, all of the PE has been transformed into KE.

On a pendulum:

On a roller coaster:

Notice the total energy remains constant. The energy is transformed from PE to KE and back to PE.

8.7 Machines A machine is a device for multiplying forces or simply changing the direction of forces.

There are 6 types of simple machines: 1. Lever 2. Screw 3. Inclined plane 4. Pulley 5. Wheel and Axel 6. Wedge

Levers A lever is a simple machine made of a bar that turns around a fixed point A fulcrum is the pivot point of a lever

Work input = Work output Fd input = Fd output Where F = force and d = distance

Mechanical Advantage Mechanical advantage is the ratio of output force to input force for a machine. OUTPUT FORCE INPUT FORCE

Mechanical Advantage Figure 8.10 compares the amount of force needed to the amount of force produced EX: a girl uses a lever to lift a rock that has a weight of 80N. She applies 10N of force to do so. The mechanical advantage = 8. Also see the question on page 116

There are three types of levers.

Type 1: fulcrum between the force and the load EX: a playground seesaw

Type 2: load is between the fulcrum and input force EX: lifting a car with a steel bar

Type 3: fulcrum at one end and load at the other EX: bicep muscles attached to bones in forearm

Pulley A pulley is a type of lever that can be used to change direction. physics/physengl/pulleysystem.htm

input output

input output

8.8 Efficiency useful work output Efficiency = input X 100 or actual mechanical advantage theoretical mechanical advantage

8.9 Energy for Life Think of cells as machines. For example: Plant Cells Photosynthesize Intestinal Cells Digest food Muscle Cells Shorten

Chapter 8 Key Terms Efficiency Energy Fulcrum Joule Kinetic energy Law of Conservation of Energy Lever Machine Mechanical advantage Mechanical energy Potential energy Power Pulley Watt Work Work-energy Theorem