Energy, Work, and Simple Machines

Chapter 10

• Energy = ability to produce a change in itself or its surroundings.

• Work transfer energy by mechanical means.

• Kinetic energy = energy of motion

• Ke = ½ mv2

• Work-Energy Theorem K = W

Work• Product of force and distance;

scalar quantity. W = Fd• Unit is J named after James

Prescott Joule. • 1 J = 1 N*m • Work is done only if object moves

in the direction of the applied force.

• Ex Prob 226 Prac Pr 227

Work and Direction of Force• Figure 10-4 p 227 shows work

can be broken into its components and that work is done only in the component in which the force is applied.

• W = Fd cos

• Since cos 90 = 0, no work is done in the vertical direction.

• Work done by the friction of grass is negative.

• Positive work done by you and means you are working.

• Ex Prob 228 Pr Prob 229

• On a Force vs Displacement Graph, the area under the curve is work. Fig 10-5 229

• Power = rate of doing work

• P = W/t

• Measured in watts (W)

• 1W = 1 J/s

• Often measured in kW because 1 W is so small.

• Ex Prob 230 Prac Pr 231

10.2 Machines• Eases load by changing

magnitude or direction of force, but it does not change the amount of work done.

• Figure 10-9 235 Example of Simple Machines

Simple and Complex Machines

• Work you do = Wi

• Work machine does = Wo

• Effort force, Fe - force you exert on machine

• Resistance force, Fr - force exerted by machine

• Mechanical Adv = Fr / Fe

• A machine can increase force, but not energy.

Simple Machines

• Pulleys & Pulley System

• Inclined Plane

• Block & Tackle

• Levers–1st Class, 2nd Class, 3rd Class

• Ideal machine transfers all energy • Wo = Wi. • Fr dr = Fe de • Fr / Fe = de / dr • MA = Fr / Fe • IMA = de /dr • Efficiency = Wo / Wi x 100%

= MA / IMA x 100%

• Lower efficiency - greater effort is needed to exert the same Fr.

• IMA = ratio of distances moved.

Compound Machines• Consists of two or more

simple machines. • Resistance force of one

becomes the effort force of the second.

• MA of complex = product of MA of each in compound

• Ex Prob 237 Pr Prob 238

• Movement by body is explained by principles of force and work.

• Lever systems four parts.

• Fig 10-12 238

• 1. Rigid bar (bone)• 2. Source of force (muscle

contraction)

• 3. Fulcrum (movable joints between bones)

• 4. Resistance (weight of body or object being moved)

• Tall person has lever with lower MA than short person.

• They must apply a greater force to move the longer lever formed by the leg bones.

• Tall people rarely have stamina in walking races.

Fig 10-9 235 Simple Machines