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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