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.
Pulleys & Pulley System Inclined Plane Block & Tackle Levers 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
Movement by body is explained by principles of force and work. 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