Work Machines Review Work Work is force exerted

Work & Machines

Review Work § Work is force exerted on an object that causes the object to move some distance § Force without moving a distance yields NO WORK!! When work is done, a transfer of energy always occurs.

1) Power and work § Power is related to work. § The amount of work done in a certain amount of time. § Power = work/time §P=W/t § Units for power is - Watt(W) § It is a Joule/second (J/s)

2) Power and Energy § Power is also related to energy. § When energy is transferred from one object to another. § Power = energy/time § P = E/t

Machine § A device that makes work easier or more effective § by changing the amount of force, the distance covered or the direction of the force

§ Mechanical Advantage (MA) -number of times a machine multiplies the effort force. § Ideal Mechanical Advantage (IMA)- theoretical performance of a machine if no energy was lost to friction. § IMA = resistance force / effort force § IMA = FR/FE § It’s a ratio- the units cancel.

Efficiency of a Machine § Since work is lost to friction, the amount of work obtained from a machine is always less than the amount of work put into it. § Efficiency = work output/ work input x 100 Remember that work = force x distance

Inclined Plane § A sloping surface that can reduce the amount of force to do work. IMA = length of incline / height of incline

Wedge § A one or two sided inclined plane that changes the direction of the force

Screw § an inclined plane wrapped around a cylinder

Lever § A rigid bar that pivots over a fulcrum (a fixed point). IMA = length of effort arm length of resistance arm Draw 3 classes of levers and give examples of each.

1 st Class Levers § Notice how § The input & output forces are in opposite directions § The fulcrum is between the input & output forces § Examples include nail remover, paint can opener scissors, seesaw

2 nd Class Levers § Notice how: § The input & output forces are in the same direction § Input force is farther away from the fulcrum than the output force § Examples include: wheel barrow, door, nutcracker

3 rd Class Lever § Notice how: § The input & output forces are in the same direction § The input force is closer to the fulcrum than the output force § Examples include rake, shovel, baseball bat and fishing pole

What Class of Lever? 3 2 1 4 5 6 7 st Class 4. 2 nd Class 1. _______ 3 rd Class 2. _______ 1 st Class 3. 1 _______ nd Class 6. rd Class 7. 1 st Class 8. nd Class 5. 2 _______ 6. 3_______ 8. 2_______ 8

Wheel and Axle § two circular objects attached together about a common axis § Wheel is the large cylinder § Axle is the small cylinder IMA = Radius of the wheel Radius of the axle

Pulley § A cord wrapped around a wheel. § IMA = the number of ropes that support the weight