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Work and Power

Work and Power. What is Work?. Work is the product of force and distance Equation: Work = Force x distance or W = Fd Work REQUIRES motion! Force and motion MUST be in the SAME direction for work to happen. What is Work? cont’d…. You walk forward dragging a box behind you.

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Work and Power

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  1. Work and Power

  2. What is Work? • Work is the product of force and distance • Equation: Work = Force x distance or W = Fd • Work REQUIRES motion! • Force and motion MUST be in the SAME direction for work to happen

  3. What is Work? cont’d…. • You walk forward dragging a box behind you. • What is the direction of the motion of the box? • What is the direction of the force on the box? • Has work been done on the box?

  4. What is Work? cont’d…. • You carry a book to your locker. • What is the direction of the motion on the book? • What is the direction of the force on the book? • Has work been done on the book?

  5. What is Work? cont’d…. • Work is measured in joules (J) or Newton-meters (N•m) • Ex: A weightlifter lifts a 1600 N barbell to a height of 2 meters. How much work has the weightlifter done? • W = Fd W = 1600N x 2m W = ? W = 3200 J F = 1600 N d = 2m

  6. What is Power? • Power is the rate of doing work • To increase power…. • Increase work or decrease time • To decrease power …. • Decrease work or increase time W P = t

  7. What is Power? cont’d …. • Power is measured in watts (W) • Before the watt, power was measured in horsepower 746 hp = 1 W

  8. Mechanical Advantage • Input force - the force you apply • Outputforce – force which is applied by the object • Mechanical Advantage – small input force produces larger output force • Often used by simple machines

  9. Ideal Mechanical Advantage (IMA) DE DR IMA= • DE = Distance traveled by effort force • DR = Distance traveled by resistance force • Friction loss is not taken into consideration • Ratio of distance traveled by effort and resistance force

  10. IMA Example IMA= Effort Length = 11.0 cm= 5.5 Resistance Length 2.0 cm Suppose you need to remove a nail from a board by using a claw hammer. If the effort length for a claw hammer is 11.0 cm and the resistance length is 2.0 cm. What is the mechanical advantage?

  11. Actual Mechanical Advantage (AMA) FR FE AMA= • FR = Magnitude of resistance force • FE = Magnitude of effort force • Frictional losses are taken into consideration • Ratio of force magnitudes

  12. AMA Example AMA= Resistance Force = 60 N= 3 Effort Force 20 N Kisha applied 20N of force to turn an ice cream freezer crank. The crank’s resistance was 60 N. What was the mechanical advantage of the crank?

  13. AMA vs IMA AMA is what actually happens IMA is what could happen without friction Since there is always some friction, AMA is less than IMA.

  14. Efficiency Work Output Work Input Efficiency = X 100% Because there is always some friction, the efficiency of any machine is always less than 100 percent.

  15. Efficiency Example X 100 = 75% • Alice and Jim calculate that they must do 1800 J of work to push a piano up a ramp. However, because they must also overcome friction, they must actually do 2400 J of work. What is the efficiency of the ramp? 1800 J 2400 J

  16. The 6 Simple Machines Screw Wedge Inclined Plane Pulley Wheel and Axle Lever

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