Work and Power What is Work Work is

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. – 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?

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?

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=? F = 1600 N d = 2 m W = 1600 N x 2 m W = 3200 J

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

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

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

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

IMA Example • 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? IMA= Effort Length Resistance Length = 11. 0 cm 2. 0 cm = 5. 5

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

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

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.

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

Efficiency Example • 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 X 100 = 75%

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