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

What is Power? W P = t 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

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 DE DR IMA= DE = Distance traveled by effort force DR = Distance traveled by resistance force

IMA Example IMA= Effort Length = 11.0 cm = 5.5 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 = 11.0 cm = 5.5 Resistance Length 2.0 cm

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

AMA Example AMA= Resistance Force = 60 N = 3 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? AMA= Resistance Force = 60 N = 3 Effort Force 20 N

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 Screw Wedge Inclined Plane Lever Pulley Wheel and Axle