Work = force X distance or W = F X d Work is done when a force moves an object over a distance Work = force X distance or W = F X d When a force is applied to an object, the force may or may not cause the object to move. If the force does not produce motion, NO WORK IS DONE! A force results in work ONLY if motion is produced.

Machines Make work easier to perform by: A device that transfers mechanical energy from one object to another Make work easier to perform by: 1) multiplying force 2) changing the direction or the distance over which a force is applied Ex: a wrench multiplies applied force when moving a tight bolt. Ex: a loading ramp attached to the back of the truck reduces the force but increases the distance the object must be moved.

Resistance and Effort Resistance = the force a machine has to overcome the force applied Using a machine CAN reduce the amount of effort needed to overcome a given amount of resistance but a machine DOES NOT decrease the amount of work

Work Suppose you lift a box weighing 450 Newtons up onto a 2 meter high platform. To lift the box straight up by yourself, you would need to apply 450 Newtons of force over a distance of 2 meters. W = F x D W = 450 N x 2 m W = 900 J

HOWEVER, if you set up a rope and pulley system to change the direction and distance of the force required, you might have to pull in 9 meters of rope only using 100 Newtons of force: So the TOTAL amount of work is the SAME using the pulley versus not using the pulley W = F x D W = (100 N) x (9 m) W = 900 J

Efficiency of Machines A machine can be made more efficient by reducing friction. Grease, oil, and wax are good ways to reduce the friction between moving parts. Sanding surfaces to make them smoother or using ball bearings between surfaces also reduces friction.

Efficiency Percent Efficiency Percent Efficiency = Work output X 100 Work input

A man expends 300 J of work to move a chair up a ramp A man expends 300 J of work to move a chair up a ramp. The amount of work produced is 120 J. What is the percent efficiency? Percent Efficiency = Work output X 100 Work input Percent Efficiency = 120 J X 100 300 J Percent Efficiency = 40 %

A student expends 150 J of work to move desk across the room A student expends 150 J of work to move desk across the room. The amount of work produced is 50 J. What is the percent efficiency? Percent Efficiency = Work output X 100 Work input Percent Efficiency = _50 J X 100 150 J Percent Efficiency = 33 %

6 Simple Machines Lever Pulley Wheel and axle Inclined plane Wedge Screw

Lever Consists of a rigid bar that can turn around a point called a fulcrum Ex: pliers, crowbar

Pulley A modified form of a lever. Can change direction of force or decrease the force needed to move a heavy object. Ex: clothes line

Wheel and axle A modified form of a lever. Consists of a large wheel with a smaller wheel (or axle) in its center. They are connected so they turn together. Turning the outer wheel uses less force but must turn a greater distance. Ex: bicycles, car steering wheels, doorknobs

What about gears? A gear is simply a modified wheel and axle.

Inclined Plane A flat surface with one end higher than the other. Ex: wheelchair ramp, truck ramp, bike ramp

Wedge Double-sided inclined plane. The effort force is applied by driving the wedge into something, like an ax into a log. Ex: ax, knives, wood nails, chisels

Screw An inclined plane wrapped around a wedge or cylinder http://www.youtube.com/watch?v=DpnvS7kM4Fs&feature=related Ex: wood screws, bolts, car jacks

Complex Machines Contains 2 or more simple machines Ex: a bike has a wheel and axle, a pulley system and a lever.