Work 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 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 EffortResistance = the force a machine has
to overcomeEffort = the force appliedUsing 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
WorkEx: Suppose you lift a box weighing 450
Newtons up onto a 2 meter high platform. To light 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 = 450 N x 2 m = 900 J
Work 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: W= f x d = 100 N x 9m = 900 J
So the TOTAL amount of work is the SAME using the pulley versus not using the pulley
6 Simple MachinesLeverPulleyWheel and axle Inclined planeWedgeScrew
LeverConsists of a rigid bar that can turn
around a point called a fulcrum Ex: pliers, crowbar
PulleyA 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 axleA 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
Inclined planeA flat surface with one end higher than
the other. Ex: wheelchair ramp, truck ramp
WedgeDouble-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
ScrewAn inclined plane wrapped around a
wedge or cylinder Ex: wood screws, bolts, car jacks
Complex MachinesContains 2 or more simple machines
Ex: a bike has a wheel and axle, a pulley system and a lever.
Efficiency of machinesMachines are never 100% efficientThe amount of work done by any
machines is ALWAYS less than the amount of work put into it
Some work is converted into heat energy and is wasted. Heat energy is produced because of rubbing together of the machine’s parts (friction).
Machine efficiencyA 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.