WORK AND MACHINES
The scientific definition for work …. Work is done when a force causes an
object to move some distance in the same direction of the force.
Example: Pushing a child on a swing,
Pulling books out of a backpack
Lifting a bag of groceries.
Work is calculated as force x distance. The unit for work is joules.
Power is the rate at which work is done. The unit for power is watt.
The formula for power is work divided by time.
A machine is a device with which you can do work in a way that is easier
or more effective.
Machines make work easier by changing force, changing direction and
changing distance.
The force you exert on a machine is called the input force.
The force exerted by a machine is called the output force.
Mechanical advantage (MA) - tells you how many times the machine
multiplies force. In other words, it compares the input force with the
output force.
The formula is output force divided by input force.
Example – MA = 500N/50N = 10
In any machine, some work is wasted overcoming friction.
The comparison of a machine’s output work to its input work is the
machine’s efficiency.
The formula you use to calculate the efficiency of a machine is
output work divided input work x 100. Your answer should include
percent (%).
Example – ME 100/200= 0.5 x 100 =50%
SIMPLE MACHINES
An inclined plane is a flat, slanted surface.
A wedge is a moving inclined plane. It is a device
that is thick at one end and tapers to a thin edge at
the other end.
A screw is an inclined plane wrapped around
a cylinder.
A lever is a rigid bar that rotates about a fixed
point - fulcrum. There are three classes of levers.
(1) first-class – load at one end, fulcrum in middle, effort at other end
examples – seesaw and scissors
(2) second-class – fulcrum at one end, load in the middle and effort at
the other end - examples – wheel barrel and bottle
opener
(3) third-class – fulcrum at one end the effort in the middle and the
load at the other end - examples - tweezer and shovel
A pulley is a rope or a chain wrapped around a wheel.
There are three types:
(1) A fixed pulley only changes the direction of the force. Fixed pulleys
do not increase force.
(2) A movable pulley is attached to the object being moved. Movable
pulleys do increase force.
(3) A block and tackle is created when a fixed and a movable pulley
are used together.
A wheel and axle is a machine composed of two wheels of different
sizes. As the wheel turns, so does the axle. But because the axle is
smaller than the wheel, it rotates through a smaller distance, which
makes the output force larger than the input force.
Examples – doorknob, ferris wheel, screwdriver, steering wheel
Simple machines in the body
Levers – 1st class - Neck the joint at the top of the neck is the fulcrum,
The muscles in the back of your neck provide the input force,
The output force is used to tilt your head back.
-
2nd class The ball of the foot is the fulcrum. The muscle in the
calf of your leg provides the input force. The output force is
used to raise your body.
3rd class – Your elbow is the fulcrum. Your bicep muscle provides
the input force. The output force is used to lift your arm.
Living levers – bones and muscles (tendons)
Working levers – teeth (the incisors).
Compound Machines
Compound machines are machines that are made of two or more simple
Machines.
Examples: block and tackle ( two or more pulleys), can opener
( wheel and axle, wedge), scissor (1st class lever and wedge), zipper
(wedges -one wedge opens the zipper by splitting the teeth apart. Two
other wedges close the zipper by pushing the teeth together)