30 terms

prentice hall exploring physical science ch. 15 3rd edition 1999

Work is done only when a force moves an object, i.e. when you push, lift, or throw an object --
WORK is A FORCE ACTING THROUGH A DISTANCE - you do work whenever you move something from one place to another
work equation
Work = Force X Distance
Force is measured in newtons
Distance is measure in meters
so the unit of work is the newton-meter
in the metric system -- the newton-meter is called the JOULE
A force of 1 newtonexerted on an object that moves a distance of 1 meter does 1 newton-meter, or 1 joule, or work
EXAMPLE of a Joule equation
if you lifted an object weighing 200 N (newtons) through a distance of 0.5 m (meters) . . .
200 N x 0.5 m = 100 J
The rate at which work is done, or the amount of work per unit of time
power equation
Power = Work/Time
can also be written like:
Power = Force X Distance/Time
The unit of work divided by a unit of time/ or the joule per second
One watt is equal to 1 joule per second (1 J/sec)
Large quantities of power are measured in kilowatts (kW)
One kilowatt equals 100 watts
example of power
This is why a bulldozer has more power than a person with a shovel. The bulldozer does more work in the same amount of time. As the process of doing work is made faster, power is increased.
an instrument that makes work easier
work input
the work that goes into the machine -- comes from the force that is applied to the machine, or the effort force.
work output
the machine exerts a force, called an output force, over some distance. The work output is used to overcome the force you and machine are working against.
resistance force
the force you and the machine are working against - often the weight of the object being moved
Do machines increase the work you put into them?
No - the work that comes out of a machine can never be greater than the work that goes into the machine -- like momentum, work is conserved.
How do machines make work easier?
Machines make work easier because they change either the SIZE or the DIRECTION of the force put into the machine.
What you increase in force you pay for in distance, and what you increase in distance is at the expense of force.
EFFICIENCY of the machine
the comparison of work output to work input is called the EFFICIENCY of the machine - thereby knowing how much work is lost to FRICTION.
mechanical advantage
the number of times a machine multiplies the effort force --
The mechanical advantage tells you how much force is gained by using the machine. The more times a machine multiplies the effort force, the easier it is to do the job.
inclined plane
a flat slanted surface. ex. ramp
an inclined plane is a simple machine with no moving parts.
The ramp decreases the amount of force you need to exert, but it increases the distance over which you must exert your force.
YOU solved the problem of how to get the snowblower into the truck for Dad when you used a table in the garage as a ramp.
an inclined plane that moves. Instead of an object moving along the inclined plane, the inclined plane itself moves to raise the object. As the wedge moves a greater distance, it raises the object with greater force. A wedge is usually a piece of wood or metal that is thinner at one end (think of a doorstop at church). The longer and thinner a wedge is, the less the effort force is required to overcome the resistance force. Example - lock or zipper
an inclined plane wrapped around a central bar or cylinder to form a spiral. A screw rotates, and with each turn moves a certain distance up or down. A screw multiplies an effort force by acting through a long distance. The closer together the threads, or ridges, of a screw, the longer the distance over which the effort force is exerted and the more the force is multiplied. Thus the mechanical advantage of a screw increases when the threads are closer together. Ex. -- corkscrew, nut and bolt, faucets, and jar lids
a rigid bar that is free to pivot, or move about, a fixed point -- when a force is applied on a part of the bar by pushing or pulling it, the lever swings about the fulcrum and overcomes a resistance force.
the fixed point that a lever pivots on.
First-class levers
crowbar, seesaw, and pliers -- levers where the fulcrum is between the effort force (your push) and the resistance force (the nail)
Second-class levers
wheelbarrows, doors, nutcrackers, and bottle openers -- The fulcrum is at the end of the lever. The resistance force is the weight of the load. The effort force (at the other end) is the force that you apply to the handles. Because the distance is decreased by the wheelbarrow, force must be increased. A second-class lever does not change the direction of the force applied to it.
Third-class levers
the fulcrum is at the END of the rod where you are holding it -- in a surf-casting rod. The effort force is applied by your other hand as you pull back on the rod. The top of the rod is the resistance force. A lever in the third class reduces the effort force but mutliplies the distance through which the output force moves. Ex. shovels, hoes, hammers, tweezers, and baseball bats. These levers cannot multiply force.
combinations of levers
a pair of scissors, a grand piano, a manual typewriter
a rope, belt, or chain wrapped around a grooved wheel. A pulley can change the direction of a force or the amount of force.
fixed pulley
a pulley that is attached to a structure -- does NOT multiply the effort force - it only changes the direction of the effort force.
movable pulley
Done when you attach a pulley to the object you are moving. For each meter the load moves, the force must pull two meters. This is because as the load moves, both the left and the right ropes move. Two ropes each moving one meter equals two meters.
combining fixed and movable pulleys
greater mechanical advantage can be obtained -- as more pulleys are used, more sections of rope are attached to the system. Each additional section of rope helps to support the object, thus less force is required. A block and tackle is an example of a pulley system.