Work & MachinesChapter 5 Notes

Work and Power Work is done if (1) an

object moves, and (2) if a force acts in the same direction that the object moves.

Work = Force times distance W = (F)(d)

Work is measured in units called Joules (J).

Time is not important when you’re measuring work. The same amount of work is done whether you do it slowly or quickly.

Power measures how fast you can do work.

Power = Work divided by time P = W / t

Another formula used to calculate power is:

P = (F)(d) / t

Work and Machines A machine is a device that

makes work easier. It does not let you do less work.

When you use a machine, two works are done! The work you do to the machine is called input work, and the work the machine does to the resistance is called output work.

The force you use on the machine is the effort force, and the distance you move is the effort distance.

The weight of the object being moved is the resistance force, and the distance the object moves is the resistance distance.

Work input = effort force times effort distance. Wi = (Fe)(de)

Work output = resistance force times resistance distance.

Wo= (Fr)(dr)

Mechanical Efficiency Machines make it easier to

do work, but you always put more work into a machine than you get out of it!

This is because some work is used to overcome friction!

Mechanical efficiency of a machine is always less than 100% because of friction.

Even though it’s easier to cut the wood with the saw than without it, the worker does more work using the saw than he would have done without it.

M.E. = work output x 100%

work input

Mechanical Advantage Mechanical advantage tells how many times a machine

multiplies your effort force. Mechanical advantage can also change the direction of

your effort force. You can find the mechanical advantage of any simple

machine by dividing the resistance force by the effort force.

Some simple machines have other formulas that can also be used to find M.A.

M.A. = resistance force

effort force

Simple Machines: Inclined Plane Simple machines do

work with one movement.

There are six kinds of simple machines: inclined plane, wedge, screw, lever, wheel and axle, and pulley.

An inclined plane is a ramp. You use less force to pull

something heavy up a ramp than you would use if you tried to lift it.

An inclined plane does not make you do less work. It lets you use less effort force, but you have to move a greater distance!

Simple Machines: Wedge and Screw

A wedge is an inclined plane that can move.

An axe is a wedge, and so is a chisel, a knife, or a wood plane. The effort force used to split wood is great. When you use a wedge, you use less effort force, but move a greater distance.

A screw is an inclined plane that is turned in a circle. Car jacks are screws.

You turn a jack handle many times to lift the car a small amount, but the force you need to turn the handle is much less than would be needed to lift the car yourself!

Simple Machines - Levers Machines that do work by

moving around a fixed point are called levers.

There are three classes of levers, depending on the location of the fulcrum, effort force, and resistance force.

The balance point of a lever is called the fulcrum.

The mechanical advantage of a lever = __effort arm length__

resistance arm length

Resistance Arm

Fulcrum

Effort Arm

Simple Machines - Levers The fulcrum is between the

effort force and the resistance force in a first class lever.

The effort force is between the fulcrum and the resistance force in a third class lever.

The resistance force is between the effort force and the fulcrum in a second class lever.

First Class LeverFulcrum

Resistance

Effort

Second Class Lever

Fulcrum

ResistanceEffort

Third Class Lever

Resistance

Effort Fulcrum

Simple Machines - Wheel and Axle A wheel and axle

consists of two circular objects that share the same center.

The larger circle is the wheel and the smaller circle is the axle.

The mechanical advantage of a wheel and axle is the radius of the wheel divided by the radius of the axle.

M.A. = radius of wheel

radius of axle

Wheel

Axle

Simple Machines - PulleysA pulley is a rope

wrapped around a grooved wheel.

The two main types of pulleys are fixed pulleys and moveable pulleys.

To find the mechanical advantage of a pulley system, count the ropes that support the resistance!

Efficiency