The Harker School

7^th Grade Science

Mrs. Raji Swaminathan

Chapter 15 Work, Power and Simple Machines (15-3 and 15-4)

15-3 Machines

Name some machines and describe how they make work faster and / or easier.

Computers, sewing machines, cars, planes, washing machines, lawn mowers etc.

Also pliers, nutcrackers, fishing poles, screw drivers, shovels, spoons, door knobs etc.

The word ‘machine’ has a well-defined meaning in science that is different from the way the word is used in everyday life.

Machine – A device that makes work easier.

See fig. 15-6 on page 377.

How do Machines make work easier?

Two types of work involved in using a machine.

The work that goes into the machine.

The work done by the machine.

Work Input (W_I) – work done on a machine.

Work Output (W_O) – work done by a machine.

Machine Vocabulary

Effort force - when you use a machine, you supply the effort force.

You put work onto the machine by exerting this force over a distance.

Effort Force (F_E) – the force applied to a machine.

Effort Distance (D_E) – the distance through which the machine moves.

Work Input = Effort Force x Effort Distance

W_I = F_E x D_E

Resistance Force (F_R) – the force applied by a machine to overcome resistance.

Resistance Distance (D_R) – the distance through which the object moves.

Work Output = Resistance Force x Resistance Distance

W_o = F_R x D_R

E.g. When you use a shovel to move a rock, your effort is opposed by the rock’s weight.

The rock’s weight is the resistance force.

How do machines make work easier?

Machines do not increase the work you put into them.

Work output = work input.(ideal machine)

Work is conserved.

Machines make work easier because they change either the size or the direction of the force put into the machine.

Any change in the size of a force is accompanied by a change in the distance through which the force is exerted.

If a machine multiplies the force you put into it, the resistance force will be exerted over a _________ distance.

If a machine exerts an resistance force over a longer distance than the effort force, the resistance force will be ______ than the effort force.

See fig. 15-7 on page 378.

Because work is conserved, if the resistance force is increased – the resistance distance is ________.

Also, if the resistance distance is increased, the resistance force is _________.

Machines make work easier by multiplying either force or distance , but never both.

Efficiency of a machine

How can you be more efficient in your daily life? ____________

The work output cannot be greater than the work input.

In most cases, the work output < the work input.

This is because some of the work the machine does is used to overcome friction.

Anything that reduces friction such as keeping a machine well lubricated increases efficiency.

The comparison of work output to work input.

Efficiency = (work output / work input) x 100

Efficiency is always expressed as a percentage.

Work output is never greater than work input.

What is the efficiency of a machine where work input is 200 J, and work output is 100 J ? ___________________

Mechanical Advantage

Mechanical Advantage: the number of times a machine multiplies the effort force.

Mechanical Advantage = Resistance Force / Effort Force

MA = F_R / F_E

The more times a machine multiplies the effort force, the easier it is to do the job.

Mechanical Advantage

When the resistance force is greater than the effort force, the mechanical advantage is ___________.

When the resistance force is less than the effort force, the mechanical advantage is ______________.

MA is equal to 1 - the machine is used to change the direction of the effort force.

MA is less than 1 - the machine is used to increase the distance an object moves or the speed at which it moves.

Questions ???

You use a pair of pliers to crack a pecan. It takes 1200 N of resistance force to crack the pecan, but you only exert 400 N of effort force with your hand on the pliers. What is the mechanical advantage of the pliers? ____________________

A worker applies an effort force of 20N to pry open a window that has a resistance force of 500N. What is the M.A. of the crowbar? ______________

Find the effort force needed to lift a 2000 N rock, using a jack with a mechanical advantage of 10. _____________

Mini QUIZ on simple machines

A simple machine does work with only one ____________.

The force applied to a machine is called the ____________.

The force applied by a machine is called the __________.

The number of times a machine multiples the effort force is the __________________ of the machine.

15-4 Simple and Compound Machines

InventorsToolbox: The Elements of Machines

There are six types of simple machines:

the inclined plane,

the wedge,

the screw,

the lever,

the pulley, and

the wheel and axle.

Inclined Plane

Inclined Plane – a ramp; a slanted surface used to raise an object.

Advantage – less effort force.

Disadvantage – more effort distance.

Mechanical advantage = Effort distance / Resistance distance.

M.A. = d_E / d_R OR ramp length / height.

Because the length of the inclined plane can never be less than the height, the M.A. can never be less than one, and the effort force is always multiplied. See fig. 15-11 on page 381.

The less slanted the inclined plane, the longer the distance over which the effort force is exerted and the more the effort force is multiplied.

The mechanical advantage of an inclined plane increases with as the slant of the plane decreases.

Wedge

Wedge – an inclined plane that moves.

Most are made of two inclined planes back to back.

Examples – knife, ax, razor blade, chisel, nail.

See fig. 15-12 on page 381.

Effort force applied to the thicker end is transferred to the thinner end.

The longer and thinner the wedge, the less effort force is required to overcome a resistance force (the object you are cutting).

You can improve mechanical advantage of a knife or an ax by ___________ it.

Mechanical Advantage is length / width or d_e / d_r..

Wedge – Lock and key

A lock uses the principle of wedge.

On a key, the edges go up and down in a certain pattern.

The edges are a series of wedges.

The wedges lift up a number of pins of different lengths inside the lock.

When all of the pins are lifted to the proper height, (accomplished by the shape of the key), the lock opens.

Howstuffworks "How Lock Works“ – animation.

Wedge - Zippers

Howstuffworks "How Zippers Work“ – animation.

Zipper – application of wedge.

The part of the zipper you pull up or down has 3 small wedges.

These wedges turn the weak effort force with which you pull into a strong force that either joins or separates the 2 rows of teeth.

Without these wedges you would not be able to use the zipper.

See fig. 15-13 on page 382.

Screw

Screw – an inclined plane wrapped around a cylinder to form a spiral.

It multiplies effort force by acting through a long distance.

E.g. wood screw, corkscrew, jar lid, faucets.

Do the activity of making a screw with paper and pencil.

Less effort is required because of the long distance through which the screw is turned.

The closer and less steep the threads or ridges of the screw are, the greater the mechanical advantage because the longer the distance over which the effort force is exerted and the more the force is multiplied.(see fig. 15-15 on page 383).

Lever

“Give me a lever and a place to stand and I will move the Earth!” - _______

Have you ….

Played on a seesaw ….

Pried open a can with a screwdriver ….

Opened or closed a door ….

Cracked open a walnut shell with a nut cracker ….

Used a wheelbarrow to move rocks and soil ….

If yes, you have worked with a LEVER.

Lever – a rigid bar that is free to pivot, or move about, a fixed point when effort force is applied.

Fulcrum – the fixed point about which the lever pivots.

E.g. use of a crowbar to remove a nail from a piece of wood.

You push down on one end of the crow bar (_________), the nail moves in the opposite direction (up).

The crowbar changes the direction of the force.

You push down on the crowbar through a longer distance than the nail moves up.

Because work is conserved, the crowbar multiplies the effort force that you apply.

In the case of the crowbar below, the fulcrum is between the effort force (your push) and the resistance force (the stone).

Three classes of Lever

Three classes of levers -- based on

the position of the effort force,

the resistance force, and

the fulcrum.

First Class Lever

First class lever -- the fulcrum is between the effort force and the resistance force.

Examples: crowbar, pliers, scissors, seesaws.(see fig. 15-17 first part on page 384).

The first class lever multiplies the effort force and changes its direction.

See fig. 15-16 on page 383. What kind of simple machine is a seesaw? ______

The distance from the effort force to the fulcrum is called the effort arm.

The distance from the resistance force to the fulcrum is the resistance arm.

The mechanical advantage of any lever = effort arm length / resistance arm length.

Mechanical Advantage for a first class lever is usually greater than one if the fulcrum is closer to the resistance force than the effort force (resistance arm < effort arm). See fig. 15.18 on page 384.

FearOfPhysics.com: Kids Playing on a Seesaw – cool website with seesaw applet!

Second Class Lever

Second class lever – the resistance force is between the effort force and the fulcrum.

Second Class Lever

Examples: wheelbarrows, doors, nutcrackers, bottle openers.(see fig. 15-17 2^nd part on page 384).

In a wheelbarrow, the effort force is exerted over the distance you lift the handles.

The load moves a much shorter distance than you actually lift the handles.

Since distance is decreased, force must be increased.

They always multiply effort force but do not change the direction of the force.

Their mechanical advantage is always greater than one.

Identify the fulcrum and the forces

Door:

Fulcrum – _________

Effort force – _____________

Resistance force - ____________

Type of lever - _____________

Third Class Lever

Activity: Make a Japanese folded fan out of a piece of paper by folding it in strips back and forth.

Grasp one end of the fan and use the fan to cool yourselves.

Third class lever -- the effort force is between the resistance force and the fulcrum.

See fig. 15-17, 3^rd part, on page 384.

A fishing rod is a 3^rd class lever.

The fulcrum is at the end of the rod, where you are holding it.

The effort force is applied by your other hand as you pull back on the rod.

At the top of the rod is the resistance force.

The effort force is applied over a short distance, but the end of the rod moves over a longer distance.

A 3^rd class lever reduces the effort force but multiplies the distance through which the output force moves.

E.g. shovels, hoes, hammers, tweezers, baseball bats.

The mechanical advantage is always less than one.

Levers – Mechanical Advantage

M.A. = Effort Arm / Resistance Arm.

Activity: Do the activity from page 385 called Levers.

For 1^st and 2^nd class levers, less effort force can move the same load, if that force is applied farther away from the fulcrum.

As long as the resistance force (load) is closer to the fulcrum than the effort force, the lever will multiply the effort force.

E.g. see fig. 15-18 on page 384.

3^rd Class Levers – M.A.

For 3^rd class levers, the resistance arm is always greater than the effort arm.

So these levers do not multiply force.

They only multiply distance.

Combination of Levers

Scissors – combination of 2 first class levers.

Fulcrum – ______________

Effort force – ______________

Resistance force – ______________

Each key on a piano is linked to a complex system of levers.

The levers transmit movement from the player’s fingers to the felt-tipped hammer, which strikes the tight piano wire and sounds a note.

They multiply movement.

A manual typewriter uses a similar principle.

Questions on Levers ???

A worker uses an iron bar to raise a manhole cover weighing 65 N. The effort arm of the lever is 60 cm long. The resistance arm is 10 cm long. What is the M.A. of the bar? ______________

Pulley

Pulley -- a chain, belt, or rope wrapped around a wheel.

They change either the direction of the force or they multiply the effort force.(see fig. 15-20 on page 386.)

Fixed Pulley

There are two types, fixed and moveable pulleys.

Fixed Pulley -- a pulley that is attached to a stationary structure like a wall.

They cannot multiply effort force.

They only change the direction of the effort force.(see fig. 15-20 on page 386).

Because work output cannot be greater than work input, if the pulley does not change the amount of force, it does not change the distance the force moves.

So the distance you pull is the same as the distance the object moves.

The mechanical advantage of a pulley system is approximately equal to the number of support ropes.

Mechanical Advantage

The mechanical advantage of a fixed pulley as in fig. 15-20 on page 386 is _____________.

Movable Pulley

Moveable Pulley -- a pulley that is attached to the object that is moved.

For each meter the load moves, the force must pull 2 meters. (as the load moves, both the left and right ropes move).(see fig. 15-21 on page 387).

To lift an object 1 meter (resistance distance), you might have to pull the rope 2 meters (effort distance).

They multiply effort force.

They cannot change the direction of the effort force.

Pulley System

A pulley system made up of several fixed and moveable pulleys is shown on fig. 15-22 on page 387.

As more pulleys are used, more sections of rope are attached to the system, which help to support the object.

What is the mechanical advantage of the largest pulley system shown in fig. 15-22 on page 387? _______________

Wheel and Axle

A wheel and axle is made up of 2 circular objects of different sizes.

Wheel – larger object.

Axle – smaller object.

The wheel turns about the axle.

Its advantage depends on whether the effort force is applied to the wheel or to the axle.

If effort force is applied to the wheel, effort force is multiplied because the wheel moves a greater distance than the axle.

The mechanical advantage is greater than one.

M.A. = radius of wheel / radius of axle

If effort force is applied to the axle, speed and distance are multiplied. The mechanical advantage is less than one.

M.A. = radius of axle / radius of wheel

Compound Machines

Compound Machines – combinations of two or more simple machines.

Examples: car, bicycle, can opener, typewriter, watch.