Simple Machines

88thth Grade Science Grade Science

Tahoma Junior HighTahoma Junior High

1

Simple Machines A simple machine is a device that makes work easier.A simple machine is a device that makes work easier.

There are six types of simple machines:There are six types of simple machines:1.1. inclined planeinclined plane2.2. wedgewedge3.3. screwscrew4.4. wheel and axlewheel and axle5.5. pulleypulley6.6. lever (1lever (1stst, 2, 2ndnd and 3 and 3rdrd Class) Class)

making a grand total of 8making a grand total of 8

2

All simple machines transfer force. Some change the direction of force, while others change the magnitude (or strength) of force. Still others change both the direction and the magnitude of force.

3

Most simple machines make work easier by allowing you to use less force to move an object. The catch, (get it? catch) however is that the force must be applied over a longer (greater) distance. Some machines make work easier by allowing you to move things farther and/or faster. In these machines, a larger force is required, but over a shorter distance.

4

Force is measured in a unit called a Newton. It is equal to the amount of force needed to accelerate a 1 kg mass at a rate of 1 meter per second, every second (1 m/s/s or 1m/s2).

The force of an object sitting on the table in front of you would be its mass times the effect of gravity (9.8 m/s2 - or simply 10 m/s2).

A 3 kg mass would exert a force downward of 30 Newtons.

3kg x 10m/s2 = 30N5

The Mechanical Advantage of a simple machine can be calculated by dividing the output force (Fout, or its downward force), by the input force (Fin, or effort put in).

MA = Fout ÷ Fin

If you take the mechanical advantage of a simple machine and multiply it by the

force applied to the machine, you get the amount of force you could use to lift

something!

6

Inclined Plane

7

An inclined plane is really just a ramp (a flat surface that slopes). This type of simple machine is the only one that does NOT move. Instead, objects are moved over it in order to raise them.

It takes less force to move an object up an inclined plane than it does to lift the object straight up.

8

The tradeoff is that the object must be moved a longer (greater) distance - the length of the inclined plane - to be raised the same height.

The mechanical advantage of the inclined plane is found by dividing the length of the ramp by the height.

MA = length height

MA = 10 ÷ 2

MA = 5 9

WorkWork = Force x Distance

If your job is to get the box into the truck, you will do the If your job is to get the box into the truck, you will do the same amount of work if you lift it straight up into the same amount of work if you lift it straight up into the truck – or - roll it up the ramptruck – or - roll it up the ramp ..

If you are using the ramp, the force needed to roll the box If you are using the ramp, the force needed to roll the box up the ramp is less but the distance it must be moved is up the ramp is less but the distance it must be moved is greater.greater.

Without the ramp, the force needed to lift the box is greater Without the ramp, the force needed to lift the box is greater

but the distance up to the truck is less.but the distance up to the truck is less.

You can NOT get Work done for free!You can NOT get Work done for free!There’s always a tradeoff (usually There’s always a tradeoff (usually

distance).distance).10

A wedge is really an inclined plane turned on its side. But instead of helping you move objects to a higher level, a wedge helps you push things apart. The blade of a knife or a shovel are both wedges. A wedge can also be round, like the tip of a nail, or the tines on your fork.

Wedge

MA = Length of Wedge Width of Wedge 11

Basically, the wedge works just like a ramp: the narrower the wedge (or the sharper the point of a wedge), the easierit is to drive it in and push things apart.

But here's the trade-off: to split something apart really wide, you have to push a narrow wedge a longer distance. 12

ScrewScrew

The screw is really a spiraling inclined plane (ramp) with a wedge at the tip. Think of a typical screw. The wedge is the pointed end. The inclined plane is the thread that wraps around the screw. Screws are used in many different places to hold things together.

13

Screw

Basically, a screw is like the ramp — and the Basically, a screw is like the ramp — and the width of the thread is like the angle of an inclined width of the thread is like the angle of an inclined plane. The less threads for a certain distance, the plane. The less threads for a certain distance, the harder it is to turn it.harder it is to turn it.

Here's the trade-off: if you've ever had to put in a Here's the trade-off: if you've ever had to put in a screw with really narrow threads, you've probably screw with really narrow threads, you've probably found that you have to turn it a really long time to found that you have to turn it a really long time to get it to go anywhere. Just like in a ramp, the get it to go anywhere. Just like in a ramp, the easier the effort, the longer the distance you easier the effort, the longer the distance you have to move something! Mechanical Advantage have to move something! Mechanical Advantage of screws is based on how many threads per inch of screws is based on how many threads per inch there are – more threads = more MA (but longer there are – more threads = more MA (but longer distance to turn).distance to turn).

14

Lever In its simplest form, a leverIn its simplest form, a lever is a stick that is free to pivotis a stick that is free to pivot or move back and forth at a or move back and forth at a certain point. Levers are certain point. Levers are probably the most commonprobably the most common simple machine because justsimple machine because just about anything that has a handle on it has a lever attached. about anything that has a handle on it has a lever attached.

The point on which the lever moves is called the fulcrum. The point on which the lever moves is called the fulcrum. By changing the position of the fulcrum, you can gain extra By changing the position of the fulcrum, you can gain extra power with less effort. power with less effort.

There are three different types of levers.There are three different types of levers.15

First Class Lever (fulcrum in middle)

A good example of a first class lever is A good example of a first class lever is a see-saw. A light person can lift a a see-saw. A light person can lift a heavier person simply by doing what heavier person simply by doing what on the see-saw? (yes! – moving on the see-saw? (yes! – moving farther away from the pivot point – or farther away from the pivot point – or fulcrum).fulcrum).

That’s the trade-off: distance for effort , That’s the trade-off: distance for effort , (because by being not only farther (because by being not only farther away from the fulcrum, you’ll also away from the fulcrum, you’ll also have to move up and down more).have to move up and down more).

16

Mechanical Advantage of a lever is easy:

Find the fulcrum and compare distances of Effort Arm (Force going in) and Resistance Arm (Force going out – or overcome).

Effort armResistance arm

17

Effort

forceResistance force

Lever’s MA = effort arm length

resistance arm length

If the effort arm had a length of 12 units and the resistance arm had a length of 6 units, the Mechanical Advantage would be 12/6 = 2. You could lift twice (out) what you apply (in).

Second Class LeverSecond Class Lever(resistance in middle)(resistance in middle)

        1) the Resistance is between the Effort and the Fulcrum.         2) the Fulcrum is at one End of the Lever.         3) the Fulcrum is usually closer to the Resistance, (load).         4) Second Class Levers produce a gain in Force.         5) Examples of Second Class Levers: Wheelbarrow and Nutcrackers.

18

Finding the Mechanical Advantage of a Second Class lever is the same as the others: compare Effort Arm length to Resistance Arm length – but it is a little tougher to figure out which is the Effort Arm and which is the Resistance Arm because they overlap.

In a Second Class lever, the Effort Arm is always longer than the Resistance Arm.

Object to lift Force you apply

Resistance arm Effort arm

19

Resistance force

Effort force

Third Class Lever(effort in middle)

1) The Effort is between the Resistance and the Fulcrum.

2) There is usually a loss in Force, but a gain in Speed and Distance.

3) Examples of Third Class Levers – broom, shovel, and fishing pole.

20

A Third Class lever is much like a Second Class lever except that the Effort Arm is always the shorter of the two. In a Third Class lever, the amount of force you can lift will always be less than what you apply.

The trade-off is that Resistance Arm will move a greater distance – and even though MA is less than 1, it’s better than nothing!

Effort arm Resistance arm

21

Effort force

Resistance force

Wheel and Axle A wheel and axle is really two machines in one because you can use each part in

different ways. The first way is to roll something along – where the wheel

reduces a lot of friction instead of simply dragging. The second way is to apply more force on an axle (which is the smaller “circle” the wheel rotates around).

Think about how hard it would be to turn a water faucet without the larger round handle (or a lever type). Wheels are actually lots of levers working together.

22

A wheel and axle is a lever thatrotates in a circle around a centerpoint or fulcrum. The larger wheel(or outside) rotates around the smaller wheel (axle). Bicycle

wheels, Ferris-wheels and gears are all examples of a wheel and axle.

Wheels can also have a solid shaft with the center core as the axle such as a screwdriver or drill bit or the log in a log rolling contest.

The Mechanical Advantage is found by taking the diameter of the wheel and dividing by the diameter of the axle.

23

The second way of using a wheel is like a lever in the round. A door knob or a faucet on a sink are really round levers, and the "fulcrum" is in the middle where the axle turns. Imagine if a door knob was replaced with a little rod. It would be much harder to open the door! Once again, there's

a trade-off: the larger the diameter of the wheel, the less effort you need to turn it, but you have to move the wheel a greater distance to get the same work done.

24

The Pulley The pulley is really a wheel and axle with a rope or chain The pulley is really a wheel and axle with a rope or chain

attached. A pulley makes work seem easier because it attached. A pulley makes work seem easier because it changes the direction of motion to work with gravity. Let's say changes the direction of motion to work with gravity. Let's say you have to lift a heavy load, like a bale of hay, up to the you have to lift a heavy load, like a bale of hay, up to the second floor of a barn. You could tie a rope to the bale of hay, second floor of a barn. You could tie a rope to the bale of hay, stand on the second floor, and pull it straight up.stand on the second floor, and pull it straight up.

Or you could put a pulley at the Or you could put a pulley at the second floor, stand at the first second floor, stand at the first floor, and lift the bale of hay by floor, and lift the bale of hay by pulling straight down. It would be pulling straight down. It would be the exact amount of work in the exact amount of work in either case, but the action of either case, but the action of pulling down feels easier because pulling down feels easier because you're working with the force of you're working with the force of gravity instead of against it.gravity instead of against it.

25

A pulley really saves effort when you A pulley really saves effort when you have more than one pulley working have more than one pulley working together. By looping a rope around two, together. By looping a rope around two, three, or even four pulleys, you can three, or even four pulleys, you can really cut down on the effort needed to really cut down on the effort needed to lift something. The trade-off? Well, as lift something. The trade-off? Well, as you increase the number of pulleys, you you increase the number of pulleys, you also increase the distance you have to also increase the distance you have to pull the rope. In other words, if you use pull the rope. In other words, if you use two pulleys, it takes half the effort to lift two pulleys, it takes half the effort to lift something, but you have to pull twice as something, but you have to pull twice as much rope (which is twice the distance). much rope (which is twice the distance). Three pulleys will result in one-third the Three pulleys will result in one-third the effort — but the distance you have to effort — but the distance you have to pull the rope is tripled!pull the rope is tripled!

Block and Tackle (many pulleys together)

Mechanical Advantage =

# of moving pulleys26