Making work easier
Imagine lifting a car by hand. Impossible! But with a jack, you can do it. A simple machine makes work easier by changing the size or direction of a force. It doesn’t reduce the total work, though. There’s always a trade-off: less force means more distance.
The six simple machines
| Machine | Description | Examples |
|---|---|---|
| Lever | Bar pivoting on a fulcrum | Seesaw, crowbar, scissors, broom |
| Wheel and axle | Wheel fixed to a smaller rod | Doorknob, steering wheel, screwdriver |
| Pulley | Wheel with a rope in a groove | Flagpole, crane, window blinds |
| Inclined plane | Sloping surface | Ramp, wheelchair ramp, slide |
| Wedge | Two inclined planes back to back | Axe, knife, doorstop, zipper |
| Screw | Inclined plane wrapped around a cylinder | Jar lid, bolt, corkscrew |
Input and output
- Input force (effort): the force you apply.
- Output force (load): the force the machine applies.
- Mechanical advantage (MA): how many times the machine multiplies your force.
MA = output force ÷ input force
If a machine lets you lift 600 N with 200 N of effort, MA = 600 ÷ 200 = 3.
Ideal mechanical advantage (IMA), ignoring friction, can be found from distances:
IMA = input distance ÷ output distance
Levers
A lever has three parts: the fulcrum, the effort and the load. The arrangement makes three classes.
| Class | Order | Examples |
|---|---|---|
| First class | Fulcrum in the middle (effort – fulcrum – load) | Seesaw, crowbar, scissors |
| Second class | Load in the middle (fulcrum – load – effort) | Wheelbarrow, nutcracker, bottle opener |
| Third class | Effort in the middle (fulcrum – effort – load) | Broom, fishing rod, your forearm |
The farther the effort is from the fulcrum, the less force you need.
Law of the lever: effort × effort arm = load × load arm
Example: A 300 N load is 0.5 m from the fulcrum. How much effort is needed 1.5 m away?
Effort = 300 × 0.5 ÷ 1.5 = 100 N
Third-class levers have MA less than 1. They don’t reduce force, but they increase speed and distance, which is useful for swinging a bat.
Pulleys
| Type | What it does | MA |
|---|---|---|
| Fixed pulley | Changes direction only | 1 |
| Movable pulley | Halves the force needed | 2 |
| Block and tackle (combined) | Large force reduction | Number of rope segments supporting the load |
Pulling down on a flagpole rope to raise a flag uses a fixed pulley: it’s easier to pull down than to lift up.
Inclined planes, wedges and screws
An inclined plane spreads the work over a longer distance:
IMA = length of ramp ÷ height
A ramp 6 m long rising 1 m has IMA = 6. You push with about 1/6 of the force, but over 6 times the distance.
A wedge turns a downward force into sideways forces that split things apart. A screw is a spiral ramp; the closer the threads, the higher the mechanical advantage.
Wheel and axle
Turning the large wheel moves the small axle with more force. The ratio of radii gives the IMA:
IMA = wheel radius ÷ axle radius
A screwdriver handle is the wheel; the shaft is the axle.
Compound machines
A compound machine combines two or more simple machines.
- A bicycle: wheels and axles, levers (brake handles), pulleys and gears (chain)
- Scissors: two levers plus two wedges (the blades)
- A can opener: lever, wheel and axle, and wedge
Efficiency
Real machines lose some energy to friction, so actual MA is less than ideal MA.
efficiency = actual MA ÷ ideal MA × 100%
Common mistakes
- Thinking machines reduce work. They reduce force but increase distance; work stays the same (or increases with friction).
- Mixing up lever classes. Ask: what is in the middle?
Try it yourself
- Use a ruler as a lever over a pencil fulcrum. Balance coins on each side at different distances. Record the pattern.
- A ramp is 10 m long and 2 m high. What is its IMA?
- Classify: tweezers, a wheelbarrow, a seesaw.
- Name three simple machines in a pair of scissors or a bicycle.
(Answers: IMA = 5; third class, second class, first class)
Quick recap
- Simple machines change the size or direction of a force.
- The six are the lever, wheel and axle, pulley, inclined plane, wedge and screw.
- Mechanical advantage = output force ÷ input force.
- Less force always means more distance; work is not reduced.
- Compound machines combine simple machines.