Key Concepts of Simple Machines

Why This Matters

Simple machines aren't just historical curiosities—they're the foundation of every mechanical system you'll encounter on your Physical Science exam. When you understand how a lever multiplies force or why an inclined plane trades distance for effort, you're grasping the core principle of mechanical advantage: the idea that machines don't create energy, they transform it. These concepts connect directly to work, force, and energy conservation—topics that appear repeatedly in multiple-choice questions and FRQs.

You're being tested on your ability to recognize force multiplication, direction change, and the work-distance tradeoff in real-world scenarios. Don't just memorize that a pulley lifts things—know that a movable pulley cuts the required force in half because you're pulling rope twice the distance. Each simple machine illustrates a specific physics principle, and understanding the "why" behind each one will help you tackle any problem the exam throws at you.

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Force Multipliers: Trading Distance for Power

These machines reduce the force needed to do work by increasing the distance over which that force is applied. The work stays the same, but the effort feels easier because it's spread out.

Lever

• Mechanical advantage depends on fulcrum placement—the closer the fulcrum is to the load, the less force you need to lift it
• Three classes exist based on the arrangement of effort, load, and fulcrum: Class 1 (seesaw), Class 2 (wheelbarrow), Class 3 (fishing rod)
• Amplifies force by allowing a small input force to move a larger load, following the principle $F_1 \times d_1 = F_2 \times d_2$

Inclined Plane

• Reduces required force by spreading the lifting effort over a longer distance—the longer the ramp, the less force needed
• Steepness tradeoff: a gentler slope means less force but more distance traveled, while steeper means more force over shorter distance
• Work remains constant because $W = F \times d$; you're just changing how that work is distributed

Screw

• Inclined plane wrapped around a cylinder—each turn moves the screw forward by one pitch (the distance between threads)
• Pitch determines mechanical advantage: smaller pitch means more turns required but less force per turn
• Converts rotational motion to linear motion, which is why screws are excellent for holding materials together with sustained pressure

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Direction Changers: Redirecting Force

These machines make work easier primarily by changing the direction of the applied force, allowing you to pull down to lift up or push in a more convenient direction.

Pulley

• Fixed pulleys change direction only—you pull down on the rope to lift a load up, but the force required stays the same
• Movable pulleys reduce force by distributing the load across multiple rope segments; each supporting rope cuts the effort in half
• Block and tackle systems combine fixed and movable pulleys to achieve high mechanical advantage, calculated by counting supporting rope segments

Wheel and Axle

• Force applied to the wheel rotates the smaller axle, multiplying torque—the larger the wheel relative to the axle, the greater the advantage
• Reduces friction compared to dragging objects, which is why wheels revolutionized transportation
• Works in reverse too: applying force to the axle (like a screwdriver handle) increases speed at the wheel's edge

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Splitting and Penetrating: Concentrated Force

These machines take a broad input force and concentrate it into a narrow edge, dramatically increasing pressure at the point of contact.

Wedge

• Converts lateral force into splitting force—the narrower the angle, the greater the splitting power but the more fragile the edge
• Double inclined plane in structure; force applied to the thick end gets redirected perpendicular to the sloped surfaces
• Applications include cutting and separating: knives, axes, doorstops, and even your front teeth function as wedges

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Quick Reference Table

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Self-Check Questions

1. Which two simple machines are both based on the inclined plane, and how does their motion differ?

2. A student uses a ramp to push a heavy box into a truck. If she makes the ramp twice as long, what happens to the force required and the distance traveled? Explain using the work equation.

3. Compare a fixed pulley and a movable pulley: which one provides mechanical advantage greater than 1, and why?

4. An FRQ shows a diagram of a car jack. Which simple machine principle does it demonstrate, and how does the pitch of its threads affect the force needed to lift the car?

5. A doorknob and a screwdriver both use the wheel and axle principle. In which device do you apply force to the "wheel" part, and how does this affect the mechanical advantage?