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Chapter 4 Machines, Work, and Energy. 4.2 Simple Machines. What is a Machine?. A machine is a device with moving parts that work together to accomplish a task. A bicycle is made of a combination of machines that work together.
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Chapter 4 Machines, Work, and Energy 4.2 Simple Machines
What is a Machine? • A machine is a device with moving parts that work together to accomplish a task. • A bicycle is made of a combination of machines that work together. • All the parts of a bicycle work as a system to transform forces from your muscles into motion. • A bicycle allows you to travel at faster speeds and for greater distances than possible on foot.
The Concepts of Input and Output • Machines are designed to do something. • The input includes everything you do to make the machine work, like pushing on the bicycle pedals. • The output is what the machine does for you, like going fast or climbing a steep hill. • The input and output may be force, power, or energy.
The Beginning of Technology • A simple machine is an unpowered mechanical device that accomplishes a task with only one movement (such as a lever). • A lever allows you to move a rock that weighs 10 times (or more) what you weigh.
Input Force and Output Force • Simple machines work with forces. • The input force is the force you apply to the machine. • The output force is the force the machine applies to what you are trying to move.
Ropes and Pulleys • A rope and pulley system is a simple machine made by connecting a rope to one or more pulleys. • You apply the input force to the rope and the output force is exerted on the load you are lifting.
Machines within Machines • Most of the machines we use today are made up of combinations of different types of simple machines.
Bicycle uses wheels and axels, levers (the pedals and kickstand), and gears. A complex machine like a VCR contains simple machines of every type including screws, ramps, pulleys, wheels, gears, and levers. Types of Machines
Ratio of Output to Input Force • The mechanical advantage of a machine is the ratio of the output force to the input force. • If the mechanical advantage of a machine is larger than one, the output force is larger than the input force. • A mechanical advantage smaller than one means the output force is smaller than the input force. • Mechanical advantage is a ratio of forces, so it is a pure number without any units.
Input and Output Work • A simple machine does work because it exerts forces over a distance. • If you are using the machine you also do work, because you apply forces to the machine that move its parts.
Definition of a Simple Machine • A simple machine has no source of energy except the immediate forces you apply. • The only way to get output work FROM a simple machine is to do input work ON the machine. • The output work done by a simple machine can never exceed the input work done on the machine.
Perfect Machines • In a perfect machine, the output work equals the input work. • Friction always converts some of the input work to heat and wear, so the output work is always less than the input work.
The Cost of Multiplying Force • The output work of a machine can never be greater than the input work. • This rule is true for all machines. • The force and distance are related by the amount of work done. • In a perfect (theoretical) machine, the output work is exactly equal to the input work.
Force or Distance • Many problems give three or four quantities: input force, input distance, output force, and output distance. • If the input and output work are equal then force x distance at the input of the machine equals force x distance at the output.
Parts of the Lever • All levers include a stiff structure that rotates around a fixed point called the fulcrum. • The side of the lever where the input force is applied is called the input arm. • The output arm is the end of the lever that applies the output force. Effort Load Fulcrum
Changing Direction • When the fulcrum is in the middle of the lever, the input and output forces are the same. • The input and output forces are different if the fulcrum is not in the center of the lever. • The side of the lever with the longer arm has the smaller force. Input work Output work di x Fi = do x Fo di = Fo do Fi
Mechanical Advantage of a Lever • The output work is the output force multiplied by the output distance. • The input work is the input distance multiplied by the input force.
Mechanical Advantage of a Lever • By setting the input and output work equal, you see that the ratio of forces is the inverse of the ratio of distances. • The larger (input) distance has the smaller force. • The ratio of distances is equal to the ratio of the lengths of the two arms of the lever.
Three Types of Levers • Levers are used in many common machines: pliers, wheelbarrow, and the human biceps and forearm. • The mechanical advantage is always the ratio of lengths of the input arm to the output arm.
How a Rope and Pulley System Works • The force in a rope is called tension and is a pulling force that acts along the direction of the rope. • The tension is the same at every point in a rope. • If the rope is not moving, its tension is equal to the force pulling on each end. • Ropes or strings do not carry pushing forces.
Mechanical Advantage • The mechanical advantage of a pulley system depends on the number of strands of rope directly supporting the load. • To make a rope and pulley system with a greater mechanical advantage, you can increase the number of strands directly supporting the load by taking more turns around the pulleys.
Gears and Ramps • Many machines require that rotating motion be transmitted from one place to another. • The transmission of rotating motion is often done with gears. • Some machines that use gears, such as small drills, require small forces at high speeds. • Other machines, such as the paddle wheel on the back of a steamboat, require large forces at low speed.
How Gears Work • The rule for how two gears turn depends on the number of teeth on each gear.
Ramps • A ramp is another type of simple machine that allows you to push a heavy object to a higher location with less force than is needed to lift the object straight up. • Ramps reduce the input force needed by increasing the distance over which the input force acts. • The output work is work done against gravity.
Mechanical Advantage of a Ramp • The input work is the input force multiplied by the length of the ramp. 10m 1m Ramp Mechanical advantage = ramp length height
Screws • A screw is a simple machine that turns rotating motion into linear motion. • A screw works just like a ramp that curves as it get higher. • The “ramp” on a screw is called a thread.