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.

Types of 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.

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.