SWMS Science Department

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1 Honors Physical Science Chapter 9 Simple Machines - Notes Section 1- Types of Simple Machines A machine is a device with moving parts that work together to accomplish a task. A machine accomplishes output work when input work is applied to it. Input: forces, energy, or power supplied to make a machine accomplish a task Output: forces, energy, or power provided by the machine Simple Machines A simple machine is an unpowered mechanical device that accomplished a task with only one movement. Types of simple machines: wheel and axle, wedge, rope and pulley, ramp, gears, and lever. Honors Physical Science Chapter 9 - Mrs. Ramirez s Class Page 1

2 Section 1- Types of Simple Machines - Continued 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. Lever: a stiff structure that rotates around a fixed point called a fulcrum Gears: a rotating wheel with teeth that transfers motion and forces to other gears or objects Most of the machines that we use today are a combination of simple machines so we refer to them as complex machines. What do simple machines do for us? Simple machines do work because they apply a force over a distance. If you are using the machine you also do work, because you apply force to the machine to make it move. The only way to get output from a simple machine is to do input work on the machine. Output work done on a machine can never exceed the input work done on the machine. In a perfect machine the output exactly equals the input work. There are no perfect machines because friction always converts some of the input work to heat and wear. Honors Physical Science Chapter 9 - Mrs. Ramirez s Class Page 2

3 Section 2 Mechanical Advantage Another way to say machines help us lift more than we can lift is to say machine multiply forces. Mechanical Advantage: the ratio of output force divided by the input force If Mechanical Advantage is >1, then output force is > input force <1, then output force is < input force =1, then output force is = input force Levers Levers are useful because you can arrange the fulcrum and the input and output arms to adjust the mechanical advantage of the lever. By changing the position of the fulcrum, you can alter the amount of input force needed compared to the output force desired. The length of the lever arm is indirectly related to the corresponding force. Using the length of the lever arms, mechanical advantage can also be calculated by dividing the length of the input arm by the length of the output arm. Honors Physical Science Chapter 9 - Mrs. Ramirez s Class Page 3

4 Section 2 Mechanical Advantage - Continued Gears Gears can transfer motion and force when the teeth of one gear press on the teeth of another gear as each gear rotates around a shaft. Gears work like rotating levers; the tip of the tooth of the gear is like the end of a lever and the shaft of the gear is like the fulcrum. Gear Ratio: the ratio of output turns to input turns for a pair of gears The gear ratio can also be calculated as the ratio of the number of teeth on the input gear versus the number on the output gear. The mechanical advantage of the pair of gears is the inverse of the gear ratio. Rope and Pulley Systems Ropes and strings carry tension forces along their length. 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. The mechanical advantage of a rope and 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. Ramps A ramp is a simple machine that allows you to raise a heavy object, such as a wheeled cart, with less force than you would need to lift is straight up. Ramps reduce the input force by increasing the distance over which the input force acts. The mechanical advantage of a ramp is the length divided by the height of the ramp. A screw is a type of ramp that turns rotating motion into linear motion. You can find the mechanical advantage of a screw by dividing its circumference by the lead ( lead of a screw is the distance it advances in 1 turn). Honors Physical Science Chapter 9 - Mrs. Ramirez s Class Page 4

5 Section 2 Mechanical Advantage - Continued Wedges A wedge is like a ramp that can work while in motion (a ramp is always stationary). A wedge has a side that slopes to a thin edge. The mechanical advantage of a wedge is inversely related to the size of the wedge angle. Sharp wedges (small angles) produce large forces. Also, when the surface of the wedge is rough (producing high friction), a wedge provides a large holding or stopping force thus preventing other objects from moving. Wheels A wheel rotates around a rod called an axle. The wheel and axle work together to move or lift loads The mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to the radius of the axle. By applying a force to the wheel to turn the axle, output force is increased. In this case, distance and speed are decreased. If you want to increase distance and speed, you apply a force to the axle in order to turn the wheel. In this case, force output is decreased. Honors Physical Science Chapter 9 - Mrs. Ramirez s Class Page 5

6 Section 3 Levers in the Human Body Honors Physical Science Chapter 9 - Mrs. Ramirez s Class Page 6