Grade 8 Science Notes - Simple Machines & Mechanisms PDF
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These notes cover simple machines, including levers, pulleys, inclined planes, wedges and screws, and their ideal mechanical advantages. The document also explains efficiency of machines and how to increase efficiency by reducing friction.
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Unit 5.0 5. 1 Simple Machines and Mechanisms Six Simple Machines - Lever - Inclined plane (ramp) Wheel and axle - Screw - Pulley - Wedge Levers A lever is a rigid bar that is supported at one point. The point it is supported by is called the...
Unit 5.0 5. 1 Simple Machines and Mechanisms Six Simple Machines - Lever - Inclined plane (ramp) Wheel and axle - Screw - Pulley - Wedge Levers A lever is a rigid bar that is supported at one point. The point it is supported by is called the fulcrum. There are three different classes of levers. Three Classes of Levers First-class lever: Always has the fulcrum between the input and output force Second-class lever: The fulcrum is at the very end so the output force is between the input force and the fulcrum. Third-class lever: The input force is between the fulcrum and the output force. Ideal Mechanical Advantage of a Lever The ideal mechanical advantage (IMA) of a lever is calculated by dividing the length of the input arm (Lin) by the length of the output arm (Lout). IMA = Lin/Lout Human Levers A lot of movements by the human body can be explained by comparing them to levers. For example, levers give us the ability to throw a ball. When you throw a ball overhand, your elbow acts as the fulcrum, and your triceps as the input force. The output force is the hand that throws the ball. Pulleys A pulley consists of a grooved wheel with a rope or cable looped around it. A pulley changes the direction of the force so it is easier to lift the load. Ideal Mechanical Advantage of a Pulley System The IMA of a pulley system is equal to the number of ropes in the pulley system. Wheel and Axle The wheel and axle consists of a shaft or axle that is attached to a larger disc called the wheel. An example of a wheel and axle is a doorknob. Ideal Mechanical Advantage of a Wheel and Axle If the input force is applied to the axle, the IMA of a wheel and axle is calculated by dividing the radius of the axle by the radius of the wheel. (axle) IMA = Ra / Rw If the input force is applied to the wheel, the ideal mechanical advantage is calculated by dividing the radius of the wheel by the radius of the axle. (wheel) IMA = Rw / Ra Inclined Planes An inclined plane is a sloping surface on which an object can move. Another name for an inclined plane is a ramp Ideal Mechanical Advantage of an Inclined Plane The ideal mechanical advantage of an inclined plane is the ratio of the length of the slope to the height of the ramp. IMA = length of ramp (l) / Height of ramp (h) The Screw A screw is an inclined plane wrapped around a rod. It is a continuous incline plane that starts at the tip of the screw and gradually gets larger as you get to the head of the screw. The Wedge A wedge is an inclined plane that travels through the object of an area. Some examples of wedges are: an axe, a needle, a knife. Questions 1. a) Pulley b) First class lever c) wedge d) inclined plane 2. Lever A First class Lever B First class Lever C Second class 3. The longer ramp would require less force 4. A mechanism has multiple simple machines working together 5. IMA=3 6. When the distance of the input force increases, the distance of the output force decreases. 5.2 Efficiency Efficiency of Machines The efficiency of a machine measures the amount of useful work done compared to the total work needed to operate the machine. The useful output work is the work that the machine is designed to perform. Work Done by Friction An example of force of friction is when the pulley wheel rotates on its shaft. Since the force of friction is applied to a distance of motion, work is done by the friction force. Work done by the force of friction transforms input energy into heat when the pulley wheel turns. Therefore, extra work must be input into the machine to compensate for the work done by friction. For this reason, the useful output work of a machine is always less than the input work. Calculating Efficiency To calculate the efficiency of a machine, the useful output work is divided by the input work. Efficiency is usually expressed as a percentage. Efficiency = Useful output work (joules) x 100% / Input work (joules) Efficiency = Wout x 100% / Win The Efficiency of Common Mechanisms When a mechanism does work, its energy is transformed from one form to another or transferred from one object to another. How to Increase Efficiency The efficiency of any machine is not 100 % because some of the input work is used to compensate for the work done by friction. If you reduce the frictional force you can increase the efficiency. The best way to reduce friction is to add a lubricant (grease, oil) to surfaces that rub together. Questions 1. A machine of mechanism cannot have an efficiency of 100% because of friction. 2. a) increase b) decrease c) decrease d) stays the same 3. 4. A lubricant would reduce the friction in a machine because it fills the gaps between the surfaces rubbing together so therefore the machine is more efficient. 5. The efficiency of the sharpener is 80% 6. 55.55% or 56%
