Chapter 7 Review

Why we use machines A machine is a device that makes work easier. A simple machine is a device that does work with only one movement. There are 6 types of simple machines… Lever Pulley Inclined Plane Screw Wedge Wheel and axle

Advantages of Simple Machines A machine makes work easier by changing the force you exerted on it in size, direction or both. When you use a simple machine, you are trying to move something that resists being moved. The weight of the object (the force that gravity exerts on the object) is what makes the object resist the work. The machine allows us to move the object regardless of the resistance.

Advantages cont’d… Two forces are involved when a machine is used to do work. Effort Force (Fe) = the force applied to the machine Resistance Force (Fr) = the force applied my the machine to overcome resistance due to gravity or friction. Ex. To open a can lid…you apply the effort force on the screwdriver handle. The screwdriver applies the resistance force to the lid.

Work done by the machine… Work done on the machine is called Work input (Win) Work done by the machine is called Work output (Wout) Win = Fe x de Wout = Fr x dr You can never get more work out of a machine than you put in. (Wout) can never be greater than (Win)

Machines cont’d… In an ideal machine, Win = Wout In this system Fe x de = Fr x dr In most cases, a machine multiplies the force applied to it, Fr is greater than Fe The machine multiplies your effort but you must moe the handle a greater distance. Mechanical Advantage (MA) = the number of times a machine multiplies the effort force MA = resistance force/effort force =Fr/Fe

6 simple machines Levers Pulleys Wheel and Axle Inclined Plane Screw Wedge

6 types of simple machines

Levers A bar that is free to pivot or turn about a fixed point. The fixed point is called a fulcrum. The part of the lever on which the effort force is applied is called the effort arm. The part of the lever that exerts the resistance force is called the resistance arm. MA = length of effort arm length of resistance arm

Pulleys A grooved wheel with a rope or a chain running along the groove. Can be fixed or movable Fixed pulley – attached to something that doesn’t move. Movable pulley – attached to the object being moved. Fixed and moveable pulleys can be combined to form a block and tackle.

Wheel and Axle A machine consisting of two wheels of different sizes that rotate together. Effort force is usually applied to the larger wheel. The resistance force is exerted by the smaller wheel, which is the axle. Radius of the wheel is the effort arm, and the radius of the axle is the resistance arm. The center of the axle is the fulcrum. MA = radius of wheel radius of axle

Inclined plane A sloping surface used to raise objects. Ex. Ramp MA = effort distance = length of slope resistance distance height of slope

Screw and Wedge Screw - An inclined plane wrapped in a spiral around a cylindrical post Wedge – an inclined plane with one or two sloping sides.

Simple machines All six types of simple machines are variations of two basic machines: lever and inclined plane.

Compound Machines A combination of two or more simple machines Often the simple machines that make up a compound machine are concealed

Compound Machines cont’d… A bicycle is an example of a compound machine Pedal – wheel and axle system Overall mechanical advantage (MA) of a bicycle is the ratio of the resistance force exerted by the tires on the road to the effort force exerted by the rider on the pedals. Seat – connected to the bike with a Screw Hand brake - Lever

Efficiency The measure of how much of the work put into a machine is changed to useful work put out by a machine. The higher the efficiency of a machine the greater the amount of work input is changed to useful work output. Efficiency = Wout x 100% = Fr x dr x 100% Win Fe x de

Power The rate at which work is done. The measure of the amount of work done in a certain amount of time. To calculate power, divide the work done by the time required to do the work. Power = Work (in Newtons) / Time (in seconds) Power = W measured in watts (W) t