Chapter 12 Work and Machines

Section 1 - What is work? Work is done on an object when the object moves in the same direction in which the force is exerted. Work - The product of force and distance when a force is used to move an object. No work without motion Chapter 12

Section 1 - What is work? The amount of work done on an object can be determined by multiplying force times distance. Work = Force x Distance work units - Joules (J) Chapter 12

Section 1 - What is work? Power is the rate at which work is done. Power equals the amount of work done on an object in a unit of time. Chapter 12

Section 1 - What is work? Power = Joules/second = watt (W) kW = 1,000 W 1 horsepower = 746 W Chapter 12

Section 2 - How Machines Do Work A device that changes the amount of force exerted or the direction in which force is exerted. A device with which you can do work in a way that is easier or more effective. A machine makes work easier by changing at least one of three factors. A machine may change the amount of force you exert, the distance over which you exert your force, or the direction in which you exert your force. Chapter 12

Section 2 - How Machines Do Work pg.415 Increase force, decrease distance Increase distance, decrease force Change direction Chapter 12

Section 2 - How Machines Do Work A machine’s mechanical advantage is the number of times a machine increases a force exerted on it. Chapter 12

Section 2 - How Machines Do Work Friction reduces mechanical advantage The efficiency of a machine compares the output work to the input work. Efficiency is expressed as a percent. Efficiency = output work/input work x 100% Problems on page 418 Chapter 12

Section 3 - Simple Machines Six types of machines Inclined plane Wedge Screw Lever Wheel & axle Pulley Chapter 12

Inclined Plane An inclined plane is a flat sloped surface. Less force exerted over longer distance Chapter 12

Inclined Plane You can determine the ideal mechanical advantage of an inclined plane by dividing the length of the incline by its height. Ideal Mechanical Advantage= Length of incline/ Height of incline. Chapter 12

Wedge A wedge is a device that is thick at one end and tapers to a thin edge at the other end. Less force exerted over longer distance Chapter 12

Wedge The ideal mechanical advantage of a wedge is determined by dividing the length of the wedge by its width. Chapter 12

Screw Inclined plane wrapped around a cylinder Less force Chapter 12

Screw The ideal mechanical advantage of a screw is the length around the threads divided by length of the screw. Chapter 12

Lever A lever is a rigid bar that is free to pivot, or rotate, on a fixed point. Fulcum - the fixed point that a lever pivots around Chapter 12

Lever The ideal mechanical advantage of a lever is determined by dividing the distance from the fulcrum to the input force by the distance from the fulcrum to the output force. Ideal mechanical advantage = Distance from fulcrum to input force / Distance from fulcrum to output force Chapter 12

3 Types of Levers Chapter 12

Wheel and Axle A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis. Chapter 12

Wheel and Axle Less force exerted over longer distance Doorknob Chapter 12

Wheel and Axle You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle. Mechanical advantage = Radius of wheel / Radius of axle Chapter 12

Pulley A pulley is made of a grooved wheel with rope or cable wrapped around it. Change amount and direction of input force Chapter 12

Pulley The ideal mechanical advantage of a pulley is equal to the number of sections of rope that support the object. Chapter 12

Section 4 - Machines in the human body Lifting levers - most of the machines in your body are levers that consist of bones and muscles. Pg.432 Levers in the body Wedges - front teeth (incisors) Break toothpick in fingers Chapter 12

Compound Machines A compound machine is a machine that utilizes two or more simple machines. The ideal mechanical advantage of a compound machine is the product of the individual ideal mechanical advantages of the simple machines that make it up. Chapter 12

Ch. 12 Vocabulary work, machine, efficiency, mechanical advantage, inclined plane, wedge, screw, lever, fulcrum, wheel and axle, pulley Chapter 12