General Science Chapter 6

General Science Chapter 6
Machines Chapter 6 April 17 General Science Chapter 6

Machine A device that makes work easier April 17 General Science Chapter 6

Simple machine A device that does work with only one movement There are six types Levers Inclined Plane Pulley Wedge Wheel & Axle Screw April 17 General Science Chapter 6

Work and machines Work is done when a force is exerted through a distance Machines make work easier by changing the size or direction of the force, or both. Opening a paint can with a screwdriver Changes size – you can use less force Changes direction April 17 General Science Chapter 6

Forces and machines Effort force (Fe) – applied to the machine The force you exert Resistance force (Fr) – applied by the machine to overcome gravity or friction April 17 General Science Chapter 6

Work and machines Work input (Win) – work done on the machine Effort force times distance it moves Win = Fe X de Work output (Wout) – work done by the machine Resistance force times distance it moves Wout = Fr X dr April 17 General Science Chapter 6

Conservation of energy
You can never get more work out than you put in If force increases, distance must decrease. April 17 General Science Chapter 6

Mechanical advantage The number of times a machine multiplies the effort force April 17 General Science Chapter 6

Mechanical advantage Can be greater than 1 Opening paint can Can be equal to 1 Raising blinds Can be less than 1 Hitting a baseball April 17 General Science Chapter 6

Example A worker applies an effort force of 10 N to pry open a window that has a resistance of 500 N. What is the mechanical advantage of the crowbar? Fr = 500 N Fe = 10 N April 17 General Science Chapter 6

You try A jack is used to lift a 2000-N rock. The effort force is 200 N. Find the mechanical advantage. April 17 General Science Chapter 6

Discuss #1 A simple machine is a device that does work with only one __________. List the 6 simple machines Give an example of each What is the force applied to a machine? What is the force applied by a machine? April 17 General Science Chapter 6

Discussion #1 What is mechanical advantage? What does it mean when the MA value is equal to 1? How do we calculate MA? April 17 General Science Chapter 6

Levers Examples Crowbars Seesaws Baseball bat April 17 General Science Chapter 6

Definitions A lever is a bar that is free to pivot, or turn, about a fixed point. A fulcrum is the fixed point of a lever. The effort arm is the part of the lever on which the force is applied. The resistance arm is the part of the lever that exerts the resistance. April 17 General Science Chapter 6

Lever Effort force Resistance arm Effort arm fulcrum April 17 General Science Chapter 6

Mechanical advantage Review, we learned that We can also use for levers April 17 General Science Chapter 6

You try You use a crowbar 160 cm long as a lever to lift a large rock. The rock is 20 cm from the fulcrum. You push down on the other end of the crowbar. What is the length of the effort arm? The resistance arm? What is the MA of the lever? April 17 General Science Chapter 6

First class levers The fulcrum is in the middle Seesaw crowbar Effort force Resistance arm Effort arm fulcrum April 17 General Science Chapter 6

Second class levers The resistance is in the middle wheelbarrow nutcracker Effort force Effort arm Resistance arm fulcrum April 17 General Science Chapter 6

Third class levers The effort is in the middle Baseball bat broom Effort force Effort arm Resistance arm fulcrum April 17 General Science Chapter 6

Discuss #2 What is a lever? What is a fulcrum? What is the effort arm? What is the resistance arm? April 17 General Science Chapter 6

Discussion #2 What are the 3 types of levers? What is an example of each type of lever? Which type usually has a MA value less than 1? April 17 General Science Chapter 6

Pulleys Pulley – grooved wheel with a rope or chain running along the groove Acts like a lever The two ends of the rope are the effort arm and the resistance arm The wheel acts like the fulcrum April 17 General Science Chapter 6

Fixed pulley Attached to something that doesn’t move Change the direction of a force MA of 1 Le Lr Fr Fe April 17 General Science Chapter 6

Movable pulley Attached to the object being moved MA greater than 1 Effort distance must be greater than resistance distance Le Lr Fr Fe April 17 General Science Chapter 6

Block and tackle System of fixed and movable pulleys Has MA equal to the number of ropes that support the resistance weight Count every rope coming off the movable pulleys April 17 General Science Chapter 6

Examples of Block & Tackle
April 17 General Science Chapter 6

Example of Block & Tackle #2
April 17 General Science Chapter 6

Wheel and axle Consists of two wheels of different sizes that rotate together The effort force is usually applied to the large wheel The small wheel, or axle, exerts the resistance force Examples: doorknob, water faucet, gears, pencil sharpener April 17 General Science Chapter 6

Wheel and axle Can be thought of as a lever attached to a shaft Radius of wheel is effort arm Radius of axle is resistance arm Center of axle is fulcrum April 17 General Science Chapter 6

Inclined plane A ramp Lifting something along an inclined plane means you cover more distance than lifting it straight up, but you get to use a smaller force April 17 General Science Chapter 6

Screw An inclined plane wrapped in a spiral around a cylindrical post. As you drive in a screw, the inclined plane slides through the wood. April 17 General Science Chapter 6

Wedge An inclined plane with one or two sloping sides Examples Chisels Knives Axe blades The material stays in place while the wedge moves through it. April 17 General Science Chapter 6

Variations All six kinds of simple machines are variations of two basic machines The lever The inclined plane April 17 General Science Chapter 6

Discuss #3 What kind of simple machine is a ramp? What is an inclined plane wrapped in a spiral around a cylindrical post? What kind of simple machine are chisels, knives, and axes? April 17 General Science Chapter 6

Discussion #3 What type of pulleys have a MA = 1? What is the difference between a fixed pulley and a movable pulley? What is a block and tackle? What two groups can simple machines be broken into? April 17 General Science Chapter 6

Compound Machine A machine that is made up of 2 or more simple machines. Examples of compound machines Fishing rod, pencil sharpener, an axe April 17 General Science Chapter 6

Rube Goldberg #1 Open window (A) and fly kite (B). String (C) lifts small door (D) allowing moths (E) to escape and eat red flannel shirt (F). As weight of shirt becomes less, shoe (G) steps on switch (H) which heats electric iron (I) and burns hole in pants (J). Smoke (K) enters hole in tree (L), smoking out opossum (M) which jumps into basket (N), pulling rope (O) and lifting cage (P), allowing woodpecker (Q) to chew wood from pencil (R), exposing lead. Emergency knife (S) is always handy in case opossum or the woodpecker gets sick and can't work. April 17 General Science Chapter 6

Rube Goldberg #2 April 17 General Science Chapter 6

Rube Goldberg #3 Honda video April 17 General Science Chapter 6

Efficiency Measure of how much of the work put into a machine is changed to useful work put out by the machine. April 17 General Science Chapter 6

Efficiency Can it ever be more than 100%? How can we increase efficiency? April 17 General Science Chapter 6

example A worker pushes a 1500 N chair up an inclined plane that is 4.0 m long and 1.0 m high. The worker exerts a force of 500 N. What is the efficiency of the inclined plane? 4 m 1 m April 17 General Science Chapter 6

You try Using a fixed pulley, you pull the rope down 1.0 m with a force of 72 N. A 65-N object is raised 1.0 m. What is the efficiency of the pulley? April 17 General Science Chapter 6

Power Power is the rate at which work is done. April 17 General Science Chapter 6

Watts Power is measured in Watts, named after James Watt, who invented the steam engine. 1 W = 1 J/s Very small unit, so we often use kW. 745.6 Watts = 1 horsepower April 17 General Science Chapter 6

Example A figure skater lifts his partner, who weighs 450 N, 1 meter in 3 seconds. How much power is required? April 17 General Science Chapter 6

You try A N elevator rises 30.0 m in 60.0 s. How much power is required? Express your answer in kW. April 17 General Science Chapter 6

Making Connections 4.184 Joules = 1 calorie Joules are units for energy and work 1 Calorie = 1000 calories A Calorie is used for foods, so if a candy bar has 250 Calories it is the same as calories. April 17 General Science Chapter 6

Example #1 Tommy eats a candy bar that has 230 Calories. How many Joules is that? 1 Calorie = 1000 calories 1 calorie = Joules so 1 Cal = 4184 Joules therefore 230 Calories = Joules April 17 General Science Chapter 6

Example #1 continued How much Power can be produced with the 230 Calorie candy bar in 1 hour? 230 Cal = Joules Use the equation below April 17 General Science Chapter 6

Example #1 continued.. April 17 General Science Chapter 6

Discuss #4 What is a compound machine? What is efficiency? What part of an axe is the lever? What class of lever is it? What are the benefits of attaching a wedge to a lever to make an axe? April 17 General Science Chapter 6

Discussion #4 What units are used to measure Power? Why is the unit kW used more often than W? What is the conversion factor for horsepower to Watts? April 17 General Science Chapter 6

General Science Chapter 6

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