Foundations of Physical Science. Unit 2: Work and Energy

Transcription

1 Foundations of Physical Science Unit 2: Work and Energy

2 Chapter 4: Machines and Mechanical Systems 4.1 Force and Machines 4.2 The Lever 4.3 Designing Gear Machines

3 Learning Goals Describe and explain a simple machine. Apply the concepts of input force and output force to any machine. Determine the mechanical advantage of a machine. Construct and analyze a block and tackle machine. Describe the difference between science and engineering. Understand and apply the engineering cycle to the development of an invention or product. Describe the purpose and construction of a prototype. Design and analyze a lever. Calculate the mechanical advantage of a lever. Recognize the three classes of levers. Build machines with gears and deduce the rule for how pairs of gears turn. Design and build a gear machine that solves a specific problem.

4 Vocabulary engineering gear engineering cycle engineers force fulcrum gear input input arm input force input gear lever machine mechanical advantage mechanical systems output output arm output force output gear prototype simple machine

5 4.1 Forces in Machines The world without machines Technology of today So what is a machine?

6 Machine A device: with moving parts that work together to accomplish a task. that multiplies forces or changes the direction of forces that employs the conservation of energy A bicycle is a good example! Input: everything you do to make the machine work, like pushing on the pedals Output: what the machine does for you, like going fast

7 Simple Machines An unpowered mechanical device, such as a: Lever Wheel and axle Block and tackle Gear Ramp

8 Simple Machines: Input and Output Lever Input force: what you apply Output force: what the lever applies to what you are trying to move Block and Tackle (Pulley) Input force: what you apply to the rope Output force: what gets applied to the load you are trying to lift Most machines we use today are made up of combinations of different simple machines

9 Mechanical Advantage The ratio of output force to input force If the mechanical advantage is > 1, the output force is greater than the input force If the mechanical advantage is < 1, the output force is smaller than the input force

10

11 How a Block and Tackle Works The forces in ropes and strings Ropes and strings carry tension forces along their length a pulling force (not a pushing force!) Every part of a rope has the same tension If friction is very small, the force in a rope is the same everywhere The forces in a block and tackle More rope, easier to pull (see diagram slide)

12 How a Block and Tackle Works Mechanical advantage More ropes, more output force than input force easier to lift! Multiplying force with the block and tackle Input force can be much less with more ropes If the mechanical advantage is 4, the input force for the machines is ¼ the output force

13

14 4.2 The Lever Archimedes GIVE ME A PLACE TO STAND AND I WILL MOVE THE EARTH Greek scientist 3 rd century BC

15 What is a Lever? Another simple machine Pliers, wheelbarrow, human biceps, forearm Your bones and muscles work as levers to perform everything from chewing to throwing a ball

16 What is a Lever? A stiff structure that rotates around a fixed point called the fulcrum We can arrange the fulcrum and the lengths of the input and output arms to make almost any mechanical advantage we need

17 How a Lever Works Fulcrum in the middle: input and output forces are the same Input arm is longer: output force is larger than the input force Input arm is 10x longer than the output arm, the output force will be 10x bigger than the input force Input arm is shorter: output force is smaller than the input force Input arm is 10x shorter than the output arm, the output force will be 10x less than the input force

18

19 4.3 Designing Gear Machines Engineering/Technology: The application of science to solve problems Scientists: study the world to learn the basic principles behind how things work Engineers: use scientific knowledge to create or improve inventions that solve problems

20 Sample Engineering Problem Conceptual design Prototype Testing the prototype Changing the design and testing again

21 Gears and Rotating Machines Many machines require that rotating motion be transmitted from one place to another. Gears change force and speed. Gears are better than wheels because they have teeth and don t slip as they turn together.

22 What is the Gear Ratio? Gears have input and output Input gear: the one you turn, or apply forces to Output gear: the one that is connected to the output of the machine Gear ratio: the ration of output turns to input turns Smaller gears turn faster; the gear ratio is the inverse of the ratio of teeth in two gears

23 Designing Machines Machines are designed to do specific things Simple machines can be combined to solve more complex problems Two pairs of gears with a 2:1 ratio can be combined to make a machine with a ratio of 4:1

24 Designing Machines Design involves trade offs Even the best designs are always being improved

25 Rube Goldberg Machine ( ) Pulitzer Prize winning cartoonist, sculptor and author