Chapter 7 Machines

Objectives

• 7.1 Explain how machines make work easier • 7.1 Calculate mechanical advantage • 7.2 Describe the six types of simple machines

Objectives

• 7.2 Calculate the ideal MA for different types of simple machines

• 7.3 Explain what the science of bionics involves

• 7.3 Contrast two methods of using electrical signals to trigger motion of a limb or other bodily process

Objectives

• 7.4 Recognize the simple machines that make up a compound machine

• 7.4 Calculate the efficiency of a machine • 7.4 Describe the relationship among work,

power, time

Machines

• A machine is a device that makes work easier– Washer, dryer, car, computer. All are supposed to

make work easier• Simple Machines: A device that does work

with only one movement– Six total types of simple machines

Levers

• A bar that is free to pivot on a fulcrum• Type 1 Fulcrum in Middle• Type 2 Load in Middle• Type 3 Force in Middle

Mechanical Advantage

• Ratio of Effort in (Force applied) to Work out (Weight Lifted)

• If a 20 N force is applied, and a 40 N weight is lifted, the MA is 2

Mechanical Advantage

• Type 2 Lever: Always lower the force required. The force needed to lift the weight is less than the weight. The MA is bigger than one. (Ratio of Arms)– Example: Wheel barrow, crowbar

Mechanical Advantage

• Type 3 Lever: Always have a MA less than 1. Takes more force to lift an equal load weight– Biceps are an example– MA is calculated by ratio of Force (Effort) arm to

Load (Distance to Fulcrum)

Mechanical Advantage

– Trade Force for Distance• MA of 1 means both the Load and the Force

move the same distance, and force = load • MA of 2 means the Force moves 2x as far but

is half as large as the load• MA of ½ means the Load moves 2x as far and

a force 2x the load is needed– Remember: Work in = Work Out• Force x Distance = Force x Distance

Mechanical Advantage

• Lever: Ratio of Effort to Resistance distance from fulcrum– This is true for all 3 types of levers

• Pulley: MA of 1 for fixed, MA > 1 for movable pulleys (determine by number of support ropes)– A grooved wheel with a rope running the groove

• Wheel and Axle: Ratio of Wheel to Axle radius– Consists of two wheels of different sizes rotating

together

Mechanical Advantage

• Screw: Ratio of circumference to pitch– An inclined plane wrapped in a spiral

• Inclined Plane: Ratio of slope to height – Flat plane = MA of infinity– Vertical plane = MA of 1– A sloping surface used to raise objects

• Wedge: Ratio of slope to base– Same as inclined plane, but serves purpose of

separation/change in direction of forces

http://1.bp.blogspot.com/_f1hxA2zCrF4/S_wV1woXmNI/AAAAAAAAAIY/Cq7doGN0taY/s1600/250px-Wedge-diagram.jpeg

Human Body

• Many levers in human body• Biceps are good example of type 3 lever (poor

MA)• How strong is the human bicep?• Your body uses 2,000 Calories (1 C = 4,184 J)– How big of a light bulb are you?

Efficiency

• Useful Energy Out / Energy put in– If you put 200 J of energy into a machine, but it

only puts out 180 J of work, it is 90 % efficient– Energy losses (energy not being used to

accomplish purpose) include• Sound• Heat• Friction

Ideal MA

• The calculations we do are for the Ideal (no Friction) Machine. In the real life, don’t get 100% of work in = work out – Energy is conserved, but did you really want to

increase the temperature of the material by friction?

Bionics

• The science of designing artificial replacements for the body– Is it fair? Should they be allowed to compete?

How good is too good? – Eye Surgery? Is this cheating?– Nanobots doing repairs?– Artificial muscles?– Artificial legs?

Compound Machines

• Combination of 2 more simple machines– An axe is an example of a wedge and lever

• To find the MA of the machine, take all the MA’s and multiply them together

Power

• Power is How much work how fast– Work divided by time

• Measured in Watts– Joule’s per second

• 500 Joules of work done in 10 seconds is equal to 50 W of power

Simple Machines

• How were the Pyramids built? • Still a mystery, some propose a Lever

(modern), others believe Inclined Planes.

Review Q’s

• A simple machine has a MA of 2.0. If the load has a weight of 50.0 Newtons and moves a distance of 4.0 meters…– A) How much force was needed to lift the load?– B) How far did the effort arm move while lifting

the load?– C) Which moved faster, the load or the effort?

Review Q’s

• A box of weight 500 N is lifted 2.0 meters. – A) How much work was done on the box?– B) If it took 5 seconds, what power was used to

lift the box?– C) If the actual energy used to lift the box was

1200 Joules, how efficient was the machine used to lift it?

Review Q’s

• A bicep is attached 3.0 cm away from the elbow. In the hand (30.0 cm away from elbow) a 25 pound weight is lifted a distance of 20 cm. – A) What is the MA of the bicep?– B) How much force does the bicep apply to lift the

weight?– C) How much does the bicep shrink in length?– D) Is the human body efficient?