Energy, Work and Simple Machines Chapter 10 Physics

Objectives: The student will be able to: explain the relationship between simple machines and work. Demonstrate a knowledge of the usefulness of simple machines. Differentiate between ideal and real machines in terms of efficiency. Describe a compound machine.

Machines Machines can do any of the following; Machines can Multiply the Force (Lever) Change the Direction of the Force (Pulley) Change the speed in which the force acts (Gears)

Force is a push or pull that changes the motion or shape of an object. Example: I push a bookshelf to move it.

Energy is the ability to do work. Example: I must have energy to run the mile.

Work is the result of force moving an object. Example – Picking up something heavy is hard work!

Gravity is the force that constantly pulls objects toward Earth.

Machines Most machines make work easier by multiplying the Force! Machines never Multiply the Work! When using a machine there is always a Work put into the machine (WIN) and a Work the machine puts out (WOUT). Ideally, the WIN = WOUT in an ideal machine (no friction).

10.2 Machines The best way to analyze what a machine does is to think about the machine in terms of input and output. 9

Machines How Does a Machine multiply the force without multiplying the Work? Answer: If a machine multiplies the Input Force (FIN), then the machine must act over a larger Displacement (dIN)! Remember, W = Fd

Machines WIN = WOUT FIN dIN = FOUT dOUT FIN will be small so dIN will be large! FOUT will be large so dOUT will be small!

Types of Simple Machines Simple Machine (SM) – a machine with one or two moving parts. There are six types of Simple Machines: 1. Lever 4. Inclined Plane 2. Pulley 5. Wedge 3. Wheel and 6. Screw Axle

Example: seesaw, crowbar, baseball bat, rake Lever –made of a board or bar set on top of a fulcrum. It is used to lift weight Example: seesaw, crowbar, baseball bat, rake

Classes of Levers “First Class Lever” Examples: Seesaw A first-class lever is a lever in which the fulcrum is located between the input effort and the output load. In operation, a force is applied (by pulling or pushing) to a section of the bar, which causes the lever to swing about the fulcrum, overcoming the resistance force on the opposite side. The fulcrum may be at the center point of the lever as in a seesaw or at any point between the input and output. This supports the effort arm and the load. Examples: Seesaw Scissors (double lever)

First Class Lever fulcrum Effort Resistance Fulcrum is between EF (effort) and RF (load) Effort moves farther than Resistance. Multiplies EF and changes its direction The mechanical advantage of a lever is the ratio of the length of the lever on the applied force side of the fulcrum to the length of the lever on the resistance force side of the fulcrum.

Examples of first class levers Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws.

Second Class Lever Effort Resistance RF (load) is between fulcrum and EF Effort moves farther than Resistance. Multiplies EF, but does not change its direction The mechanical advantage of a lever is the ratio of the distance from the applied force to the fulcrum to the distance from the resistance force to the fulcrum.

Three Lever Classes Always multiplies a force. Second class lever E R Explanation Three Lever Classes Second class lever Resistance is located between the effort force and the fulcrum. Always multiplies a force Example: Wheelbarrow E R F Always multiplies a force. 8/23/04

Examples of Second class levers In a second class lever the input effort is located at the end of the bar and the fulcrum is located at the other end of the bar, opposite to the input, with the output load at a point between these two forces. Examples: Paddle Wheelbarrow Wrench

Examples of second-class levers Examples of second-class levers include: nut crackers, wheel barrows, doors, and bottle openers.

Third Class Lever EF is between fulcrum and RF (load) Does not multiply force Resistance moves farther than Effort. Multiplies the distance the effort force travels The mechanical advantage of a lever is the ratio of the distance from the applied force to the fulcrum to the distance of the resistance force to the fulcrum

Classes of Levers Examples: Hockey Stick Tweezers Fishing Rod “Third Class Lever” Examples: Hockey Stick Tweezers Fishing Rod For this class of levers, the input effort is higher than the output load, which is different from second-class levers and some first-class levers. However, the distance moved by the resistance (load) is greater than the distance moved by the effort. In third class levers, effort is applied between the output load on one end and the fulcrum on the opposite end.

Explanation Three Lever Classes Third class lever Effort force located between the resistance and the fulcrum. Effort arm is always shorter than resistance arm MA is always less than one Example: Broom E R F There is an increase distance moved and speed at the other end. Other examples are baseball bat or hockey stick. 8/23/04

Examples of Third Class Levers Examples of third-class levers include: tweezers, arm hammers, and shovels. Third class lever in human body.

Example : flagpole, clothesline, cranes, fishing reel Pulley – made of rope and string wound around a reel to change the direction of a force. Example : flagpole, clothesline, cranes, fishing reel

Example: steering wheel, doorknob, screwdriver Wheel and Axel – a wheel that turns on a post to help move things quickly and easily Example: steering wheel, doorknob, screwdriver

Inclined Plan – a slanted surface to make lifting easier Example: ramp, stairs

Example: knife, door wedge, ax Wedge – two inclined planes together used to raise an object or split an object. Example: knife, door wedge, ax

Example: drill bit, screws Screw – an inclined plane wrapped around a pole or shaft that is used to hold materials together or drill holes. Example: drill bit, screws

Compound Machine – two or more simple machines working together. Examples: Bike Car

Mechanical Advantage Mechanical Advantage (MA) – is the number of times a machine multiplies the Input Force (FIN) Example: MA = 2 Means the machine doubles the force you put into it. MA = 10 Means the machine multiplies the force put into it by 10.

10.2 Mechanical Advantage Mechanical advantage is the ratio of output force to input force. For a typical automotive jack the mechanical advantage is 30 or more. A force of 100 newtons (22.5 pounds) applied to the input arm of the jack produces an output force of 3,000 newtons (675 pounds)— enough to lift one corner of an automobile.

Mechanical Advantage MA >1 (Machine multiplies the force) MA < 1(Machine multiplies the distance) MA = 1(Machine does not multiply either force or distance. Probably only changes the direction to the force.)

Mechanical Advantage To find the Mechanical Advantage (MA) of a machine, we take the ratio of the Resistance Force (Fr) to the Effort Force (Fe) Effort Force (Fe) – is the force applied to the machine Resistance Force (Fr) – is the force the machine applies to the object

10.2 Mechanical Advantage MA = Fo Fi Output force (N) Mechanical Input force (N)

Ideal Mechanical Advantage (IMA) The Ideal Mechanical Advantage (IMA) is the largest possible MA a machine can have if the machine operated without friction. To find the Ideal Mechanical Advantage (IMA) of a machine you take the ratio of the Effort Distance (de) over the Resistance Distance (dr)

Calculating MA and IMA To calculate MA we use the Forces (Fr and Fe). Since Friction is a force, Friction affects MA. MA = Fr/Fe To calculate IMA we use the distances (dr and de). Friction does not affect IMA. IMA = de/dr MA has No Unit!! It’s a number telling how many times the force is multiplied!

Ideal Mechanical Advantage and Actual Mechanical Advantage The Actual Mechanical Advantage (MA) is always less than the IMA (MA < IMA) because of Friction. Machines are designed with an IMA Machines are tested to find Actual MA

Input/Effort and Output/Resistance Note from this point on: Effort = Input (FIN = Fe and dIN = de) Resistance = Output (FOUT = Fr and dOUT = dr) WIN = WOUT (Ideal Machine) Fede = Frdr

Compound Machines Compound Machine – any combination of two or more simple machines Examples: Axe, Shovel, Scissors Compound Machines have a higher Mechanical Advantage (MA) because they are made up of multiple machines

Mechanical Advantage of Compound Machines To calculate the Mechanical Advantage (MA) of a Compound Machine (CM), you multiply the Mechanical Advantages of all the Simple Machines in the Compound Machine MACM = MASM#1 x MASM#2 x MASM#3 x …

Efficiency Efficiency is the ratio of the useful work you get out of a machine (WOUT) over the work you put into a machine (WIN) In an ideal world (no friction); WOUT = WIN therefore; WOUT/WIN = 1

Efficiency In the real world (with friction); WOUT < WIN therefore; We express Efficiency as a Percentage by multiplying the ratio by 100% Ideal World Efficiency = 100% Real World Efficiency < 100%

Efficiency We can use different equations for Efficiency Eff = (WOUT/WIN) x 100% Eff = (Frdr/Fede) x 100% Eff = (MA/IMA) x 100%

Efficiency and Machines Simple Machines have a small MA but work with a high Efficiency. Compound Machines have a high MA but work with a lower Efficiency. The more complicated the machines the greater the MA but the lower the Efficiency!

The Human Machine Levers – Muscles and Tendons Wedges – Teeth and Finger Nails Your Body uses many Simple and Compound machines to create Mechanical Advantage Human Walking Machine Page 273

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Practice Question Using a single fixed pulley, how heavy a load could you lift?

Practice Question Using a single fixed pulley, how heavy a load could you lift? Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.

Practice Question Give an example of a machine in which friction is both an advantage and a disadvantage.

Practice Question Give an example of a machine in which friction is both an advantage and a disadvantage. The use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency.

Practice Question Why is it not possible to have a machine with 100% efficiency?

Practice Question Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100% efficient.

Practice Question What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input.

Practice Question What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input. The effort force is the force applied to a machine. Work input is the work done on a machine. The work input of a machine is equal to the effort force time the distance over which the effort force is exerted.

Elaboration Simple Machines Transparency 10-2 Lever Lab Energy Lab – work input and Output energy Levers in the Human Body Page 272 #s 25 and 26

Closure Kahoot – 10.2