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Chapter 5 Lesson 2. What is a machine? AA machine is a device that makes doing work easier MMachines can be simple. SSome, like knives, scissors,

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Presentation on theme: "Chapter 5 Lesson 2. What is a machine? AA machine is a device that makes doing work easier MMachines can be simple. SSome, like knives, scissors,"— Presentation transcript:

1 Chapter 5 Lesson 2

2 What is a machine? AA machine is a device that makes doing work easier MMachines can be simple. SSome, like knives, scissors, and doorknobs, are used everyday to make doing work easier.

3 Making Work Easier MMachines can make work easier by increasing the force that can be applied to an object. AA second way that machines can make work easier is by increasing the distance over which a force can be applied. MMachines can also make work easier by changing the direction of an applied force.

4 Increasing Force AA car jack is an example of a machine that increases an applied force. TThe upward force exerted by the jack is greater than the downward force you exert on the handle.

5 HHowever, the distance you push the handle downward is greater than the distance the car is pushed upward. TThe jack increases the applied force, but doesn't increase the work done.

6 Force and Distance TThe work done in lifting an object depends on the change in height of the object. TThe same amount of work is done whether the mover pushed the furniture up the long ramp or lifts it straight up. IIf work stays the same and the distance is increased, then less force will be needed to do the work.

7 Changing Direction SSome machines change the direction of the force you apply. TThe wedge-shaped blade of an ax is one example.

8 The Work Done by Machines WWhen you use an ax to split wood, you exert a downward force as you swing the ax toward the wood. TThe blade changes the downward force into a horizontal force that splits the wood apart.

9 WWhen you use a machine such as a crowbar, you are trying to move something that resists being moved. IIf you use a crowbar to pry the lid off a crate, you are working against the friction between the nails in the lid and the crate.

10 YYou also could use a crowbar to move a large rock IIn this case, you would be working against gravity—the weight of the rock.

11 Input and Output Forces TTwo forces are involved when a machine is used to do work. TThe force that is applied to the machine is called the input force. FF in stands for the effort force. TThe force applied by the machine is called the output force, symbolized by F out.

12 TTwo kinds of work need to be considered when you use a machine—the work done by you on the machine and the work done by the machine. TThe work done by you on a machine is called the input work and is symbolized by W in. TThe work done by the machine is called the output work and is abbreviated W out.

13 Conserving Energy WWhen you do work on the machine, you transfer energy to the machine. WWhen the machine does work on an object, energy is transferred from the machine to the object. TThe amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine.

14 Work  Conservation of Energy can never get more work out than you put in trade-off between force and distance W in = W out F e × d e = F r × d r

15 Ideal Machines SSuppose a perfect machine could be built in which there was no friction. NNone of the input work or output work would be converted to heat. FFor such an ideal machine, the input work equals the output work.

16 SSuppose the ideal machine increases the force applied to it. TThis means that the output force, F out, is greater than the input force, F in. RRecall that work is equal to force times distance.

17 IIf F out is greater than F in, then W in and W out can be equal only if the input force is applied over a greater distance than the output force is exerted over.

18 Mechanical Advantage TThe ratio of the output force to the input force is the mechanical advantage of a machine. TThe mechanical advantage of a machine can be calculated from the following equation.

19 Force  Effort Force (F e ) force applied to the machine “what you do”  Resistance Force (F r ) force applied by the machine “what the machine does”

20 Mechanical Advantage  Mechanical Advantage (MA) number of times a machine increases the effort force MA > 1 : force is increased MA < 1 : distance is increased MA = 1 : only direction is changed

21 Mechanical Advantage WWindow blinds are a machine that changes the direction of an input force. AA downward pull on the cord is changed to an upward force on the blinds.

22 TThe input and output forces are equal, so the MA is 1.

23 Ideal Mechanical Advantage TThe mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA. TThe IMA can be calculated by dividing the input distance by the output distance.

24 Efficiency  Efficiency measure of how completely work input is converted to work output always less than 100% due to friction

25 Calculating Efficiency IIn an ideal machine there is no friction and the output work equals the input work. So the efficiency of an ideal machine is 100 percent. TThe efficiency of a real machine is always less than 100 percent.

26 Increasing Efficiency MMachines can be made more efficient by reducing friction. This usually is done by adding a lubricant, such as oil or grease, to surfaces that rub together. AA lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily.

27 Mechanical Advantage  A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA? GIVEN: F e = 20 N F r = 500 N MA = ? WORK : MA = F r ÷ F e MA = (500 N) ÷ (20 N) MA = 25 MA FrFr FeFe

28 Mechanical Advantage  Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: F e = ? F r = 2000 N MA = 10 WORK : F e = F r ÷ MA F e = (2000 N) ÷ (10) F e = 200 N MA FrFr FeFe


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