Chapter 5

1. Chapter 5 Lesson 2

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

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

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

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

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

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

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

9. The Work Done by Machines • When you use a machine such as a crowbar, you are trying to move something that resists being moved. • 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. The Work Done by Machines • You also could use a crowbar to move a large rock • In this case, you would be working against gravity—the weight of the rock.

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

12. Input and Output Forces • 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. • The work done by you on a machine is called the input work and is symbolized by Win. • The work done by the machine is called the output work and is abbreviated Wout.

13. Conserving Energy • When you do work on the machine, you transfer energy to the machine. • When the machine does work on an object, energy is transferred from the machine to the object. • 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 Win = Wout Fe × de = Fr × dr

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

16. Ideal Machines • Suppose the ideal machine increases the force applied to it. • This means that the output force, Fout, is greater than the input force, Fin. • Recall that work is equal to force times distance.

17. Ideal Machines • If Fout is greater than Fin, then Win and Wout can be equal only if the input force is applied over a greater distance than the output force is exerted over.

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

19. Force • Effort Force (Fe) • force applied to the machine • “what you do” • Resistance Force (Fr) • 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 • Window blinds are a machine that changes the direction of an input force. • A downward pull on the cord is changed to an upward force on the blinds.

22. Mechanical Advantage • The input and output forces are equal, so the MA is 1.

23. Ideal Mechanical Advantage • The mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA. • 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 • 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. • The efficiency of a real machine is always less than 100 percent.

26. Increasing Efficiency • 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 lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily.

27. Fr Fe MA 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: Fe = 20 N Fr = 500 N MA = ? WORK: MA = Fr ÷ Fe MA = (500 N) ÷ (20 N) MA = 25

28. Fr Fe MA Mechanical Advantage • Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: Fe = ? Fr = 2000 N MA = 10 WORK: Fe = Fr ÷ MA Fe = (2000 N) ÷ (10) Fe = 200 N