Chapter 8 Work and Machines

1. Chapter 8Work and Machines Section 1 Work and Power

2. To Work or Not to Work If you had a choice what would you prefer to do, work or not work? Give some examples of work.

3. The Scientific Meaning of Work • Work occurs when a force causes an object to move in the direction of the force. • The object must move for work to occur!

4. Force and Motion in the Same Direction • You’ll know work is being done on an object if two things occur: • 1. the object moves as the force is applied • 2. the direction of the objects motion is the same as the direction of the force applied.

5. Force and Motion in the Same Direction

6. How do we calculate work? • Work = force x distance Force expressed in Newtons Distance is expressed in meters So the expression for work is the Newton meter or simply the joule • Work = # Newtons x # meters = # Joules

7. Practice • A man applies a force of 500N to push a truck 100m down the street. How much work does he do? • In which situation do you do more work? a. You lift a 75 N bowling ball 2m off the floor. b. You lift two 50 N bowling balls 1m off the floor

8. Power-How Fast Work Is Done • Power is the rate at which work is done • To calculate power (P) divide the amount of work done (W) by the time (t) it takes to do that work or P = W/ t Unit for work is joule Unit for time is second So the unit for power is joules / second or J/s Or more commonly called a watt (W)

9. Chapter 8Work and Machines Section 2 What is a Machine?

10. Machines - Making Work Easier • A machine is something that makes work easier by changing the size or direction of a force

11. Machines - Making Work Easier • Work input: the work you do on a machine • Work output: the work done by the machine • Output force: the force that the machine applies that opposes the forces of you and the machine • Machines do not increase the amount of work that is done because work output can never be more than work input

12. Machines - Making Work Easier • Machines make work easier because they change the size or direction of the input force • If force decreases then distance must increase and vice versa

14. Mechanical Advantage • A machine’s mechanical advantage tells how many times a machine multiplies force…it compares the input force to the output force Mechanical Advantage (MA) =output force input force

15. Mechanical Advantage The larger the mechanical advantage, the easier the machine makes your work!

16. Mechanical Advantage • The output force is the same as the input force if the machine changes direction of a force and the MA is 1. • The input force is greater than the output force allows lower force over a longer distance and has a MA less than 1. • The output force is greater than the input force gives a MA of greater than 1.

17. Mechanical Efficiency While output cannot be greater than input the input is always greater than the output because some of the work done by the machine is to overcome gravity. The less work a machine has to do to overcome friction the more efficient it is. Mechanical Efficiency = work output x 100 work input

18. Mechanical Efficiency Mechanical Efficiency = work output x 100 work input Mechanical efficiency tells what percentage of the work input gets converted into work output. So reducing friction in a machine makes it more efficient.

19. Chapter 8Work and Machines Section 3 Types Of Machines

20. Types of Machines: Levers • Lever: a simple machine consisting of a bar that pivots at a fixed point called a fulcrum • There are three classes of levers: First class levers Second class levers Third class levers

21. Types of Machines: Levers • First class levers: always change the direction of the input forces

22. Types of Machines: Levers • Second class levers: do not change the direction of the input forces; output force is greater than the input force

23. Types of Machines: Levers • Third class levers: do not change the direction of the input forces; and do not increase the input force

24. Types of Machines: Inclined Planes • An inclined plane is a simple machine that is a straight, slanted surface • A ramp is an example of an incline plane • MA of an inclined plane:

25. Types of Machines: Wedges • A wedge is a double inclined plane that moves • A wedge applies an output force that is greater than the input force • MA of wedges: • The longer and thinner the wedge, the better.

26. Types of Machines: Screws • A screw is an inclined plane that is wrapped in a spiral • MA of screws: same advantage as a long inclined plane

27. Types of Machines: Wheel and Axle • A wheel and axle is a simple machine consisting of two circular objects of different sizes

28. Types of Machines: Wheel and Axle • The MA of this machine can be determined by dividing the radius of the wheel by the radius of the axle

29. Types of Machines: Pulleys A pulley is a simple machine consisting of a grooved wheel that holds a rope or a cable. A load is attached to one end of the rope and an input force is applied to the other end. There are two types of pulleys: 1) fixed 2) movable

30. Types of Machines: Pulleys

31. Types of Machines: Pulleys

32. Compound Machines • Compound machines are made of two or more simple machines. • The block and tackle we just talked about is a compound machine

33. Compound Machines

34. Mechanical Efficiency of Compound Machines Generally, the more moving parts a machine has, the lower its mechanical efficiency What would happen if the mechanical efficiency of a car was too high?