1. Work and Simple Machines Chapter 3 Physical Science

2. Ch3 L.1 Work and Power What must happen for work to be done? How does doing work on an object change its energy? How are work and power related?

3. Ch3 L.1 Bellwork Work: the transfer of energy that occurs when a force is applied over a distance Power: the rate at which work is done.

4. Is work done? Object MUST move for work to be done A force that does NOT make an object move does NO work

5. Calculating work Need force and distance W=F*d W= work (in Joules) F= force (in newtons) d= distance (in meters) If work= force*distance then a Joule= Newton*meter Newton= kg*m/s 2 so if you multiply that by a meter you get: Joule= kg*m 2 /s 2 Distance= distance moved while the force is applied ONLY

6. Work practice A student pushes a desk 2.0m across the floor using a constant force of 25.0N. How much work does the student do on the desk? A child pushes a toy truck 2.5m across a floor with a constant force of 22N. How much work does the child do on the toy truck? 50J of work is done on a box moving it 10m. How much force is applied?

7. Factors that affect work Look at what direction the force is and what direction the object is moving The force that acts in the direction of the motion does work If force is in same direction as the movement of the object, it’s easy. Just multiply force and direction to calculate work What if force is not exactly in the same direction as the movement? ONLY the force in the same direction as the movement is calculated

8. Lifting objects The force required to lift the object is the same as the weight of the object W=w*d W= work w= weight d= distance

9. Work and Energy Doing work on an object transfers energy to it If you do work on an object and make it move, you increase the kinetic energy (energy due to motion) If you do work on an object by lifting it, you increase the potential energy (increases as height increases)

10. What is power? Changing the TIME it takes to do the work does NOT change the work. One person moving an object faster than another person does NOT change the work. This changes the POWER!! Power is the rate at which work is done. The person who lifted the object faster had more power than the person who lifts the same object more slowly.

11. Calculate power P=W/t P= power (in watts)Watt= kg*m 2 /s 3 OR 1 W= 1 J/s W= work (in joules) Joule= kg*m 2 /s 2 OR 1 J= 1 N*m t= time (in s) A boy does 18J of work in 2.0s on his backpack as he lifts it from a table. How much power did the boy use on the backpack? A child pulls a wagon, doing 360J of work in 8.0s. How much power is exerted?

12. Ch3 L.1 Homework p.92 1-10 Study for quiz Outline Ch3 L.1

13. Ch3 L.2 Using Machines What are three ways a machine can make doing work easier? What is mechanical advantage? Why can’t the work done by a machine be greater than the work done on the machine?

14. Ch3 L.2 Bellwork Mechanical advantage: the ratio of a machine’s output force produced to the input force applied. Efficiency: the ratio of the output work to the input work

15. What is a machine? Shovel Pencil sharpener Clock Bike Ramp Door knob Scissors Screw driver Car Machines do NOT change the amount of work that needs to be done, they just change the way the work is done.

16. Input to output A machine works by using an input force (a force you apply) and changing that to become an output force Same idea applies to input work and output work Input work= input force* input distance (how much force you apply and how much the part of the machine moved in the direction of that force) Output work=output force*distance machine moves in direction of output force

17. How do machines make work easier to do? Change the size of the force Person using a hammer applies a smaller force Hammer applies larger force to nail Change the distance the force acts Person using a hammer applies that force over a longer distance Hammer applies force over a shorter distance Change the direction of a force You pull back on the hammer The hammer pulls up on the nail

18. Changing the size of a force The total work doesn’t change If the force increases then the distance has to decrease If the force decreases then the distance has to increase In the image, the force the man applies is small but the distance he applies it over is long The board moves a small distance but a large force is applied to it

19. Change the distance the force acts The force applied by a machine (output force) decreases as the distance over which the force acts (output distance) increases Initial force is greater but distance is smaller Rake acts over a larger area but the force is smaller Same idea as changing the size of a force

20. Change the direction of a force Change the direction of the input force Pulley is an example Equal output and input forces act over equal distances Just in different directions Input force Input distance Output distance Output force =X X Input force Input distance Output force Output distance Input work = Output work

21. Mechanical advantage The ratio of a machine’s output force produced to the input force applied How many times larger or smaller the output force is than the input force MA (no units)= output force/ input force MA=F out /F in Values > 1= output force is greater than input force It’s easier to do a task with machine < 1= output force is less than input force Harder to do a task with machine = 1 → output force and input force are equal Same with or without machine Ideal mechanical advantage= MA if no friction existed. Not in real life.

22. Mechanical advantage calculations A carpenter applies 525N to the end of a crowbar. The force exerted on the board is 1,575N. What is the mechanical advantage of the crowbar? While raking leaves, a woman applies an input force of 32N to a rake. the rake has an output force of 16N. What is the mechanical advantage of the rake?

23. Efficiency Output work of a machine never exceeds the input work of the machine friction converts some work to thermal energy (heat) and it can’t be used to do work Efficiency: ratio of output work to input work ALWAYS less than 100% Lubricating increases efficiency Efficiency (in %)= Output work (in Joules)/Input work (in Joules) x100% efficiency= W out /W in x100%

24. Efficiency calculations A mechanic does 78J of work pulling the rope on a pulley to lift a motor. The output work of the pulley is 64J. What is the efficiency of the pulley? A carpenter turns a handle to adjust a saw blade. The input work is 55J and the output work is 51J. What is the efficiency of the blade adjuster?

25. Ch3 L.2 homework p.100 1-10 Study for quiz Outline Ch3L2

26. Ch3 L.3 Simple machines What is a simple machine? How is the ideal mechanical advantage of simple machines calculated? How are simple machines and compound machines different?

27. Ch3 L.3 Bellwork Simple machine: do work using only one movement Lever: a simple machine made up of a bar that pivots, or rotates, about a fixed point. Fulcrum: The point about which a lever pivots Wheel and axle: an axle attached to the center of a wheel and both rotate together. Inclined plane: flat, sloped surface. A ramp. Wedge: sloped surface that moves. Screw: an inclined plane wrapped around a cylinder. Pulley: a simple machine that is a grooved wheel with a rope or a cable wrapped around it.

28. Six types of simple machines Lever Wheel and axle Inclined plane Wedge Screw Pulley

29. Levers First class Fulcrum is between the input force and the output force Direction of input force is always different from output force Second class Output force between the input force and the fulcrum Output force and input force are in same direction Makes the output force greater than the input force Third class Input force is between output force and the fulcrum Output force is less than input force Both forces act in the same direction

30. Mechanical advantages of levers IMA of a lever equals length of input arm divided by length of output arm IMA= length of input arm (in meters)/ length of output arm (in meters) 1st-class levers- location of fulcrum determines the mechanical advantage >1 → length of input arm > output arm input arm 2nd-class levers- input arm is always longer than output arm (MA is always greater than 1) 3rd-class levers- input arm is always shorter than output arm (MA is always less than 1)

31. Levers in the human body Muscles provide the input force Neck= first class lever Fulcrum= joint connecting skull to spine Input force from neck muscles Output force applied to head and supports head’s weight Foot (when standing on toes)= second class lever Fulcrum= ball of food Input force= muscles on back of lower leg Arm (forearm)= third class lever Fulcrum= elbow INput force from muscles near elbow

32. Wheel and Axle Axle attached to the center of a wheel and both rotate together Axle= shaft Screw driver is an example Handle= wheel (larger diameter) Axle= shaft that is attached to handle Both handle and shaft rotate when handle turns

33. Mechanical advantage of a wheel and axle Length of input arm is the radius of the wheel Length of output arm is the radius of the axle IMA= r wheel /r axle Apply input force to the handle (wheel). Output force is applied to screw by the axle Wheel is larger than axle, mechanical advantage is greater than 1

34. Inclined plane Ramp which is a flat, sloped surfaces Takes less force to move an object up along an inclined plane than to lift it straight up Mechanical advantage of inclined plane IMA= length of inclined plane/height of inclined plane Increasing the length and decreasing the height increases the IMA Longer or less slope= less force needed to move an object

35. Wedges Sloped surface that moves Type of inclined plane with one or two sloping sides Shape of wedge gives output force a different direction than input force

36. Screw Inclined plane wrapped around a cylinder Screw threads change input force to an output force Output force pulls screw into material

37. Pulleys Simple machine that is a grooved wheel with a ripe of a cable wrapped around it Fixed pulleys- only changes the direction of the force Movable pulleys and pulley systems- decrease the force needed to lift an object Increases the distance over which the force acts Mechanical advantage of pulleys IMA= equal to the number of sections of rope pulling up on the object

38. Compound machine Two or more simple machines that work together Gears: wheel and axle with teeth around the wheel Two or more work together to form a compound machine Direction of motion changes from gear to gear Different sized gears turn at different speeds Smaller= faster; larger= slower Input force applied to large gear is reduced when applied to a smaller gear Efficiency: multiplying efficiencies of each simple machine together Each simple machine decreases the overall efficiency of the compound machine

39. Ch3 L.3 homework p.111 1-7 Study for quiz Outline Ch3 L.3 Study for test