1. Chapter 3
Work & Machines

2. 3.1 Work and Power
Work is the transfer of energy that occurs when a force is applied over a distance.
Transfer of kinetic energy
Applying a force does NOT always result in work being done.
If you apply a force to an object and it doesn’t move, no work was done.
If the applied force is not in the same direction as the movement of the object NO work is done.

3. 3.1 Work and Power
Is work being done?

4. 3.1 Work and Power Work depends on force as well as distance.
Would you do more work by hiking up the hill or climbing up the side of the cliff?
Because the climber ends up the same place, the same amount of work is done. The hiker goes a shorter distance climbing the cliff but uses more force. The hiker goes a longer distance but uses less force hiking up the hill. d1 d2 d2 F2 F1

5. 3.1 Work and Power
Joule – the unit used to express energy; equivalent to the amount of work done by a force of 1 N acting through a distance of 1 m in the direction of the force
W = F x d
Distance
Meters
(m) (m)
Work
Joule (J)
Force
(N)
Newton (N)

6. 3.1 Work and Power
Using a force of 10 N, you push a shopping cart 10m. How much work did you do?
You do 225 J of work pushing a box with 75 N. How far did you push the box?
Lifting a dog 2 m off the ground requires 12 J of work. How much force was required?

7. 3.1 Work and Power
Power- the rate at which work is done or energy is transformed
Watt- the unit used to express power; equivalent to joules per second
Increasing Power
Do more work in a given time
Do the same work in less time
P = W t

8. 3.1 Work and Power
A stage manager at a play raises a curtain by doing 5,976 J of work on the curtain in 12 s. What is the power output of the stage manager?
You do150 J of work on a box to move it up a ramp with 15 W of power. How long did it take you to move the box?

9. 3.1 Work and Power
A light bulb is on for 12 s, and during that time, it uses 1,200 J of electrical energy. What is the power (wattage) of the light bulb?
You and a friend together apply a force of 1,000 N to a car, which makes the car roll 10 m in 90 s. How much work did you and your friend do together? What was the power output?

10. 3.2 Using machines Changing the size of a force
A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions:
Changing the size of a force
Changing the direction of a force
Changing the distance over which a force acts

11. 3.2 Using Machines Work in, work out
Work Input- the work that you do on a machine.
You apply a force, called the input force, to the machine through a distance.
Work Output- the work a machine does by converting on changing the input work.
Work output can NEVER be greater than work input!!!

12. 3.2 Using machines HOW CAN MACHINES HELP?
Look at the picture of the paint can and screw driver:
Work done by screwdriver on lid = work you do on screwdriver
Work output can never be greater than work input.
Machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work.

13. 3.2 Using Machines Force distance trade off
When force or distance decreases, the other must increase.
Greater force, less distance
Less force, greater distance

14. 3.2 using machines
Mechanical Advantage – the ratio of a machine’s output force produced to the input force applied
It can be less than 1, _equal_ to 1, or __greater_ than 1
The ideal mechanical advantage is the advantage if no friction existed
Never happens because all objects experience friction.

15. 3.2 Using machines
A grocer uses a handcart to lift a heavy stack of canned food. Suppose he applies an input force of 40 N to the handcart. The cart applies an output force of 320 N to the stack of canned food. What is the mechanical advantage of the handcart?
A pulley with a mechanical advantage of 50 is being used to lift a dresser to the 2nd floor of a building. Suppose a input force of 30 N is applied to the pulley. What is the output force the pulley is applying to the dresser?

16. 3.2 using machines Mechanical Efficiency
The output of a machine is always less than the work input. Why??
Some of the work done by the machine is used to overcome the friction created by the use of the machine. The less work a machine has to do to overcome friction, the more efficient the machine is.
Calculating Efficiency Mechanical
Efficiency equals work output divided by work input multiplied by 100.
Efficiency =

17. 3.2 using machines
What is the mechanical efficiency of a machine whose work input is 100 J and work output is 30 J?
A machines mechanical efficiency is 66%. The work input is 150J. What is the machines work output.

18. 3.3 Simple Machines
A simple machine is a machine that does work in _one_ movement
The six simple machines are:

19. Input = effort Output = load
3.3 Simple Machines
Levers- A simple machine that consist of a bar that pivots at a fixed point called a fulcrum.
There are 3 classes of levers – depend on the location of the fulcrum, the load, and the input force.
Input = effort
Output = load

20. 3.3 simple machines First-Class Levers
The fulcrum is between the input force and the load.
Mechanical advantage: Can be less than, 1 greater than 1, or equal to 1
Examples: scissors, seesaw, tongs

21. 3.3 simple machines Second-Class Levers
The load of a second-class lever is between the fulcrum and the input force.
Mechanical advantage: greater than 1
Examples: wheelbarrow, botte opener, crow bar

22. 3.3 simple machines Third-Class Levers
The input force in a third-class lever is between the fulcrum and the load.
Mechanical advantage: less than 1
Examples: golf club, broom, hammer

23. 3.3 Simple Machines How much to levers help?
The mechanical advantage of a lever equals the length of the input arm divided by the length of the output arm
Ideal mechanical advantage = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑖𝑛𝑝𝑢𝑡 𝑎𝑟𝑚 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑜𝑢𝑡𝑝𝑢𝑡 𝑎𝑟𝑚
IMA = Lin
Lout

24. 3.3 Simple Machines
A first class lever requires you to push 2m down with a force of 20N to lift a 60N rock 0.5m.
What is the Work input?
What is the Work output?

25. 3.3 Simple Machines
A first class lever requires you to push 2m down with a force of 20N to lift a 60N rock 0.5m. 3. What is the ideal mechanical advantage? 4. What is the efficiency?

26. 3.3 simple machines
Pulley- a simple machine that consists of a grooved wheel over which a rope, chain, or wire passes

27. 3.3 Simple machines Pulleys that cannot move
Fixed Pulley
Pulleys that cannot move
Only the direction of the force changes
Ex. Window blinds
Moveable
Pulley
Pulley attached to the object being lifted
Increase distance but decreases force
Ex. Lifting furniture

28. 3.3 Simple machines a combination of fixed and moveable pulleys
- A fixed and moveable pulley are used together.
Mechanical advantage of the block and tackle depend on the number of wheels.
Pulley System
a combination of fixed and moveable pulleys
Ex. crane

29. 3.3 Simple machines
The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.
A pulley system tries to increase mechanical efficiency as much as possible.

30. 3.3 Simple machines Wheel & Axel
A wheel and axle is a simple machine consisting of an axel attached to the center of a wheel and both rotate together

31. 3.3 Simple machines
The larger size of the wheel compared to the axle results in an IMA _>_ than 1

32. 3.3 Simple machines
The handle of a screwdriver has a diameter of 0.05m, and I apply 12N of force to it. The other end has a diameter of 0.01 m and can create 48N of force.
What is the Work input?
What is the Work output?

33. 3.3 Simple machines
The handle of a screwdriver has a diameter of 0.05m, and I apply 12N of force to it. The other end has a diameter of 0.01 m and can create 48N of force.
What is the ideal mechanical advantage?
What is the efficiency?

34. 3.3 Simple machines Inclined plane – flat, slope surface
Common inclined planes:
Ramp
Wedge
screw

35. 3.3 Simple machines

36. 3.3 Simple machines
A ramp is 6 meters long and has an ending height of 1.8m. I push with a force of 100N to move the 250N box.
What is the Work input?
What is the Work output?

37. 3.3 Simple machines
A ramp is 6 meters long and has an ending height of 1.8m. I push with a force of 100N to move the 250N box.
What is the mechanical advantage?
What is the efficiency?

38. 3.3 Simple Machines
Compound machines are machines that are made of two or more simple machines.
Mechanical Efficiency
Low because compound machines have more moving parts than simple machines do, thus there is more friction to overcome
What simple machines are found in a bike?