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Ch. 5 – Work & Machines. I. Work. A. Work:. Exerting a force over a certain distance;. a form of energy. (SI units = Joules). 1. For work to be done an object must move in the direction of the force applied. 2. Equation: W = Fd.

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Ch. 5 – Work & Machines

I. Work

A. Work:

Exerting a force over a certain distance;

a form of energy

(SI units = Joules)

1. For work to be done an object must move in the direction of the force applied

2. Equation: W = Fd

3. Example: If you lift a 600 N box 12 m in the air, how much work did you do?

a. F = 600 N; d = 12 m; W = ?

c. W = 600 N x 12 m

b. W = Fd

d. W = 7200 Joules

B. Power:

The amount of work done in a certain amount of time.

1. Equation: P = W / t -or- P = Fd / t

2. SI units for power = Watts

3. Example: If a machine moves a 500 N crate 8 m in 10 seconds, how much

power is exerted?

a. F = 500 N; d = 8 m; t = 10 sec

b. P = Fd / t

c. P = (500 N x 8 m) / 10 s

d. P = 400 Watts

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II. Using Machines

A. Machine:

A device that makes work easier

1. Machines either change the force involved (crowbar),

or change the direction of the force (flagpole)

2. When you increase the distance covered, the force exerted decreases

3. Effort Force:

The force you put into a machine (Fe), “input force”

4. Resistance Force:

The force that the machine puts out (Fr), “output force”

5. You can’t get more energy out of a machine than you put into it;

you can get more force out of it, though! (Law of Conservation of Energy)

a. Some energy always transfers into friction and heat!

b. Ideal Machine: a machine that the work (energy) in = work (energy) out

B. Mechanical Advantage (MA):

The number of times a machine multiplies the force

you put into it. (MA’s a ratio, so NO units!!!!!!!)

1. (Equation: MA = Fr / Fe)

2. Example: When you exert a 35 N force on a crowbar, it exerts a

225 N force, what is the MA of the crowbar?

c. MA = 225 N / 35 N

a. Fr = 225 N; Fe = 35 N

b. MA = Fr / Fe

d. MA = 6.4

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C. Efficiency:

A measure of how the work (energy) you put into a machine

compares to the work (energy) you get out of it

1. Efficiency = (Work output / work input) x 100 = ___%

-or- Efficiency = (Frdr / Fede) x 100 = ___%

2. Efficiency is always less than 100% (Conservation of energy)

3. Reducing friction increases efficiency (oil, wheels, etc)

  • Example: You push with a 150 N force 6 meters up a ramp. If the box
  • weighs 200 N and the height of the ramp is 3 m, what is the eff.?

a. Fr = 200 N; Fe = 150 N; dr = 3 m; de = 6 m

b. Efficiency = (Frdr / Fede) x 100 = ___%

c. Eff = ((200x3) / (150x6)) x 100

d. Efficiency = 66%

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III. Simple Machines (6):

Devices that do work with only one movement

A. Lever:

A bar that is free to pivot, or turn, about a fixed point

1. Parts of a lever

a. Fulcrum:

The fixed point of a lever (turning point)

b. Effort arm:

Where the effort force is applied

c. Resistance arm:

Where the lever exerts the force on resistance

2. IMA = length of effort arm / length of resistance arm

3. Classes of levers

(scissors, crowbar, seesaw)

a. 1st: R – F – E

b. 2nd: E – R – F

(wheelbarrow, nutcracker)

c. 3rd: R – E – F

(arms, legs, shoveling snow)

B. Pulley:

Grooved wheel with a rope or chain running along the groove

1. Can be fixed or movable

a. Fixed:

Attached, doesn’t move, same force needed, just new direction

b. Movable:

Moves, attached to moving object

c. IMA = number of ropes pulling UP on the object

2. Block & Tackle:

Any combination of fixed and movable pulleys

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C. Wheel and Axle:

Consists of two wheels of different sizes that rotate together

1. Larger wheel is for the effort force, smaller wheel (axle) is for resistance

2. IMA = radius of wheel / radius of axle

D. Inclined Plane:

A sloping surface used to raise objects

  • Work (energy) is the same, whether slope is steep or gradual,
  • only the force needed changes

2. IMA = length of slope / height of slope

E. Screw and Wedge

1. Screw:

An inclined plane wrapped around a cylinder

2. Wedge:

Inclined plane used to pry apart an object

F. Compound Machine:

A device that involves two or more simple machines

Honda Rube Goldberg Ad