Chapter 5 Work & MachinesChapter 5 Work & MachinesI. Work

A. What is work?

1. Work is the transfer of energy that occurs when a force

makes an object move.

2. There are two conditions that have to be satisfied for

work to be done on an object:

a. the applied force must make the object move

b. the movement mist be in the same direction as the

applied force

B. Work and Energy

1. When work is done, a transfer of energy always occurs.

2. Energy is the ability to cause change or to do work.

3. Energy is always transferred from the object that is

doing the work to the object on which the work is done.

4. The amount of work done depends on the amount of

force exerted and the distance over which the force is

applied.

Work Equation: WW==FFdd

work (joules)= applied force (Newtons) X distance (meters)

C. Power

1. Power is the amount of work done in one second/the

rate at which work is done or energy transferred.

2. When work is done, power can be calculated from the

equation: Power=work/time

P (in watts)=W (in joules)/t (in seconds)

3. When energy is transferred, power can be calculated

from the equation: Power=energy transferred/time

P (in watts)=E (in joules)/t (in seconds)

II. Using Machine A. What is a machine? 1. A machine is a device that makes doing work easier. 2. Machines can make work easier by a. increasing the force that can be applied to an object b. increasing the distance over which a force can be applied c. changing the direction of an applied force B. The Work Done by Machine 1. Two forces are involved when a machine is used to do work. a. you exert a force on the machine (input force) b. the machine exerts a force on the object (output force)

2. Two kinds of work need to be considered when you use

a machine:

a. the work done by you on the machine (input work)

b. the work done by the machine (output work)

3. Because energy is always conserved, Wout is never

greater than Win.

4. The machine does not transfer all of the energy it

receives to the object, some of the energy changes to

heat due to friction therefore Wout is always smaller than

Win

5. For an ideal machine, input work=output work

C. Mechanical Advantage 1. Mechanical advantage is the ratio of the output force to the input force 2. Mechanical Advantage Equation:

MA= Fout/Fin

3. The mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA. 4. IMA is calculated by dividing the input distance by the output distance D. Efficiency 1. Efficiency is a measure of how much of the work input into a machine is changed into useful output work by the machine

2. A machine with high efficiency produces less heat from friction so more of the input work is changed to useful output work,

3. Calculating Efficiency

efficiency= Wout/Win x 100

4. Efficiency of an ideal machine would be 100%,

whereas the efficiency of a real machine would be less

than 100% because friction causes the output work to

always be less than the input work.

5. Machines can be made more efficient by reducing

friction. This is done by adding a lubricant.

6. A lubricant (oil or grease) fills in the gaps between the

surfaces enabling the surfaces to slide past each other

more easily.

III. Simple Machines

1. A simple machine is a machine that doe work with only one

movement of the machine.

A. Types of Simple Machine (6 types)

a. lever-a bar that is free to pivot or turn around a fixed point

b. pulley-is a grooved wheel with a rope, chain, or cable running along the

groove

c. wheel & axle-is a simple machine consisting of a shaft or axle attached to

the center of a large wheel, so that the wheel and axle rotate together

d. inclined plane- sloping surface such as a ramp that reduces the amount of

force required to do work

e. screw-an inclined plane wrapped in a spiral around a cylindrical post

f. wedge-an inclined plane with one or two sloping sides

A. Levers

1. The fixed point the lever pivots on is called the fulcrum.

2. There are (3) classes of levers. The difference among

the (3) classes of levers depend on the locations of the

fulcrum, the input force, and the output force

3. First-Class Lever The fulcrum is between the input force and the

output force

4. Second-Class Lever The output force is between the fulcrum and

the input force; output force is always greater than the input force

5. Third-Class Lever The input force is between the fulcrum and the

output force; the output force is always less than the input force

6. The ideal mechanical advantage , or IMA, can be

calculated for any machine by dividing the input

distance by the output distance. IMA=Lin (m) / Lout (m)

B. Pulleys

1. Different types of pulleys

a. A fixed pulley is attached to something that does not

move, such as a ceiling or wall

b. A movable pulley is a pulley in which one end of the

rope is fixed and the wheel is free to move

c. The block and tackle a system of pulleys consisting of

fixed and movable pulleys

C. Wheel & Axle

1. In wheel and axle, the input force is applied to the

wheel and the output force is exerted by the axle

2. Calculating ideal mechanical advantage of Wheel and

Axle: IMA=rw (radius of wheel)/ra (radius of axle)

3. IMA can be increased by increasing the radius of the

wheel.

4. Gears are a type of wheel and axle with the wheel

having teeth around the rim; When the teeth of two

gears interlock, turning one gear causes the other

gear to turn.

5. If the input force is applied to the larger gear, and it

rotates clockwise, the smaller gear rotates

counterclockwise.

D. Inclined Planes

1. Calculating mechanical advantage of an inclined plane

IMA= length of slope (m)/height of slope (m)

2. The IMA of an inclined plane for a given height is

increased by making the plane longer.

E. The Screw

1. Input force is applied by turning the screw; IMA of a

screw is related to the spacing of the threads

4. IMA of a screw is larger if the threads are closer

together; if the IMA is larger, more turns are needed to

drive it into some material

E. The Wedge

1. A wedge is also a simple machine where the inclined

plane moves thru an object or material

F. Compound Machines

1. Two or more simple machines that operate together