Chapter 5 Work & Machines

Document Sample

```					    Chapter 5 Work & Machines
I. 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: W=Fd
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
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

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 16 posted: 8/31/2012 language: English pages: 14
How are you planning on using Docstoc?