chapter 8 machines. machines make work easier by changing the force you exert in size or direction,...
TRANSCRIPT
•Machines make work easier by changing the force you exert insize or direction, or both.•But it does not change the amount of work done. •A machine is a device that makes work easier.
SIMPLE MACHINES
Do work
with one
movement
COMPOUND MACHINES
Are a combination
of two or more simple machines.
MACHINES
SIMPLE MACHINES
INCLINED PLANETYPE
LEVER TYPE
•Inclined Plane•Screw•Wedge
•Lever•Wheel & Axle•Pulley
COMPOUND MACHINE
AXE
Wedge and
Lever
EFFORT FORCE (Fe) is the force you exert on a machine*
RESISTANCE FORCE (Fr) is the force applied by the machine to overcome resistance.*
EFFORT DISTANCE (de) is the distance moved by the effort force.*
RESISTANCE DISTANCE (dr) is the distance moved by the resistance force.
WORK INPUT (Win) is the work put into the machine
eein dFW
WORK OUTPUT (Wout) is the work that the machine puts out.
rrout dFW
MECHANICAL ADVANTAGE (MA) is the number of times a machine multiplies the effort force.
e
r
FF
MA
IDEAL MECHANICAL ADVANTAGE (IMA)
r
e
dd
IMA
EFFICIENCY
%100
%100
ee
rr
in
out
dF
dF
W
WEfficiency
An ideal machine has an efficiency of 100%. Machines have efficiency of < 100%
LEVERSA lever is a bar that is free to pivot, or turn, about a fixed point.The fixed point of a lever is called the fulcrum. EFFORT FORCE
FULCRUMRESISTANCE
FORCE
LEVERSThe part of the lever on which the effort force is applied is called the effort arm.The part of the lever that exerts the resistance force is called the resistance arm.
RESISTANCE ARM
EFFORT ARM
FIRST CLASS LEVER
The fulcrum is always in between the effort force and the resistance force. Examples: seesaw, hammer, hair clip, bottle opener, crow bar and cart
SECOND CLASS LEVERThe resistance is located between the effort force and the fulcrum. They always multiply effort force. Examples: nutcracker, wheelbarrow, hole puncher,bottle opener, lemon squeezer
THIRD CLASS LEVERThe effort is located between the resistance force and the fulcrum. They do not multiply effort force. Examples: rake, broom,fishing pole, your arm, andhockey stick
THIRD CLASS LEVER
RESISTANCE ARM
EFFORT ARM
EFFORT FORCE
RESISTANCE FORCE
Effort arm is always shorter than the resistance arm, so it cannot multiply force. It’s MA isalways less than 1.
LEVERS WITH A HUMAN TOUCH
Your neck muscles support the effort force to hold up the resistance weight of your head.
First Class Lever
Tip of your feet-FulcrumWeight -ResistanceMuscles- Effort
Second Class Lever
PROBLEM #1
A lever uses a force of 250 N to move a rock that weighs 3000 N. Find the mechanical advantage of using the lever.
12250
3000
N
N
F
FMA
e
r
A simple machine allows a worker to move an effort distance 45 cm in order to move a resistance distance 20 cm. What is the ideal mechanical advantage of the machine?
PROBLEM #3
25.220
45
r
e
d
dIMA
PULLYING WITH PULLEYSA pulley is a grooved wheel with a rope or a chain running along the groove. A pulley works something like a first class lever. Instead of a bar a pulley has a rope. The axle of the
pulley acts like the fulcrum. The two sides of the pulleyare the effort arm and the resistancearm.
SINGLE FIXED PULLEY•It is lifted the same distance as it is pulled at the other end of the rope.•Example: flagpole, blinds.•Force is not multiplied. There is only a change in the direction of the force applied.
W F
SINGLE MOVABLE PULLEYS
•Force is multiplied.IMA = 2, since two wires are supporting the resistance. The effort distance must be twice as large as the resistance distance. Example:construction cranes.
W
F
BLOCK AND TACKLE
•It is combination of fixed and movable pulley.•It can produce large enough force to lift automobiles.
Wheel and axle is a simple machine consistingof two wheels of different sizes that rotate together.
Examples: doorknob, faucets, handles, steering wheel of a car etc.
An effort force is applied to the larger wheel. The smaller wheel, called the axle, exerts the resistance force.
It allows us to use less force to do work.
WHEEL AND AXLE
Axleof Radius Wheelof RadiusIMA
Gear is a modified wheel and axle machine. It is wheel with teeth along its circumference. Effort is exerted on one of the gears and the other gear turns.
WHEEL AND AXLE
Slanting boards and ramps are called inclined planes. They are sloping surfaces used to raise objects. W=weight, H=height, L=length, F=force
F
W
H
L
INCLINED PLANE
What is the efficiency of a machine with work input 1000 J and work output 700 J?
%%xJ
Jefficiency
%xWin
Woutefficiency
701001000
700
100
PROBLEM #8
Screw is another form of inclined plane. The threads form a tiny ramp that runs from its tip to near its top. As you turn the screw, the threads seem to pull the screw into the wood.
SCREWS
When a screw is turned ,it moves forward. At the same time, it presses against the material around it. Many turns are needed to move the screw a short distance. But the screw produces a far greater force than the force needed to turn it.
SCREWS