WORK
A force
acting over a distance to move something is the definition of work. In order for any work to be done there must
be movement.
Work= force x
distance
The formula for work is:
Work=N∙M or Joule (J)
POWER
Power is
the rate at which work is done, or the amount of work per unit of time.
The formula for power is:
Power = work OR
power=force x distance
Time
Time
Power is measured in watts and they are named after James
Watt (inventor of the first steam engine).
Watts(w)=1J/S
One thousand watts is equal to one kilowatt (kw). The electric company measures the electric
power used in your home in kwh.
Horsepower is the power that one horse could do = 745.56 w, originally it was the power necessary for a
strong horse to move a 750N object one meter in one second. Horsepower is commonly used to measure the
power of engines and motors.
MACHINES
Machines
are devices that help us to do work easier!
Some examples of early machines are stones (used for tools), tree
branches (pry up heavy objects), carts with wheels (carry objects). Machines
make work easier because they change the size or the direction of the
applied force. So, in other words,
machines either lessen the amount of force you have to apply and/or they change
the direction and distance an object has to move. There are always two forces involved when
using machines to do work.
Effort
force (FE) and Resistance
force (FR)
(FE) effort
force is force that is applied to a machine.
(FR) resistance
force is force applied by a machine.
(DE) effort
distance is the distance through which a machine moves or distance through
which the effort force is applied to a machine.
(DR) resistance
distance is the distance through which the resistance force is applied or
the distance through which the object moves.
(WI) work
input is work done on a machine equal to the effort force times the
distance through which the force is applied.
(WO) work output is work that is done by a machine equals resistance
force times the distance through which the force is applied.
Formula for work input WI=FE
x DE
Formula for work output WO=FR x DR
Work output (WO) can never be greater than the work input (WI). Although
machines make work easier, they do not multiply work.
MECHANICAL ADVANTAGE & EFFICIENCY
Mechanical Advantage is the
number of times a machine multiplies the force.
Formula for
mechanical advantage is:
Mechanical
Advantage = Force of resistance \ Force of effort
MA
= FR/FE
Sample Problems: If a
crowbar allows you to exert only 20
newtons of force to raise a
200 newton
object, what would its mechanical advantage be?
MA
= FR/FE
MA
= 200N / 20 N
MA
= 10.0 (There is NO Unit for MA)
The mechanical advantage of a machine can be greater than one, equal to one, or less than
one. It depends on how it changes the
force you apply.
Efficiency is the
comparison of work output to work input and is always expressed as a
percentage.
Efficiency
= (Work output/ Work input) x 100
E=
(WO x WI) x 100
The efficiency of a machine can NEVER be greater than
100! Machines always have parts that rub
on something, so you always loose some effort to overcome FRICTION.
Due to this, no matter how much effort force you put into a
machine you can never get a greater work output from the machine!
INCLINED PLANES
A slanted
surface used to raise an object is called an inclined plane. When an
inclined plane is used a smaller effort force is required to move the object
but the object is moved over a greater distance.
The formula
for finding the mechanical advantage of an inclined plane is:
MA= length of the plane / height
The length
of an inclined plane can never be shorter
than its height! Therefore, the
mechanical advantage of an inclined plane is always more than one.
SCREW
The threads of a screw are like an
incline plane wrapped around a cylinder to form a spiral. The closer the threads of a screw are the
greater the mechanical advantage of the screw, because the longer the incline
plane.
WEDGE
A wedge is
an inclined plane that moves. The longer
and thinner the wedge is, the effort force is required to do work.
A kind of
wedge is an inclined plane, double wedge and single wedge. The use for these wedges are to cut, split or
fasten. (a tack, a nail, a knife, an axe
and a chisel).
LEVERS
The formula
for levers is: MA = effort arm force /
resistance arm length
Resistance is the object moved and effort is the force
placed.
SYMBOLS: F=fulcrum
R=resistance E=effort
Resistance = object moved Fulcrum = pivoting point Effort = force is placed
Some examples of levers are a bottle opener, a
fishing pole, a seesaw, a broom, a pair of pliers, and a wheelbarrow.
WHEEL AND AXLE
A lever
that rotates in a circle is a wheel and axle.
A wheel and axle is made of two wheels of different sizes. The axle is the smaller wheel. The effort force is applied to the wheel. Some examples of wheels and axles used
everyday are a ferris wheel, wheel chair and the wheels of a car.
The formula
for the mechanical advantage of a wheel and axle is:
MA= radius of
wheel
radius of axle
A small
force on the wheel produces a larger force on the axle. A large movement of the wheel edge produces a
small movement of the axle.
PULLEYS
A pulley is
a chain, belt or rope wrapped around a wheel.
Pulleys can change either the direction and/or amount of effort force.
TYPES OF PULLEYS:
FIXED
PULLEY: a fixed pulley is a pulley
that is attached to a stationary object (wall, ceiling, etc.). A fixed pulley can not multiply effort force,
but it can change the direction of the force which might make life easier.
The
mechanical advantage of a fixed pulley is one.
Effort
force = resistance force
MOVEABLE
PULLEYS: a moveable pulley is
suspended from a rope and can move with the effort force. Moveable pulleys can multiply effort force
but they cannot change direction of effort force.
The
mechanical advantage of a moveable pulley is greater than one. To find the mechanical advantage of a pulley
system you would count the number of
supporting sections of rope.
The two
parts of a pulley are the rope and the grooved wheel. Some examples of everyday pulleys are an
elevator, a crane, or window shades.
