# Work and Machines

Document Sample

```					1. How would the effort exerted by a
backpacker over level ground compare to
the effort in climbing a steep hill?
2. How would the weight of the backpack
affect the amount of force needed to
move it?
Work and Machines

Chapter 5
Section 1
Work
Work—transfer of energy
that occurs when a force
makes an object move.
(Force is a push or pull)
Doing Work

       To determine if work is being done 2
conditions must be satisfied:
    The object has to move
    The motion of the object must be in the
same direction as the applied force.
       Figure 1 and 2, page 126-127.
Work and energy are related

   When work is done, a transfer on energy
always occurs.
   Energy is transferred from the object doing
the work to the object on which the work is
done.

Box being
given
energy
Work

       Work is done on an object only when a
force is being applied to the object and the
object moves.
       The amount of work done depends on 2
things:
    the amount of force exerted
    the distance over which the force is applied
Calculating Work

    Work = Force x distance
 W = F (N) x d (meters)

    Work is measured in Joules
Math skills activity
Practice problems, page 128
Power

   Power—amount of work done in a certain
amount of time.

   It is the rate at which work is done
How Do You Calculate Power?

       P = W/t (seconds)

       Power is measured in watts
    1 W = 1 J/s (very small unit)
    One kilowatt (kW) = 1000 W
Conversions of Power
Btu = 1,055 watts          Horsepower = 746
(for heating and cooling    watts
units)                  (for motors and engines)
Since work and energy are related,
power can also be calculated:
Math skills Activity—page 130

Practice problems, page 130

Section 1 Assessment-page 131
Questions 1-6.
Who is more powerful?
   Sally and Pete do the same amount of
work. Sally does the work in 2.3 hours
and Pete does it in 2.5 hours. Who is
Using Machines
Section 2
Using Machines
The car may weigh a
lot, but you don’t have
   A machine makes   to use nearly that
doing work        much force to lift it
easier.           with a jack.

   They may
multiply the
applied force.
Machines may increase the distance over
which a force is applied.

In this case, the amount of force necessary to push the
chair up the ramp was decreased.
Machines may change the direction
a force has to be applied.

The nail comes up as the person
pulls to the side.
   Since work is equal to force times distance
(W = F/d), the same amount of work can be
done by applying a small force over a long
distance as can be done applying a large
force over a short distance.

Small force over ling   Large force over
distance           short distance
Two forces are involved when a
machine is used to do work:
   effort force              resistance force

force applied to the       force applied by the
machine                    machine to overcome
resistance.
(Fe)
(Fr)
When prying a nail out of a piece of wood with
a claw hammer, you exert force on the handle
of the hammer, and the claw exerts the
resistance force
Amount of energy the machine
transfers to the object cannot by
greater than the amount of energy
transferred to the machine.
   Win                    Wout

work done by            work done by the
you on a                machine is called
machine is called       output work.
input work.
   A machine cannot create energy so
output work (W out) is never greater than
input work (W in)

   Some energy transferred is changed to
heat due to friction.

   An ideal machine is theoretical; it does
not take friction into account.

   Ideal Machine:    Win = Wout

   Mechanical                    To calculate:
number of times a
machine multiplies
the effort force

Some machines don’t multiply the force that is
applied to them. For example, mini blinds—the
effort force equals the resistance force, the MA of
the mini blinds is 1
Efficiency
   Efficiency measures of how much of the
work put into a machine is changed into
useful output work by the machine.

   To calculate:

   Efficiency of a machine is always less
than 100%
   Friction changes the useful work of a
machine into thermal energy.

   Adding lubricant such as oil and
graphite to reduce friction can make
a machine more efficient.

Oil fills the space
between surfaces so
high spots don’t rub
against each other.
Section 2 Assessment
Question 1-6
Page 137
Simple Machines
Section 3
Simple Machine

A machine that does work with only
one movement is a simple machine.
Two basic kinds of simple machines

   Lever                      Inclined plane
   Lever can be modified      Inclined plane can be
into:                       modified into:
   Pulley                     Screw
   Wheel and axle             wedge
Lever

   Lever is a bar that is      Effort arm is part of
free to pivot about a        the lever on which
fixed point called the       effort force is applied.
fulcrum.                    Resistance arm is
part of the lever that
exerts the resistance
force.
Three classes of levers are
based on positions of effort
force, resistance force, and
fulcrum.
First-class lever
   Fulcrum is located between the effort force and
resistance forces
   Multiplies and changes direction of force.
Second class lever

   Resistance force is
located between the
effort force and
fulcrum
   Always multiplies
force
Third class lever

   Effort force is
between the
resistance force and
fulcrum
   Does not multiply
force but does
increase distance
over which force is
applied.
Levers in human body

First class   Third class   Second class
Calculating ideal mechanical
Pulley

A pulley is a grooved wheel with
a rope, chain, or cable running
along the grove.
Types of pulleys

   A fixed pulley is
attached to
something that
doesn’t move
   Force is not            4N   4N
multiplied but
direction is changed.
   IMA = 1
Types of pulleys

   A movable pulley has
one end of the rope
fixed and the wheel
free to move
   Multiplies force
   IMA = 2
Types of pulleys

   Block and tackle—
system of pulley
consisting of fixed
and movable pulleys
   IMA = number of
ropes supporting
resistance weight.

The 100 N weight is divided
equally among the four
rope segments, so you
only have to use a 25 N
force to lift weight.
Wheel and Axle

   Wheel and axle is a
machine with two wheels
of different sizes rotating
together.
   Modified lever form
   IMA = radius of wheel
axle

Effort distance
   Gears are a modified form
of a wheel and axle.
Inclined Plane
   Inclined plane—
sloping surface that
reduces the amount
of force required to
do work.

   Less force is required
if a ramp is longer
and less steep.
Screw

   A screw is an inclined      Wedge is an inclined
plane wrapped in a           plane with one or two
spiral around a              sloping sides.
cylindrical post.
Compound Machine
   A compound machine uses a combination of
two or more simple machines.

1    Lever

2    Wheel and axle

3    Inclined plane

4    Lever

55   Pulley

6    Lever

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 1 posted: 4/12/2011 language: English pages: 45