EA 2
2011
Mechanical Engineering
What is Mechanical Engineering?
Mechanical engineering is the second largest and
one of the oldest disciplines; broadest of all
engineering disciplines.
Mechanical engineers apply the principles of
mechanics and energy to the design of machines
and devices:
ENERGY
and
MOTION
What will you study as an ME student?
Forces, motion, structures
: statics, dynamics, kinematics,
mechanics of solids and fluids.
Energy
: thermodynamics, heat transfer
Materials
: materials engineering & processing, manufacturing.
Machines
: graphics, design, machine elements, controls.
Economics
: engineering economic analysis, cost engineering.
Human and Social Studies
: arts, humanities, social sciences,
history, government, ethics, law.
Overall foundation
: math, physics, chemistry, biology, analysis
skills, communication skills, computation skills.
Mass vs. Weight
A 1.0

kg mass is suspended from a
spring scale in an effort to determine
its weight. The scale reads just short
of 10.0 N

close enough to call it
9.8 N.
Mass
refers to
how much stuff
is
present in the object.
Weight
refers to the force with
which gravity pulls upon the object
F = m * a
9.81 N/m
2
Check Understanding
1. Complete the following table showing the relationship between mass and weight.
Object
Mass (kg)
Weight (N)
Melon
1 kg
9.8 N
Apple
.1 kg
0.98 N
Pat
25 kg
245 N
Fred
100 kg
980 N
2. Different masses are hung on a spring scale calibrated in Newtons.
a.
The force exerted by gravity on 1 kg = 9.8 N.
b.
The force exerted by gravity on 5 kg = __
49
____ N.
c.
The force exerted by gravity on __
10
_____ kg = 98 N.
d.
The force exerted by gravity on 70 kg = ___
686
____ N.
Energy and Momentum
Work and Energy
•
Machines and Mechanical Advantage
•
Work
•
Energy and Conservation of Energy
1.
Calculate the mechanical advantage for a lever or rope
and pulleys.
2.
Calculate the work done in joules for situations
involving force and distance.
3.
Give examples of energy and transformation of energy
from one form to another.
4.
Calculate potential and kinetic energy.
5.
Apply the law of energy conservation to systems
involving potential and kinetic energy.
Objectives
machine
energy
input force
output force
ramp
gear
screw
rope and pulleys
closed system
work
lever
friction
mechanical system
simple machine
potential energy
kinetic energy
chemical energy
mechanical energy
mechanical
advantage
joule
energy
conservation of
energy
electrical energy
input output
input arm output
arm
fulcrum
Vocabulary Terms
Key Question:
How do simple
machines work?
Machines and Mechanical
Advantage
The ability of humans to build buildings and
move mountains began with our invention
of
machines
.
In physics the term “
simple machine
”
means a machine that uses only the forces
directly applied and accomplishes its task
with a single motion.
Machines
•
The best way to analyze what a machine
does is to think about the machine in terms
of
input
and
output
.
Machines
Mechanical Advantage
•
Mechanical advantage
is the
ratio of output force to input
force.
•
For a typical automotive jack the
mechanical advantage is 30 or
more.
•
A force of 100
newtons
(22.5
pounds) applied to the input arm
of the jack produces an output
force of 3,000
newtons
(675
pounds)
—
enough to lift one
corner of an automobile.
Mechanical Advantage
MA =
F
o
F
i
Output force (N)
Input force (N)
Mechanical
advantage
Mechanical Advantage of a Lever
MA
lever
=
L
i
L
o
Length of input arm
(m)
Length of output arm
(m)
Mechanical
advantage
Calculate position
•
Where should the fulcrum of a lever be
placed so one person weighing 700 N
can lift the edge of a stone block with a
mass of 500 kg?
The lever is a steel bar three meters long.
Assume a person can produce an input force equal to
their own weight.
Assume that the output force of the lever must equal
half the weight of the block to lift one edge.
Wheels, gears, and rotating
machines
•
Axles and wheels provide advantages.
•
Friction occurs where the wheel and axle touch or where the wheel
touches a surface.
•
Rolling motion creates less wearing away of material compared with
two surfaces sliding over each other.
With gears the trade

off is
made between
torque
and
rotation speed.
An output gear will turn with
more
torque when it rotates
slower than the input gear.
Ramps and Screws
•
Ramps
reduce input force by
increasing the distance over
which the input force needs to
act.
•
A
screw
is a simple machine that
turns rotating motion into linear
motion.
•
A thread wraps around a screw at
an angle, like the angle of a
ramp.
Work
Key Question:
What are the
consequences of
multiplying forces in
machines?
Work
•
In physics,
work
has a very specific
meaning.
•
In physics, work
represents a
measurable change
in a system, caused
by a
force.
Work
•
If you push a box with a force of
one
newton
for a distance of
one meter
, you
have done exactly
one joule
of work.
Work (force is parallel to distance)
W = F x d
Distance (m)
Force (N)
Work (joules)
Work (force at angle to distance)
W = Fd cos (
q
)
Distance (m)
Force (N)
Work (joules)
Angle
Work done against gravity
W =
mgh
Height object raised (m)
Gravity (m/sec
2
)
Work (joules)
Mass (g)
Why the path doesn't matter
Calculate work
A crane lifts a steel beam with
a mass of 1,500 kg.
Calculate how much work is
done against gravity if the
beam is lifted 50 meters in
the air.
How much time does it take
to lift the beam if the motor of
the crane can do 10,000
joules of work per second?
Energy and Conservation of Energy
Energy
is the ability to make things change.
A system that has energy has the ability to do work.
Energy is measured in the same units as work
because energy is transferred during the action of
work.
Potential Energy
E
p
= mgh
Height (m)
Mass (kg)
Potential Energy
(joules)
Acceleration
of gravity (m/sec
2
)
A cart with a mass of
102 kg is pushed up a
ramp.
The top of the ramp is
4 meters higher than
the bottom.
How much potential
energy is gained by the
cart?
If an average student
can do 50 joules of
work each second, how
much time does it take
to get up the ramp?
Potential Energy
Energy of motion is called
kinetic energy
.
The kinetic energy of a moving object depends on
two things: mass and speed.
Kinetic energy is proportional to mass.
Kinetic
Energy
Mathematically, kinetic energy increases as the
square
of speed.
If the speed of an object doubles, its kinetic
energy increases four times. (mass is constant)
Kinetic
Energy
E
k
=
1
mv
2
2
Speed (m/sec)
Mass (kg)
Kinetic Energy
(joules)
Kinetic
Energy
Kinetic energy becomes important in calculating
braking distance.
Kinetic Energy
A car with a mass of 1,300
kg is going straight ahead at
a speed of 30 m/sec (67
mph).
The brakes can supply a
force of 9,500 N.
Calculate:
a) The kinetic energy of the car.
b) The distance it takes to stop.
Calculate Kinetic Energy
As energy takes different forms and changes things
by doing work, nature keeps perfect track of the
total.
No new energy is created and no existing energy is
destroyed.
Law of Conservation of Energy
Energy and Conservation of Energy
Key Question:
How is motion on a
track related to
energy?
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