Topics:
MECH1200
• Introduction to Chapter 3: Mechanical Power
Transmission Fundamentals
• Energy
• Force
• Inertia
• Acceleration
• Friction
• Work
• Note: Refer to textbook Mechanical Principles and Systems for Industrial
Maintenance, Richard Knotek and Jon Stenerson, Prentice Hall, 2006
1
Introduction
MECH1200
• Usually mechanical systems consist of the
following elements:
- A prime mover: such as en electric motor
or an internal combustion engine.
- Linking components: such as shafts, gears,
belts, joints..etc.
- Driven components such as wheels.
2
Energy
MECH1200
Exists in many forms: kinetic, potential, heat,
electrical, chemical, ..etc.
• Energy cannot be created nor destroyed, but it
changes from one form to another.
Law of conservation of energy - 1st law of
thermodynamics
• Some common units of energy:
 Joule (J)
 Foot-pound force (ft.lb) = 1.356 Joules
 KiloWatt-hour (KWH) = 1000 Watt.h = 3600 kJ
 Calorie = 4.2 kJ = 4200 J
3
Force
MECH1200
Force: causes an object that has a mass to
change velocity.
• A vector quantity: It has direction and
magnitutde.
• Some common units of force:
Newton (N)
Pound force (lbF) = 4.45 N
4
Inertia
MECH1200
• Inertia: the resistance of an object to any change
in its motion state.
• Example 1: Object A (1 kg) and object B (10 kg) are
each acted upon by a force of 1 N to the right. which
would move at a higher acceleration?
1N
A
1kg
1N
B
10 kg
Object B has a larger inertia, thus more resistance to
change in its rest state, so it has a lower acceleration.
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MECH1200
Newton’s Laws of Motion
Newton’s First Law (Law of Inertia):
A body in rest will stay in rest and a body in motion will
stay in motion unless an external force is acted upon it.
Newton’s Second Law (conservation of momentum):
The net force acting on an object is equal to the rate of
change of its momentum.
Forcenet = mass × acceleration
Newton’s Third Law (action-reaction law):
For every action there is a reaction equal in magnitude
and opposite in direction.
FA = - FB
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MECH1200
Acceleration & Deceleration
• The change of speed of a body. If the speed decreases, then it is
a deceleration.
•
•
1.
2.
From Newton’s 2nd Law: 𝐹 = 𝑚𝑎
Example 2:
What is the acceleration of object A if a force of 2 N acts on it?
What is the acceleration of object B if a force of 20 N acts on it?
2N
A
1kg
B
10 kg
Both objects will move at the same acceleration and
acquire the same final speed.
Both objects will have an acceleration of 2 m/s2
20 N
7
MECH1200
Acceleration & Deceleration
• 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =
𝐹𝑖𝑛𝑎𝑙 𝑠𝑝𝑒𝑒𝑑−𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑝𝑒𝑒𝑑
𝑇𝑖𝑚𝑒
𝑚/𝑠 2
• Example 3:
• A vehicle is moving at a speed of 20 ft/s. After 4
seconds, the vehicle was moving at a speed of 15
ft/s. what is the acceleration of the vehicle?
• Solution:
• 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =
𝑓𝑡
𝑠
𝑓𝑡
𝑠
15( )−20( )
4 (𝑠)
=−
𝟓
𝒇𝒕
𝐬
𝟒 𝐬
= −1.25 ft/s2
Thus the vehicle decelerates by 1.25 ft/s2.
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Angular Speed
MECH1200
• Angular speed: a measurement to describe an object rotating
about an axis.
• Units of measurement:
 RPM
 Rad/s
Surface velocity (ft/min) = RPM× π × diameter (ft)
= RPM ×0.262 × diameter (in inches)
Example 4: what is the surface velocity of a 20 in diameter rotor
that rotates at 1500 rpm?
Solution:
𝑆𝑝𝑒𝑒𝑑 = 1500
1
𝑚𝑖𝑛
× 𝜋 × 20 𝑖𝑛 ×
1 𝑓𝑡
12 𝑖𝑛
= 1500 × 0.262 × 20 = 7860 ft/min
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Friction
MECH1200
• The resistance to a mechanical force.
• Applies to solids and fluids
• Types of friction:
1. Static friction: when the contacting surfaces are at
rest.
2. Sliding friction: when one or both of contacting
surfaces is/are at motion.
3. Rolling friction: when the surfaces are separated by
rollers.
How to quantify friction?
Coefficient of friction relates force to mass of object
under force.
10
Work
•
•
•
•
MECH1200
Work = force × distance ( W = F × D)
Units of work: N.m or ft.lb
Example 5:
A force of 10 ft.lb is applied on a body at rest that has a
mass of 4 lb. Then the force was removed after the body
moved 5 ft.
10 N
4 kg
a) What is the acceleration that the body acquires while the
force is applied?
F = ma thus a = F/m = 10 / 4 = 2.5 ft/s2
b) What is the work performed on the body?
W = F×D = 10 × 5 = 50 ft.lb
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Exercise
MECH1200
1. Which of the following are vectors and which
are scalar quantities:
Temperature
Pressure
Velocity
Acceleration
Energy
Force
Inertia
scalar
scalar
vector
vector
scalar
vector
scalar
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Exercise
MECH1200
2. Match each of the following terms to its unit
of measurement:
Mass
Kilowatt.hour
Energy
Newton
Velocity
kilowatt
Acceleration
ft/s
Force
Kg
Power
RPM
Angular speed
m/s2
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Exercise
MECH1200
3. What is the acceleration (g) of the earth’s
gravitational force at sea level?
Solution:
9.81 m/s2 = 32.2 ft/s2
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