Force in Mechanical Systems
Objectives
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Define force, and describe how forces are measured.
Describe what happens when forces on an object are
balanced and when they are unbalanced.
Explain the meaning of Newton's first law of motion.
Define scalar, vector, weight, mass, and torque.
Determine the resultant force on an object when two
or more forces act on it.
Solve problems involving force, lever arm, and
torque.
Ways to transmit mechanical forces!
Measuring Force
Metric System (SI- Systems International)
 English
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Quantity
Unit
Symbol
Length
meter
m
Mass
kilogram
kg
Time
second
s
Electric Current
ampere
A
Temperature
Kelvin
K
Amount of substance
mole
mol
Luminous intensity
candela
cd
Units
Conversions
English
 Length – foot (ft)
 Force - Pound (lb)
 SI
 Length – meter (m)
 Force - Newton (N)
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1 pound = 16 ounces
1 pound = 4.45 newtons
1 kilogram = 1000 grams
1 kg = 9.8 N = 2.2 lb
1 slug = 32.2 lb
1 slug = 14.59 kg
Definitions
Takes into account gravity
Force
 Weight
Not the same thing
 Mass
Amount of matter in an object
 Vector – has magnitude and direction
 Scalar – quantity described by only a
magnitude. (temp., time, pressure, mass)
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Drawing Forces
Draw as an arrow.
 Length of the arrow is proportional to the
magnitude of the vector.
 Head points in the direction of the vector.
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1 inch = 20 lb.
20º
Terms
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Balanced forces
Unbalanced forces
Net force
Equilibrium
Newton’s First Law of Motion (law of inertia)
 Every object will remain at rest, or will
continue to move in a straight line with constant
speed, unless the object is acted on by a net
force.
Inertia – the property of an object to resist changes
in its motion.
Adding Forces
1. Draw the 40 lb. Force
2. Add the 30 lb. Force to the 40 lb. Force (head to tail method)
3. Draw the resultant force
4. Determine magnitude and direction of resultant
5. Form conclusion.
Example
Terms
Torque – the quantity that causes rotation in
mechanical systems. (cw or ccw)
  =F x L (Greek letter “tau”)
Torque = Applied Force x length of lever arm
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A force is a push or a pull.
Summary
 Force is a vector. It has both magnitude and
direction. Its magnitude is measured in pounds or
in newtons.
 Newton's first law says that an object will remain
at rest or will continue to move in a straight line
unless it is acted on by a net force.
 Unbalanced forces result in a net force acting on
an object. Balanced forces result in no net force
acting on an object.
 If two forces act on an object and the forces act
in a straight line, the magnitude of the resultant is
either the sum of or the difference between the
two forces' magnitudes.
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Summary (cont.)
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If two forces act on an object and the forces do not act in
a straight line, the resultant can be found using the headto-tail method of vector addition.
The mass of an object is a measure of the object's
inertia. The weight is a measure of the force exerted on
the object by gravity.
A torque is exerted on a body when a force is applied
and the line of action of the force does not pass through
the body's axis of rotation. The torque equals the force
times the lever arm
If no net torque is exerted on a body, it will remain at
rest or will continue to rotate at a constant rate.
(TLS # 16)