2 Mechanical Equilibrium
Different Types of Forces
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Gravity (also called weight)
Friction
Applied Force
Air Resistance
Tension
Spring Force
Normal Force (also called support force)
• http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Cl
ass/newtlaws/u2l2b.html#spring
2 Mechanical Equilibrium
2.3 Definition of Forces
1. Gravity
Force of attraction due to mass of an object
2. Tension Force
Pulled force applied to a rope, wire or cable
3. Spring Force
Elastic, compressed or stretched force
4. Applied Force
Any push or pull on an object
5. Friction
Resistance of two objects in contact moving past
each other
6. Air Resistance
Resistance of an object moving through the
atmosphere
7. Normal force
Force that pushes back on one object resting on
another
2 Mechanical Equilibrium
2.3 Support Force
For an object at rest on a horizontal surface, the
support force (normal force) must equal the
object’s weight (gravity).
2 Mechanical Equilibrium
2.1 Force
Tension, Weight and Spring
The upward tension in the
string has the same magnitude
as the weight of the bag, so the
net force on the bag is zero.
The bag of sugar is attracted to
Earth with a gravitational force
of 2 pounds or 9 newtons.
•1 lb = 4.45 N
Spring
force (in
scale)
Weight (of
bag)
•9 N of weight = 9 N of tension
and spring force
Tension
(in
string)
2 Mechanical Equilibrium
2.3 Support Force
Support Force (also called
normal force)
The table pushes up on the book
with as much force as the
downward weight of the book.
Fnorm = Fgrav
Net force
Weight (also called gravity)
2 Mechanical Equilibrium
2.3 Support Force Physics
The book lying on the table compresses atoms in the table and
they squeeze upward on the book. The compressed atoms
produce the support force.
2 Mechanical Equilibrium
2.3 Support Force
The upward support
force is as much as the
downward pull of
gravity.
2 Mechanical Equilibrium
2.3 Support Force
The upward support
force is as much as the
downward pull of
gravity.
2 Mechanical Equilibrium
2.3 Support Force
think!
What is the net force on a bathroom scale when a 110-pound
person stands on it?
2 Mechanical Equilibrium
2.3 Support Force
think!
What is the net force on a bathroom scale when a 110-pound
person stands on it?
Answer: Zero–the scale is at rest. The scale reads the
support force, not the net force.
2 Mechanical Equilibrium
2.3 Support Force
think!
Suppose you stand on two bathroom scales with your weight
evenly distributed between the two scales. What is the reading
on each of the scales? What happens when you stand with
more of your weight on one foot than the other?
2 Mechanical Equilibrium
2.3 Support Force
think!
Suppose you stand on two bathroom scales with your weight
evenly distributed between the two scales. What is the reading
on each of the scales? What happens when you stand with
more of your weight on one foot than the other?
Answer: In the first case, the reading on each scale is half
your weight. In the second case, if you lean more on one
scale than the other, more than half your weight will be read
on that scale but less than half on the other. The total support
force adds up to your weight.
2 Mechanical Equilibrium
2.3 Support Force
For an object at rest on a horizontal surface,
what is the support force (or normal force) equal
to?
•The object’s weight (or gravity)!
2 Mechanical Equilibrium
2.1 Force
Tension and Weight
A stretched spring is under a “stretching force”
called tension.
Pounds and newtons are units of weight, which
are also units of force.
Weight is a force.