Transcript ch02 equilibrium and forces 2012

2 Mechanical Equilibrium
2.1 Force
• What is a force?
Measured quantity
SI Unit
Symbol
Force
Newton
N
A force is
a push or a
pull.
2 Mechanical Equilibrium
2.1 Force
Net Force
The combination of all forces acting on an
object is called the net force.
2 Mechanical Equilibrium
2.1 Force
An unbalanced force is needed
to change an object’s state of
motion.
A force is
a push or a
pull.
2 Mechanical Equilibrium
2.1 Force
Net Force
The combination of all forces acting on an object is
called the net force.
The net force on an object changes its motion (as long
as the net force is greater than zero).
Measured quantity
SI Unit
Symbol
Force
Newton
N
2 Mechanical Equilibrium
2.1 Force
Balanced vs. Unbalanced Force
Balanced force occurs when the net force = 0 N
(the object experiences no change in motion)
Unbalanced force occurs when the net force > 0 N
(the object experiences a change in motion)
•It takes an unbalanced force to
change the motion of an object!
2 Mechanical Equilibrium
2.1 Force
Net Force
When the girl holds the
rock with as much force
upward as gravity pulls
downward, the net force
on the rock is zero.
•Fnorm = Fgrav
Normal
Force
Gravity
2 Mechanical Equilibrium
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
•The blue arrows
are called
vectors (they
indicate size and
direction)
2 Mechanical Equilibrium
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
•The blue arrow in
the net force column
is a type of vector
called the resultant
(5 N + 10 N = 15 N)
2 Mechanical Equilibrium
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
2 Mechanical Equilibrium
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
•10 N – 5 N = net
force of 5 N to the
right
2 Mechanical Equilibrium
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
2 Mechanical Equilibrium
2.1 Force
Net Force
The net force depends on
the magnitudes and
directions of the applied
forces.
•5 N – 5 N = net
force of 0 N …which
is equilibrium!
2 Mechanical Equilibrium
2.1 Force
Force Vectors can be drawn to scale
This vector represents a force of 60 N to the right.
N
•Vectors show magnitude (size) and
direction
2 Mechanical Equilibrium
Resultant
• The sum of two or more vectors
• Can show the magnitude and direction of
the net force.
2 Mechanical Equilibrium
Resultant (forces in same direction
and in opposite directions)
40 N
20 N
Net force = 60 N to the right
Resultant?
40 N
20 N
Net force = 20 N to the right
Resultant?
2 Mechanical Equilibrium
Resultant (pulling or pushing at an
angle)
2 Mechanical Equilibrium
Resultant (pulling or pushing at an
angle)
2 Mechanical Equilibrium
Identify the resultant and the
horizontal and vertical forces
2 Mechanical Equilibrium
2.1 Force
How can you change an object’s
state of motion?
•Apply a net force greater than zero.
2 Mechanical Equilibrium
An object in mechanical
equilibrium is stable, without
changes in motion.
2 Mechanical Equilibrium
Things that are in balance with one
another illustrate equilibrium.
Things in mechanical equilibrium are
stable, without changes of motion.
The rocks are in mechanical equilibrium.
•http://library.thinkqu
est.org/27638/leanin
g.html
2 Mechanical Equilibrium
• http://sciencefair.math.iit.edu/projects/pisa/
•An object
will fall if its
center of
mass is no
longer
situated
within its
base
2 Mechanical Equilibrium
Different Types of Forces
•
•
•
•
•
•
•
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
2. Tension Force
Force of attraction due to mass of an object
Pulled force applied to a rope, wire or cable
3. Spring Force
4. Applied Force
Elastic, compressed or stretched force
5. Friction
6. Air Resistance
7. Normal force
Any push or pull on an object
Resistance of two objects in contact moving
past each other
Resistance of an object moving through the
atmosphere
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.