# 2 Outline ```2 Outline
• Newton’s 1st Law &amp; equilibrium
• diagramming forces
• Homework:
• RQ: 1, 7, 10, 11, 12, 14, 16.
• Ex: 1, 3, 9, 15, 22, 27, 31.
1
Force
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“push” or a “pull” in a direction
SI unit is newton (N)
US customary unit is pound (lb)
1 lb = 4.45 N
2
Net Force
• is sum of all forces acting on an object
• The net force can be zero (and often is)
even when several forces act.
• Sign Convention for Forces:
• + denotes upward or rightward direction
• - denotes downward or leftward direction
• If forces act in the same or opposite
direction, the net force is the algebraic
sum of force-values.
3
Ex. Net Force for Up &amp; Down
Applied Forces
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Fill in the missing information
F1
F2
Net Force
-2
+5
+3
-10
-10
0
-10
+5
-5
0
-10
+10
+30
-30
0
Newton’s 1st Law of Motion
• When the net force on an object is zero,
the object will either
• a) continue in a state of rest, or
• b) maintain a constant speed &amp; direction.
5
Mechanical Equilibrium
• State in which the net force acting on an
object is zero
• This also means the object is not
accelerating
6
Example of Mechanical Equilibrium and
Newton’s 1st Law. What happens to the
speed of the block?
7
Common Forces Encountered
• Outward forces (compressed spring or any
object under compression)
• Inward forces (stretched spring or any
object under tension)
• Downward force of Weight
• Reaction forces (object on top of spring)
• Frictional forces (oppose motion)
8
What is the unknown tension? Assume
the man weighs 800 N and the platform
has negligible weight.
Net Force = +170 + ???? + (-800) = 0
9
Inertia (Mass)
• the property of objects to resist changes in
motion.
• measurement of is called “mass”
• Scientific unit is “kilogram” (kg)
• US customary unit is “slug”
10
What is important here, the size or the mass
of the books? Would a stiff but light box of
the same size protect her as much?
Why would a very light
size as the books protect
her from the hammer
blow?
11
When the pellet fired
into the spiral tube
emerges, which path
will it follow? (Neglect
gravity.)
1. A
2. B
3. C
12
2 summary
• Galilean and Newtonian ideas about force
and motion prevailed over Aristotle’s
ideas.
• A net force of zero leaves the motion of an
object unchanged (called equilibrium even
when moving)
• Pages 3-5 in practicing physics.
13
Motion Diagrams
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Are velocity-position diagrams
Length determines speed (zero velocity is a “dot”)
Arrow determines direction
force required to change velocity
Example: slowing, reversing direction
14
2 agenda
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lecture
practicing physics:
p3 static equilibrium
p4 equilibrium rule
p5 vectors and equilibrium
• lab: vector forces with spring scales
15
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