2 Outline
• ideas about motion
• Newton’s 1st Law & 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 & 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 & 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
foam pad of the same
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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•
•
•
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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