2 Outline

1
2 Outline

• ideas about motion
• Newtons 1st Law equilibrium
• diagramming forces
• Homework
• RQ 1, 7, 10, 11, 12, 14, 16.
• Ex 1, 3, 9, 15, 22, 27, 31.

2
Force

• push or a pull in a direction
• SI unit is newton (N)
• US customary unit is pound (lb)
• 1 lb 4.45 N

3
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.

4
Ex. Net Force for Up Down Applied Forces

• Fill in the missing information
• F1 F2 Net Force
• -2 5 3
• -10 0
• -10 5
• -10 10
• -30 0

-10
-5
0
30
5
Newtons 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.

6
Mechanical Equilibrium

• State in which the net force acting on an object
  is zero
• This also means the object is not accelerating

7
Example of Mechanical Equilibrium and Newtons
1st Law. What happens to the speed of the block?
8
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)

9
What is the unknown tension? Assume the man
weighs 800 N and the platform has negligible
weight.
Net Force 170 ???? (-800) 0
10
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

11
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?
12
When the pellet fired into the spiral tube
emerges, which path will it follow? (Neglect
gravity.)

• A
• B
• C

13
2 summary

• Galilean and Newtonian ideas about force and
  motion prevailed over Aristotles ideas.
• A net force of zero leaves the motion of an
  object unchanged (called equilibrium even when
  moving)
• Pages 3-5 in practicing physics.

14
Motion Diagrams
0

• Are velocity-position diagrams
• Length determines speed (zero velocity is a
  dot)
• Arrow determines direction
• force required to change velocity
• Example slowing, reversing direction

15
2 agenda

• lecture
• practicing physics
• p3 static equilibrium
• p4 equilibrium rule
• p5 vectors and equilibrium
• lab vector forces with spring scales