### What is the net force?

```Homework See Supplemental Chapter 1. Read pages 3-end. #7-14, 16, 18, on pg 8
Due
Tonight’s
HW
Ch. 4 Notes – read pages 43-55. As you read, only fill out the
BULLET portion using the Cornell Notes handout. (you will get
a copy of this at end of class)

Do Now
EXTRA INFO:
o We will do the KEY POINTS and SUMMARY sections in class
o If you need more paper, you can download from my website, or
alter loose leaf, to make more.
If you pull a crate across a factory floor at a constant speed in
a constant direction…
- What is the size of the force of friction on the crate
- What is the net force?
- Is it in mechanical equilibrium? If so, which type?
(static or dynamic?)
If you pull a crate across a factory floor at a constant
speed in a constant direction…
- What is the size of the force of friction on the
crate compared with your pull if your pull force is 50 N?
- What is the net force?
- Is it in mechanical equilibrium? If so, which
type? (static or dynamic?)
50 N of pull
force
???? of
friction
force
Box is moving at a constant speed and direction to the left.
15.
If you pull a crate across a factory floor at a constant speed in a constant
direction, what is the size of the force of friction on the crate compared with your pull if
your pull force is 50 N? What is the net force? Is it in mechanical equilibrium? If so,
which type?
 Both the pull force and the friction force are equal in
magnitude, but are in opposite directions. Thus, the net
force is zero and the object is in dynamic equilibrium.
50 N of pull
force
50 N of
friction
force
Box is moving at a constant speed and direction to the left
(NET FORCE = 0….so it is in Dynamic equilibrium)
“The Equilibrium Rule”
 Take a copy of “The Equilibrium Rule” and READ
through the solved sample problem.
COMMENT with any “aha” moments, questions, or
areas that you don’t understand.
 We will discuss as a class and then complete…finished
reading /commenting? Try to do some of the
problems!
Take out the HW that was due
today…
 We are going to GO OVER it quickly via PowerPoint
 If you have questions, STOP and ask!
 Use a different color pen (if you would like) when
making corrections
7.
How much tension is in a rope that holds up a 20-N bag of
apples at rest?
 Since the bag is at rest, it is in mechanical equilibrium.
Therefore, there must be 20N of tension force to
balance the 20N of weight force due to gravity.
 This is referred to in the book as static equilibrium.
8. What does F = 0 mean?
 The sum of all forces acting on a body are balanced (or
net force is equal to zero)
9.
What is the net force on an object at rest?
 Zero. All forces are balanced.
10. When you do pull-ups and you hang at rest, how
much of your weight is supported by each arm?
 Each arm supports one-half of your weight.
11. What is the angle between the support force and
the surface an object rests on?
 90 degrees
12.
What two forces compress a spring inside a weighing scale when
you weight yourself?
support force of the floor (pushing upward).
13. When you are at rest and supported by a pair of scales,
how does the sum of the scale readings compare with your
weight?
14. Can an object be moving and still be in
(mechanical) equilibrium?
 Yes. If the object moves at a constant speed in a
straight line, then the net force on the object is equal
to zero.
 This is referred to in the book as dynamic
equilibrium.
16. Distinguish between static equilibrium and
dynamic equilibrium.
 An object at rest is said to be in static equilibrium.
 An object moving in a straight line at a constant speed
is said to be in dynamic equilibrium.
 For each case, the net force = 0, thus the forces acting
are balanced…in other words, the object is NOT
changing its state of motion
18. Consider Nellie in Fig. 2.11. Name the 3 forces that act on her.
What is your evidence they cancel to zero?
 Tension of 1 rope, tension of the second rope, and her
weight
 She is in mechanical equilibrium (not changing her
state of motion!!!), so the resultant of the rope tensions
must equal / cancel out her weight….net force must be
zero.
```