1. Chapter 9
Static
Equilibrium

2. Conditions for Equilibrium
• What are the conditions for equilibrium?

3. What if…
• We look at a book sitting on a table
• Is it moving?

4. Equilibrium
• The book is experiencing forces but is not
moving
• It is in equilibrium
• Latin for equal forces

5. Braces
• The wire band
around the teeth has
a tension TF of 2.0 N
along it. What is the
resultant force on
the highlighted
tooth?
• Draw a force
diagram of the tooth.

6. Braces
• The wire band
exerts the FT in each
direction along the
wire.
• Since the forces are
equal and in
opposite directions,
the FR will be along
the line that bisects
the angle between
them.

7. Braces
• Call the direction of
FR the y axis
• What will be the
sum of the forces in
the x direction?
• What will be the
sum of forces in the
y direction (FR)?
• Result of this?

8. Equilibrium

9. Chandelier
• Calculate the
tension FA and FB
in the two cords
that are connected
to the vertical cord
supporting the 200
kg chandelier.
• Draw a free-body
diagram of what?

10. Chandelier
• Pick the point where the three cords meet
• Sum in the forces in the x and y direction
• Resolve FA into components

11. Equilibrium

12. Equilibrium?

13. Equilibrium
• The forces add up to
zero, but…
• The forces produce a
net torque that will
rotate the object
• Couple: A pair of
equal forces acting in
opposite directions
but at different point in
on object

14. Equilibrium

15. Lever
• A bar is placed over
a small rock to act as
a lever on the large
rock
• The lever should
allow a smaller FP
• What if FP is still not
large enough?

16. Mechanical Advantage

17. Homework
• Read Section 9.1