1. Conceptual Physics Fundamentals
Chapter 3:
EQUILIBRIUM AND FRICTION
Edited for TWHS Physics

2. This lecture will help you understand:
Net Force
The Equilibrium Rule
Equilibrium of Moving Things
The Force of Friction

3. Equilibrium and Linear Motion
“When you’re over the hill, that’s when you pick up speed.”
—Quincy Jones

4. Net Force
Net force is
the combination of all forces that change an object’s state of motion.
example: If you pull on a box with 10 N and a friend pulls oppositely with 5 N, the net force is 5 N in the direction you are pulling.

5. Net Force CHECK YOUR NEIGHBOR
A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is
A. 5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
A. 5 N to the left.

6. Net Force CHECK YOUR ANSWER
A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is
A. 5 N to the left.
5 N to the right.
25 N to the left.
25 N to the right.
A. 5 N to the left.

7. Net Force Vector quantity
a quantity whose description requires both magnitude (how much) and direction (which way)
can be represented by arrows drawn to scale, called vectors
length of arrow represents magnitude and arrowhead shows direction
examples: force, velocity, acceleration

8. The Equilibrium Rule The equilibrium rule
the vector sum of forces acting on a non-accelerating object equals zero
in equation form: F = 0

9. The Equilibrium Rule example: a string holding up a bag of flour
two forces act on the bag of flour:
–tension force acts upward
–weight acts downward
In equilibrium, the Forces are equal in magnitude and opposite in direction
when added, cancel to zero
bag of flour remains at rest

10. The equilibrium rule, F = 0, applies to
CHECK YOUR NEIGHBOR
The equilibrium rule, F = 0, applies to
A. vector quantities.
scalar quantities.
both of the above.
neither of the above.
A. vector quantities.

11. The equilibrium rule, F = 0, applies to
CHECK YOUR ANSWER
The equilibrium rule, F = 0, applies to
A. vector quantities.
scalar quantities.
both of the above.
neither of the above.
Explanation:
Vector addition takes into account + and - quantities that can cancel to zero. Two forces (vectors) can add to zero, but there is no way that two masses (scalars) can add to zero.
A. vector quantities.

12. Support Force
Support force (normal force) is an upward force on an object that is opposite to the force of gravity.
example: a book on table a compresses atoms in the table, and the compressed atoms produce the support force

13. The Support Force CHECK YOUR NEIGHBOR
When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read
A. your weight.
half your weight.
zero.
more than your weight.
B. half your weight.

14. The Support Force CHECK YOUR ANSWER
When you stand on two bathroom scales, with one foot on each scale and with your weight evenly distributed, each scale will read
A. your weight.
half your weight.
zero.
more than your weight.
Explanation:
You are at rest on the scales, so F = 0. The sum of the two upward support forces is equal to your weight.
B. half your weight.

15. Equilibrium of Moving Things
a state of no change with no net force acting
static equilibrium
example: hockey puck at rest on slippery ice
dynamic equilibrium
example: hockey puck sliding at constant speed on slippery ice

16. Equilibrium of Moving Things
Equilibrium test
whether something undergoes changes in motion
example: A refrigerator at rest is in static equilibrium. If it is moved at a steady speed across a floor, it is in dynamic equilibrium.

17. A bowling ball is in equilibrium when it
Equilibrium of Moving Things
CHECK YOUR NEIGHBOR
A bowling ball is in equilibrium when it
A. is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
C. both of the above.

18. A bowling ball is in equilibrium when it
Equilibrium of Moving Things
CHECK YOUR ANSWER
A bowling ball is in equilibrium when it
A. is at rest.
moves steadily in a straight-line path.
both of the above
none of the above
C. both of the above.

19. The Force of Friction Friction
occurs when objects rub against one another
applies to solids, liquids, and gases
acts in a direction to oppose motion
example: When an object falls down through air, the force of friction (air resistance) acts upward.

20. The Force of Friction
depends on the kinds of material and how much they are pressed together
is due to tiny surface bumps and to “stickiness” of the atoms on a material’s surface
example: friction between a crate on a smooth wooden floor is less than that on a rough floor

21. The force of friction can occur
CHECK YOUR NEIGHBOR
The force of friction can occur
A. with sliding objects.
in water.
in air.
all of the above
D. all of the above.

22. The force of friction can occur
CHECK YOUR ANSWER
The force of friction can occur
A. with sliding objects.
in water.
in air.
all of the above
Comment:
Friction can also occur for objects at rest. If you push horizontally on your book and it doesn’t move, then friction between the book and the table is equal and opposite to your push.
D. all of the above.

23. The Force of Friction Friction is a Passive Force
It exists as a response to an Applied Force
As the Applied Force increases, the Friction Force also increases, until it reaches its maximum
When the Applied Force is greater than the maximum Friction Force, the Fnet X will be unbalanced, and the object will slide.
The equation for maximum Force of Friction is μ●FN = coefficient of Friction x Normal Force