1. Unit 4: Forces and Newton’s Laws of Motion
Why physics teachers don’t monitor recess

2. I. Force Equation FORCES CAUSE ACCELERATION
Equation (see Reference Tables):
Example: What net force is required to accelerate a 1500 kg race car at + 3 m/s2?
FORCES CAUSE ACCELERATION

3. II: Weight vs. Mass MASS IS A CONSTANT MEASUREMENT
Then what is its acceleration?
Equation (see reference tables):
Example: Find the weight of a 35 kg mass on Earth.
WEIGHT IS KNOWN AS THE FORCE DUE TO GRAVITY
a = g = 10 m/s2 (ON EARTH)

4. III. Normal Force and Free Body Diagrams
Forces acting on a book Forces acting on a book resting
resting on a flat desk on a desk at an angle
FORCE FROM THE SURFACE PUSHING BACK ON OBJECT
ALWAYS ACTS PERPENDICULAR TO THE SURFACE

5. IV: Apparent Weight (REST OR CONSTANT VELOCITY)
Example: An elephant with a mass of 1000 kg is standing on a scale inside an elevator that is at rest. What is the scale reading (apparent weight of elephant)?
THE NORMAL FORCE = WEIGHT WHEN ON A FLAT SURFACE IN EQUILIBRIUM
(REST OR CONSTANT VELOCITY)

6. IV: Apparent Weight
What happens to your “weight” if the elevator moves?
Video (3 min)
Elevator Safety – Bill Nye video (4 min Start at 2:30)
Nellie in the Elevator (3 min)

7. V. Change in “Weight”
Example: The elevator from the above example begins to move.
A) What would happen to the scale reading if the elevator accelerates up?
THE NORMAL FORCE (APPARENT WEIGHT) CAN CHANGE IF THE OBJECT IS ACCELERATING

8. V. Change in “Weight”
Example: The elevator from the above example begins to move.
B) What would happen to the scale reading if the elevator accelerates down?

9. V. Change in “Weight”
Example: The elevator from the above example begins to move.
C) What would happen to the scale reading if it was moving at a constant speed up?

10. Part B: Weight vs. Mass
V. Change in Weight
Aboard the Vomit Comet video (4 min)
What happens to your “weight” if the elevator moves?
Video (3 min)
Elevator Safety – Bill Nye video (4 min Start at 2:30)
Nellie in the Elevator (3 min)

11. Part C: Newton’s Laws of Motion
VI. Newton’s First Law of Motion
Inertia:
Units:
Examples:
1. Which object has more inertia: 2 kg pumpkin moving at m/s or 10 kg turkey at rest?
 2. What happens to the inertia of an object, if the mass increases?
TENDENCEY FOR AN OBJECT TO REMAIN AT REST OR CONSTANT VELOCITY (a = 0) (INTERIA = MASS) kg
10 kg TURKEY (MORE MASS)
INERTIA INCREASES (INERTIA = MASS)

12. Part C: Newton’s Laws of Motion
VI. Newton’s First Law of Motion
Newton’s 1st Law of Motion:
Which of the following problems have an unbalanced force (net force does not = 0)?
a. A car is traveling at 20 meters per second, and the driver steps on the brake.
b. A chair is dragged across the floor at a constant velocity.
c. A student is sitting still in a chair.
d. A plane makes a turn at a constant speed.
e. An object is dropped vertically toward the earth.
AN OBJECT WILL REMAIN AT REST OR
AT CONSTANT VELOCITY UNTIL AN UNBALANCED FORCE ACTS ON IT

13. Seatbelt Law Video (2 min)
Part C: Newton’s Laws of Motion
XII. Newton’s First Law of Motion
Newton’s 1st Law of Motion and the seatbelt
Seatbelt Law Video (2 min)
Table Cloth Stunt from Mythbusters Part min

14. Part C: Newton’s Laws of Motion
VII. Newton’s Second Law of Motion
Newton’s 2nd Law of Motion:
Equation: (see reference tables)
ACCELERATION IS DIRECTLY PROPORTIONAL TO THE FORCE AND INVERSELEY PROPORTIONAL TO MASS

15. Part C: Newton’s Laws of Motion
VII. Newton’s Second Law of Motion
Examples:
1. What net force is required to accelerate a 1500 kg truck at 5 m/s2?

16. Part C: Newton’s Laws of Motion
VII. Newton’s Second Law of Motion
Examples:
2. A. What is the net horizontal force? B. What is the acceleration (magnitude and direction) of the block below?

17. Part C: Newton’s Laws of Motion
VII. Newton’s Third Law of Motion
Newton’s 3rd Law of Motion:
Action/Reaction Pairs:
David Ortiz hits a baseball with a force of 200 N. How much force does the baseball exert on the bat?
FOR EVERY ACTION (FORCE) THERE IS AN EQUAL AND OPPOSITE REACTION (FORCE)
200 N

18. Part C: Newton’s Laws of Motion
VII. Newton’s Third Law of Motion
What are “action” and “reaction”
“action” and “reaction” are names of _______________
Forces __________ come in ___________
FORCES
ALWAYS
PAIRS

19. Part C: Newton’s Laws of Motion
VII. Newton’s Third Law of Motion
Action/Reaction Forces
Since a force is an interaction between _________ objects,
____________ objects are involved in ______________
Action force:
Reaction force:
TWO
TWO
EVERY FORCE
A PUSHES B
B PUSHES A

20. Part C: Newton’s Laws of Motion
VII. Newton’s Third Law of Motion
D. “Equal”
In Newton’s 3rd Law, “equal” means equal in ______________
Action and reaction forces are exactly the same _________
Action and reaction forces occur at exactly the same ________
SIZE AND TIME
SIZE (MAGNITUDE)
TIME

21. a m Fnet Fnet = Part C: Newton’s Laws of Motion = DIRECTION SAME FORCE
VII. Newton’s Third Law of Motion
E. “Opposite”
In Newton’s 3rd Law, “opposite” means ______________
F. Don’t Forget about Mass
The __________________ acting on objects of ______________
will produce _______________________
DIRECTION
Same force
Fnet
Fnet
SAME FORCE
DIFFERENT MASSES a = = m a m
DIFFERENT ACCELERATIONS

22. a m Fnet Fnet = Part C: Newton’s Laws of Motion =
VII. Newton’s Third Law of Motion
Don’t Forget about Mass
Example
A bug flies into a moving truck. How does the force exerted on the truck compare to the force exerted on the bug?
Same force
Fnet
Fnet a = = m a m
THE SAME

23. Part C: Newton’s Laws of Motion
Friction
Normal Force:
Frictional Force:
Equation (See reference tables):
Force from surface pushing on object
(always perpendicular to surface)
Force that opposes motion
(always acts parallel to surface)

24. Part C: Newton’s Laws of Motion
Friction
Static Friction:
If you pull on an object and it does not move, how does your applied force compare to the force of static friction?
FORCE OF FRICTION THAT PREVENTS OBJECTS TO START MOTION
EQUAL, BUT OPPOSITE DIRECTION
(NOTE: this is not Newton’s 3rd Law)

25. Part C: Newton’s Laws of Motion
Friction
Static Friction:
Maximum static friction (Ff, static max) occurs:
Ff, static Ff, static max
OBJECT JUST BEGINS TO MOVE

26. Part C: Newton’s Laws of Motion
Friction
Static Friction:
Example: What is the minimum horizontal force a man has to pull on a 200 N wood crate on a wood floor to start it to move? (use reference tables)
Phone Book Friction Mythbusters (6 min video)

27. Part C: Newton’s Laws of Motion
Friction
Kinetic Friction: Physics of Curling (4 min)
How ABS Work (1 min)
Ff, kinetic Ff, static
Equation:
FORCE OF FRICTION THAT OPPOSES MOTION OF A MOVING OBJECT

28. Part C: Newton’s Laws of Motion
Kinetic Friction:
Example: Jim exerts a 36 N horizontal force as he pulls a 5 kg sled across a cement sidewalk at a constant speed. What is the coefficient of kinetic friction between the sidewalk and the metal sled?

29. Applications of Friction
Friction Welding
Ice Storm in Portland
ABS Brakes (Kinetic vs. Static Friction) (video)
Physics of Curling (start at 2:30 video)
Sled wax (Christmas Vacation)

30. Part C: Newton’s Laws of Motion
Practice Problems
Complete the following problems on a whiteboard and jot down the correct answer once checked
Newton’s 2nd Law:
2,000 kg
2.4 kg
100 m/s
A) 2 N B) 1 m/s2

31. Part C: Newton’s Laws of Motion
Practice Problems
Complete the following problems on a whiteboard and jot down the correct answer once checked
Friction:
58.9 N
A) drawing B) 0.66
A) drawing, B) 98.1 N, C) 98.1 N, D) Zero, not moving vertically, E) N, F) N, G) 3.04 m/s/s
A) 2 N B) 1 m/s2