1. I. Newton’s Laws of Motion
Ch. 12 Motion & Forces
I. Newton’s Laws of Motion
“If I have seen far, it is because I have stood on the shoulders of giants.”
- Sir Isaac Newton
(referring to Galileo)

2. Newton’s First Law Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

3. F = ma Newton’s Second Law Newton’s Second Law of Motion
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
F = ma

4. Newton’s Third Law Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

5. III. Defining Force Force Newton’s First Law Friction

6. A. Force Fkick Fgrav Force
a push or pull that one body exerts on another
What forces are being exerted on the football?
Fkick
Fgrav

7. A. Force Balanced Forces
forces acting on an object that are opposite in direction and equal in size
no change in velocity

8. A. Force Net Force unbalanced forces that are not opposite and equal
velocity changes (object accelerates)
Fnet
Ffriction
Fpull N W

9. Calculating Net Force from Images
Net force is the sum of forces acting on an object
If forces are acting in the same direction they combine (add together)
If forces are acting in opposite directions they reduce (subtract)
If net force is zero the motion of the object will NOT change
If net force is not zero the object will accelerate (this may be positive or negative acceleration).

10. What is the net force on the objects below?
Opposite = subtract
8N – 3N = 5 N
Same = addition
5N + 2N = 7 N
2 N + 5N – 8N = 1 N
7N + 2N – 3N = 6N

11. B. Newton’s First Law Newton’s First Law of Motion “Law of Inertia”
tendency of an object to resist any change in its motion
increases as mass increases
Newton’s First Law of Motion
An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

12. ConcepTest 1
TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it.
FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.

13. ConcepTest 2
You are a passenger in a car and not wearing your seat belt.
Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door.
Which is the correct analysis of the situation? ...
1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above
2. Starting at the time of collision, the door exerts a leftward force on you.

14. F = ma C. Newton’s Second Law Newton’s Second Law of Motion
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
F = ma

15. F = ma a Newton’s Second Law F m F: force (N) m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2

16. a Calculations F m F = ? F = ma m = 40 kg F = (40 kg)(4 m/s2)
What force would be required to accelerate a 40 kg mass by 4 m/s2?
GIVEN:
F = ?
m = 40 kg
a = 4 m/s2
WORK:
F = ma
F = (40 kg)(4 m/s2)
F = 160 N m F a

17. a Calculations F m m = 4.0 kg a = F ÷ m F = 30 N a = (30 N) ÷ (4.0 kg)
A 4.0 kg shot-put is thrown with 30 N of force. What is its acceleration?
GIVEN:
m = 4.0 kg
F = 30 N
a = ?
WORK:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2 m F a

18. D. Gravity
Gravity
force of attraction between any two objects in the universe
increases as...
mass increases
distance decreases

19. B. Gravity
Who experiences more gravity - the astronaut or the politician?
Which exerts more gravity - the Earth or the moon?
more mass
less distance

20. D. Gravity Would you weigh more on Earth or Jupiter?
Jupiter because...
greater mass
greater gravity
greater weight

21. Free Fall When gravity is the only force acting on an object
Represented by the letter g
Near Earth’s surface g = 9.8 m/s2

22. W = mg B. Gravity Weight the force of gravity on an object MASS WEIGHT
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
MASS
always the same
(kg)
WEIGHT
depends on gravity
(N)

23. D. Gravity Accel. due to gravity (g)
In the absence of air resistance, all falling objects have the same acceleration!
On Earth: g = 9.8 m/s2
elephant
feather

24. g Calculations W m W = 557 N m = W ÷ g m = ? m = (557 N) ÷ (9.8 m/s2)
Mrs. J. weighs 557 N. What is her mass?
GIVEN:
W = 557 N
m = ?
g = 9.8 m/s2
WORK:
m = W ÷ g
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg m W g

25. ConcepTest Is the following statement true or false?
An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.
False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

26. E. Newton’s 3rd Law
For every action force, there is an equal and opposite reaction force.

27. 3rd law: Action / Reaction
The action force and the reaction force occur to different objects so the force is not balanced.
Not all action and reaction forces produce motion.
Unbalanced forces equal changes in motion

28. Newton’s third law Kicking a soccer ball Leaning against a wall
Action force = your foot hits the ball
Reaction force = the ball pushes against your foot.
Ball moves b/c action force is larger than reaction force
Leaning against a wall
Action force = you pushing against a wall
Reaction force = wall pushing against you
Nothing moves b/c the action force equals the reaction force.

29. F. Friction
Friction is a force that opposes the motion of objects that touch as they move past each other.
Friction acts at the surface where object are in contact
Four main types of friction: static friction, sliding friction, rolling friction and fluid friction.

30. C. Friction
Friction
is the force that opposes motion between 2 surfaces
depends on the:
types of surfaces
force between the surfaces

31. C. Friction Friction is greater... between rough surfaces
when there’s a greater force between the surfaces (e.g. more weight)
Pros and Cons?

32. Static Friction
Static friction is the friction force that acts on objects that are not moving.
Static friction always acts in the direction opposite to that of the applied force
Prevents objects from sliding.
Example: pushing a dresser that does NOT move

33. Sliding Friction
Sliding Friction is a force that opposes the direction of motion of an object as it slides over a surface.
Occurs when there is enough force to overcome the static friction
There will be a net force in the direction of motion
Pushing a desk that slides against the floor.

34. Rolling Friction
Rolling friction is the force that acts on rolling objects.
Rolling friction is about 100 to 1000 times less than the force of static or sliding friction
Skate boarding down a hill

35. Fluid Friction
Fluid friction opposes the motion of an object through a fluid.
Fluid examples: air, water, quick sand, and cake batter
Air Resistance is the fluid friction action on an object moving through the air.
What are the forces acting on a falling leaf?

36. Air Resistance Fair Fgrav Terminal Velocity
maximum velocity reached by a falling object
reached when… Fgrav = Fair
Fair
no net force
 no acceleration
 constant velocity
 still falling
Fgrav

37. Projectile Motion
Projectile motion the motion of a falling object after it is given an initial forward velocity.
this is a curved path
Objects with different mass fall at the same rate.
The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

38. Momentum
Momentum is the product of an object’s mass and its velocity, an object with large momentum is hard to stop
momentum for any object at rest is zero
Law of conservation of momentum if not net forces acts on a system, then the total momentum of the system does not change
In a closed system the loss of momentum of one object equals the gain in momentum of another object--- momentum is conserved