1. Newton’s Laws
The Study of Dynamics

2. Isaac Newton Arguably the greatest scientific genius ever.
Came up with 3 Laws of Motion to explain the observations and analyses of Galileo and Johannes Kepler.
Discovered that white light was composed of many colors all mixed together.
Invented new mathematical techniques such as calculus and binomial expansion theorem in his study of physics.
Published his Laws in 1687 in the book Mathematical Principles of Natural Philosophy.

3. What is a Force? NO not THE Force
A force is a push or pull on an object. It may be from gravity, electrical, magnetic, or muscle efforts.

4. A net force causes an object to accelerate…
To speed up
To slow down
To change direction

5. ~Arise because of direct ~Gravity
Contact Forces Field Forces
~Arise because of direct ~Gravity
contact between objects. ~Electric Act over a distance.
ex: friction, tension ~Magnetic

6. Free body diagram (FBD)
Shows all the forces acting on an object.
support force
friction force Applied force
weight

7. Newton’s First Law
A body in motion stays in motion at constant velocity and a body at rest stays at rest unless acted upon by a net external force.
This law is commonly referred to as the Law of Inertia.

8. Implications of Newton’s 1st Law
If there is zero net force on a body, it cannot accelerate, and therefore must move at constant velocity, which means
it cannot turn,
it cannot speed up,
it cannot slow down.

9. What is Zero Net Force? SF = 0 The table pushes up on the book. FT
A book rests on a table.
Physics
Gravity pulls down on the book. FG
Even though there are forces on the book, they are balanced.
Therefore, there is no net force on the book.
SF = 0

10. Diagrams
Draw a force diagram and a free body diagram for a book sitting on a table.
Physics N W
Force Diagram N W
Free Body Diagram

11. Sample Problem
A monkey hangs by its tail from a tree branch. Draw an FBD representing all forces on the monkey FT FG

12. Sample Problem
Now the monkey hangs by both hands from two vines. Each of the monkey’s arms are at a 45o from the vertical. Draw an FBD representing all forces on the monkey.
Fa1
Fa2 FG

13. Mass and Inertia
Chemists like to define mass as the amount of “stuff” or “matter” a substance has.
In physics mass is a measure of inertia, which is the ability of a body to resist acceleration by a net force.

14. Demonstration
A heavy block hangs from a string attached to a rod. An identical string hangs down from the bottom of the block. Which string breaks
when the lower string is pulled with a slowly increasing force?
when the lower string is pulled with a quick jerk?
Top string breaks due to its greater force.
Bottom string breaks because block has lots of inertia and resists acceleration. Pulling force doesn’t reach top string.

15. Newton’s Second Law
A body accelerates when acted upon by a net external force.
The acceleration is proportional to the net force and is in the direction which the net force acts.

16. Newton’s Second Law ∑F = ma
where ∑F is the net force measured in Newtons (N)
m is mass (kg)
a is acceleration (m/s2)

17. Units of force Newton (SI system)
1 N = 1 kg m /s2
1 N is the force required to accelerate a 1 kg mass at a rate of 1 m/s2
1 N=.225 lb (about the weight of a stick of butter)

18. Working 2nd Law Problems
Draw a free body diagram.
Set up 2nd Law equations in each dimension.
SFx = max and/or SFy = may
Identify numerical data.
x-problem and/or y-problem
Substitute numbers into equations.
“plug-n-chug”
Solve the equations.

19. Sample Problem
In a grocery store, you push a 14.5-kg cart with a force of 12.0 N. If the cart starts at rest, how far does it move in 3.00 seconds?

21. Sample Problem
A catcher stops a 92 mph pitch in his glove, bringing it to rest in 0.15 m. If the force exerted by the catcher is 803 N, what is the mass of the ball?

23. Sample Problem
A 747 jetliner lands and begins to slow to a stop as it moves along the runway. If its mass is 3.50 x 105 kg, its speed is 27.0 m/s, and the net braking force is 4.30 x 105 N a) what is its speed 7.50 s later? b) How far has it traveled in this time?

25. Newton’s Third Law
For every action there exists an equal and opposite reaction.
If body A exerts a force F on body B, then body B exerts a force of -F on body A.

26. Examples of Newton’s 3rd Law

27. Sample Problem
You rest an empty glass on a table. a) How many forces act upon the glass? b) Identify these forces with a free body diagram. c) Are these forces equal and opposite? d) Are these forces an action-reaction pair? Identify the action-reaction force pairs.

29. Sample Problem
A force of magnitude 7.50 N pushes three boxes with masses m1 = 1.30 kg, m2 = 3.20 kg, and m3 = 4.90 kg as shown. Find the contact force between (a) boxes 1 and 2 and (b) between boxes 2 and 3.

31. Mass and Weight
Many people think mass and weight are the same thing. They are not.
Mass is the amount of matter in an object, measured by its inertia, or resistance to acceleration.
Weight can be defined as the force due to gravitation attraction.
W = mg

32. Sample Problem
A man weighs 150 pounds on earth at sea level. Calculate his a) mass in kg. b) weight in Newtons.

34. Apparent weight
If an object subject to gravity is not in free fall, then there must be a reaction force to act in opposition to gravity.
We sometimes refer to this reaction force as apparent weight.

35. Elevator rides
When you are in an elevator, your actual weight (mg) never changes.
You feel lighter or heavier during the ride because your apparent weight increases when you are accelerating up, decreases when you are accelerating down, and is equal to your weight when you are not accelerating at all.

36. Going Up? v = 0 a = 0 v > 0 a > 0 v > 0 a = 0 v > 0W
Wapp
Ground
floor
Normal feeling
v = 0
a = 0 W
Wapp
Just
starting up
Heavy feeling
v > 0
a > 0 W
Wapp
Between
floors
Normal feeling
v > 0
a = 0 W
Wapp
Arriving at
top floor
Light feeling
v > 0
a < 0
Going Up?

37. Going Down? v = 0 a = 0 v < 0 a < 0 v < 0 a = 0 v < 0
Wapp
Top
floor
Normal feeling
v = 0
a = 0 W
Wapp
Beginning
descent
Light feeling
v < 0
a < 0 W
Wapp
Between
floors
Normal feeling
v < 0
a = 0 W
Wapp
Arriving at
Ground floor
Heavy feeling
v < 0
a > 0
Going Down?

38. Sample Problem
An 85-kg person is standing on a bathroom scale in an elevator. What is the person’s apparent weight a) when the elevator accelerates upward at 2.0 m/s2? b) when the elevator is moving at constant velocity between floors? c) when the elevator begins to slow at the top floor at 2.0 m/s2?