Newton

1
Newtons Laws
Easy as apple pie!
2
Force

• Force? a push or a pull
• Causes object to start or stop moving or change
  direction

force of table
g
3
Newtons 1st Law

• Greeks thought that the natural state of an
  object was at rest.
• ex. A ball stops rolling
• without friction an object would never stop!
• Newtons 1st? an object in motion tends to stay
  in motion an object at rest tends to stay at rest

4
Newtons 1st Law

• Inertia? property of matter that resists a
  change in motion
• An object with great mass has high inertia

5
Newtons 2nd Law
f m x a
A
FA 1000 kg x 0.05 m/s/s
FA
50N
B
FB 2000 kg x 0.05 m/s/s
FB
100N
6
Newtons 2nd Law

• Force mass x acceleration
• An object will only accelerate if there is an
  unbalanced force

7
Newtons 3rd Law

• For every action there is and equal and opposite
  reaction
• The astronaut pushes on the rock and the rock
  pushes on the astronaut

8
Newtons 3rd Law

• For every action there is and equal and opposite
  reaction
• A bird pushes down on the air and the air pushes
  up on the bird

9
Newtons 3rd Law
Note Equal and opposite forces does not result
in equal acceleration if the masses are different.
10
Newtons Laws

• 1st Law (inertia objects tend to do what they
  are doing)
• cannon ball will rest until a force is put on it
• ball will roll straight until ramp puts a force
  on it
• 2nd Law (f m x a)
• greater force put on ball accelerates it more
• greater mass of ball but greater force on water
• 3rd Law (every action has an equal but opposite
  reaction)
• ball moves right, cannon recoils left
• ball move down, water splashes up
• Newton's Laws - YouTube

11
Gravity
force of hand
g
With the upward force of the floor equilibrium is
attained and there is no motion
Without the upward force of the hand there is not
equilibrium of forces and motion occurs
12
Gravity air resistance

• Opposes downward motion of falling objects.
• Larger surface areas increases air resistance

13
Which Law???
Fma Inertia
14
Which Law???
The pellet has entered the egg at the left side
but not exited yet. One second later there was
raw egg all over the work table and the
backdrop. Fma, Inertia
15
Which Law???
Action-reaction Inertia
16
Which Law???
Fma Inertia Action-reaction
17
Which Law???
Inertia Fma Action-reaction
18
Free Body Diagrams

• A force diagram, which is also known as a free
  body diagram, is a sketch in which all the force
  vectors acting on an object are drawn with their
  initial points at the location of the object.

19
Opposition to Motion

• Friction? a force that opposes motion
• Caused by rough surfaces of all materials

razor's edge
20
3 Types of Friction

• 1. Sliding? when solid objects grind over each
  other
• puck and ice
• 2. Rolling? wheels spinning on an axle
• skateboards eventually roll to a stop
• 3. Fluid? liquids or gases slow the motion of a
  solid
• wind resistance
• oil a squeaky hinge
• pushes a surfer

21
Free-body diagrams

• Free-body diagrams are used to show the relative
  magnitude and direction of all forces acting on
  an object.

22

• This diagram shows four forces acting upon an
  object. There arent always four forces, For
  example, there could be one, two, or three
  forces.

23
Problem 1

• A book is at rest on a table top. Diagram the
  forces acting on the book.
• In this diagram, there are normal and
  gravitational forces on the book.

24
Problem 2

• An egg is free-falling from a nest in a tree.
  Neglect air resistance. Draw a free-body diagram
  showing the forces involved.

25

• Gravity is the only force acting on the egg as it
  falls.

26
Problem 3

• A flying squirrel is gliding (no wing flaps) from
  a tree to the ground at constant velocity.
  Consider air resistance. A free body diagram for
  this situation looks like

27

• Gravity pulls down on the squirrel while air
  resistance keeps the squirrel in the air for a
  while.

28
Problem 4

• A rightward force is applied to a book in order
  to move it across a desk. Consider frictional
  forces. Neglect air resistance. Construct a
  free-body diagram. Lets see what this one looks
  like.

29

• Note the larger applied force arrow pointing to
  the right since the book is accelerating to the
  right. Friction force opposes the direction of
  motion. The force due to gravity and normal
  forces are balanced.

30
Problem 5

• A skydiver is descending with a constant
  velocity. Consider air resistance. Draw a
  free-body diagram.

31

• Gravity pulls down on the skydiver, while air
  resistance pushes up as she falls.

32
Problem 6

• A man drags a sled across loosely packed snow
  with a rightward acceleration. Draw a free-body
  diagram.

33

• The applied force arrow points to the right and
  is larger than the frictional force since the
  object is accelerating. Since the sled is on the
  ground, the normal and gravitational force are
  balanced.

34
Problem 7

• A football is moving upwards toward its peak
  after having been booted by the punter. Draw a
  free-body diagram. (Neglect air friction)

35

• The force of gravity is the only force described.
  (no air resistance).

36
Problem 8

• A car runs out of gas and is coasting down a
  hill.

37

• The car is coasting down the hill, there is
  dragging friction of the road (left pointing
  arrow) as well as gravity and normal forces, but
  no applied force.

38
Net Force

• Now lets take a look at what happens when
  unbalanced forces do not become completely
  balanced (or cancelled) by other individual
  forces.
• An unbalanced forces exists when the vertical and
  horizontal forces do not cancel each other out.

39
Example 1

• Notice the upward force of 1200 Newtons (N) is
  more than gravity (800 N). The net force is 400 N
  up.

40
Example 2

• Notice that while the normal force and
  gravitation forces are balanced (each are 50 N)
  the force of friction results in unbalanced force
  on the horizontal axis. The net force is 20 N
  left.

41
Another way to look at balances and unbalanced
forces
42
Balanced or unbalanced?
Accelerating Unbalanced
Constant velocity balanced
deccelerating Unbalanced
43
Balanced or Unbalanced?
B
unbalanced
C
unbalanced
A
balanced
44
Which objects are moving?