Newton

1

• Newtons Laws of Motion

2
Isaac Newton (1642-1727)
3
Newtons First Law

• The Law of Inertia
• A body remains at rest or moves in a straight
  line at a constant speed unless acted upon by a
  net force.
• Objects do not accelerate unless a net force is
  applied.
• Tendency of objects to resist changes in motion
  and keep on doing what they are doing
• The more mass of an object, the more inertia

4
Example of Inertia

• If you are in a car that stops suddenly, inertia
  causes you to continue moving forward
• Therefore a force is required to change your
  motion
• That force is exerted by the seatbelt
• If not that it would have been the windshield

5
More Examples

• A hockey puck will only stop when a hockey stick
  stops it
• A ball is stopped when it is caught

6
What makes the World go round?

• NOTHING!!! An object in motion tends to stays in
  motion

7
Mass vs. Weight

• the amount of matter in an object
• a constant property
• a measure of the inertia
• measured in kilograms (kg)

• the force upon an object due to gravity
• weight mass ? accel. due to gravity
• Fw mg
• g9.8 m/s2
• measured in Newtons (N)

8

• The weight of a 10 kg brick is...
• A) 98 N
• B) 10 kg
• C) 9.8 kg
• D) 10 N
• E) 98 kg

9
Weight
Location
Mass
Earth
18.4 kg
180 N
Moon
18.4 kg
30 N
Space
18.4 kg
0 N
10
Mass and Weight

• On the Moon, the force of gravity is only 1/6 as
  strong as on the Earth.
• In space you are practically weightless but your
  mass remains unchanged.
• Your mass does not depend on where your are.
• e.g. Earth, Moon, or space

11
Newtons Second Law

• The acceleration of an object is directly
  proportional to the net force acting on the
  object, and inversely proportional to the mass of
  the object.

12
Newtons Second law of Motion mathematically
Net Force (mass)(accel) Fnet ma
13
When the acceleration is g we have Free Fall
m
2m
F
2F
14
Practice Problems

• 1. What is the force necessary to give a 4 kg
  object an acceleration of 5 m/2?
• 20 N
• 2. A force of 60 N accelerates an object by 4
  m/s2. Find the objects mass.
• 15 kg

15
Newtons Second Law

• So if you double the force what happens to the
  acceleration?
• It also doubles
• If you apply the same force to twice the mass
  what happens to acceleration?
• Results in half the acceleration

16
NEWTON'S 2nd LAW OF MOTION
a
F
F
a
F
a
F
a
F
a
F
a

17
Newtons Third Law

• Action-Reaction
• When one object exerts a force on another object,
  the second object exerts a force of equal
  strength in the opposite direction on the first
  object.

18
Newtons Third Law of Motion
For every action, there is always an equal
(magnitude) and opposite (direction) reaction.
By action or reaction, we mean a force.
Action/reaction forces do not act on the same
object.
19
Reaction road pushes on tire
Action tire pushes on road

20
Reaction gases push on rocket
Action rocket pushes on gases

21
Action earth pulls on kid
Reaction kid pulls on earth

22
Identify at least six pairs of action-reaction
force pairs in the following diagram
23
Tug-a-war

• If Fido and Rover play tug-a war, how do their
  pulls compare?
• If each dog pulls with 50 N of force, what is the
  tension force in the middle of the rope (between
  the dogs)?