Newton

1
Newtons Laws

• Three Laws of Motion

2
Aristotles Motion

• Natural Motion is up or down
• Down for falling objects
• Up for smoke
• Circular for heavenly bodies since without end
• Violent Motion
• Due to imposed forces such as wind pushing a ship
  or someone pulling a cart
• Natural state of motion is rest
• A force is needed to keep something moving

3
Aristotles Basic Error

• Friction not understood as a force

4
Galileos Motion

• Force is a push or a pull
• Friction is a force that occurs when objects move
  past each other
• Friction due to tiny irregularities
• Only when friction is present is a force required
  to keep something moving

5
Galileos Inclined Planes

• Ball rolling downhill speeds up
• Ball rolling uphill slows down
• He asked about ball on smooth level surface
• Concluded it would roll forever in absence of
  friction

6
Inertia

• Resistance to change in state of motion
• Galileo concluded all objects have inertia
• Contradicted Aristotles theory of motion
• No force required to keep Earth in motion around
  sun because no friction

7
Newton

• Born 1665
• Built on Galileos ideas
• Proposed three laws of motion at age of 23

8
Newtons First Law
Ourtesy www.lakeheadu.ca/alumni/ hockey.gif

• Every object continues in its state of rest, or
  of motion in a straight line at constant speed,
  unless compelled to change that state by forces
  exerted on it.
• Also called Law of Inertia things move according
  to their own inertia
• Things keep on doing what they are doing
• Examples Hockey puck on ice, rolling ball, ball
  in space

9
Mass

• Amount of inertia depends on amount of massor
  amount of material (number and kind of atoms)
• Measured in kilograms
• Question Which has more mass, a kilogram of
  lead or a kilogram of feathers?
• Mass vs. Volume volume is how much space
  something occupies

10
Experiencing Inertia

• Inertia is resistance to shaking
• Which is easier to shake, a pen or a person?
• Why is it so hard to stop a heavy boat?

11
Mass vs. Weight

• Mass is intrinsic property of any object
• Weight measures gravitational force on an object,
  usually due to a planet
• Weight depends on location of object
• Question 1 How does mass of a rock compare when
  on Earth and on moon?
• Question 2 How does its weight compare?

12
Review Mass vs. Weight

• What is mass?
• Answer quantity of matter in something or a
  measure of its inertia
• What is weight?
• Answer Force on a body due to gravity

13
Weight of 1 Kilogram

• 9.8 Newtons
• About 2.2 pounds
• Compare the weight of 1 kg nails with 1 kg
  styrofoam
• Answer Same

14
Weight Examples

• What does a 70 kg person weigh?
• W mg 70 kg x 9.80 N/m2 686 N
• An object weighs 9800 n on Earth. What is its
  mass?
• m W/g 9800 / 9.8 m/s2 1000 kg

15
Inertia in a Car

• Discuss three examples of inertia in a car

• Car hitting a wall
• Car hit from behind by a truck
• Car going around a corner

16
Newtons Second Law

• Law of Acceleration
• The acceleration produced by a net force on an
  object is directly proportional to the magnitude
  of the net force, and is inversely proportional
  to the mass of the body.
• Acceleration net force mass
• F ma
• Acceleration is in direction of net force

17
Net Force

• Net Force means sum of all forces acting
• Sum is Vector sum

F2
F1
Resultant force
18
Understanding the Second Law
Force

• The cause of acceleration is
• _________ resists acceleration
• The greater the force, the ________ the
• The greater the mass, the _________ the
  acceleration.

Mass
greater
acceleration
less
19
Units

• F ma
• Unit of force is the Newton (N)
• 1 N 1 kg m/s2

20
F ma is Three Equations

• F and a are vectors
• So F ma equation is really three
• SFx max SFy may SFz maz

21
Examples

• What force is required to accelerate a 1000 kg
  car at 2.0 m/s2 ?
• Answer F ma 1000 kg x 2.0 m/s2 2000
  N.
• What is the acceleration of a 145 g baseball
  thrown with a force of 20.0 N?
• a F/m 20/0.145kg 138 m/s2

22
Newtons Third Law

• Forces always come in pairs
• Two forces on different objects
• Whenever one object exerts a force on a second
  object, the second exerts an equal and opposite
  force on the first
• Example hammer hits nail

23
Example pushing on wall

• What are the forces when you push on a wall?
• You exert force on wall
• You accelerate in the opposite direction
• Wall must have exerted a force on you in the
  direction you accelerated (by 2nd Law)

24
Example person walking

• Foot exerts force backward on ground
• Ground exerts force forward on foot

25
Example Throwing ball

• Pitcher exerts force on ball
• Ball exerts equal and opposite force on pitcher
• Why doesnt pitcher move?

26
Example Rocket

• Rocket engine exerts rearward force on gas
  molecules
• Molecules exert forward force on rocket.

27
Horse and Cart

• Horse exerts force on cart
• Cart exerts equal and opposite force on horse
• Net force zero
• Cart cant move
• Huh?

The net force is NOT zero. Forces on different
objects cannot be added to make zero
28
Book on Table

• The mass of the book is one kg. What is the
  force (magnitude and direction) on the book?
• 9.8 N upward

29
More Examples

• Can you think of some more examples of Newtons
  Third Law in Action?

30
Review Newtons Laws of Motion

• Newtons First Law
• Every object continues in its state of
  rest, or of motion in a straight line at constant
  speed, unless compelled to change that state by
  forces exerted on it.
• Newtons Second Law
• The acceleration produced by a net force
  on an object is directly proportional to the
  magnitude of the net force, and is inversely
  proportional to the mass of the body.
• Newtons Third Law
• Whenever one object exerts a force on a
  second object, the second exerts an equal and
  opposite force on the first

31
Concept Check 1

• Which of these statements about an object in free
  fall is false?
• (a) It accelerates
• (b) It is in equilibrium
• (c) It exerts an upward force on the Earth
• (d) Its mass equals the net force on it divided
  by its acceleration
• (e) Only one force acts on it