Newton's 1st Law

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Newtons 1st Law

• inertia
• an object at rest has a tendency to stay at rest
• and an object in motion has a tendency to stay in
  motion
• UNLESS a net force is applied.

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• In motion means what?
• Constant speed
• Same direction

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Newtons 1st Law

• inertia
• The simpler way
• Objects maintain velocity.
• UNLESS a net force is applied.

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• Mass and ________ are similar.
• Mass is a measure of ____________.

inertia
inertia
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• What does having a large mass mean?
• You have a lot of inertia.
• What are the consequences of having a lot of
  inertia?
• Hard to get moving or hard to stop moving.
• But moving is a banned word!
• Hard to change your speed and hard to change your
  direction
• Or hard to change your velocity

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• Large mass means
• Lots of inertia means
• Hard to change what?
• Hard to change what over time
• Hard to
• ACCELERATE!

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• What is mass?
• Measured in
• What is weight?
• A force
• A force caused by gravity
• The force of the Earths gravitational pull on
  mass.
• Measured in
• NEWTONS (and lbs)

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What is a Force?

• An interaction between two objects
• push or a pull
• measured in Newtons or pounds
• the cause of a change in speed or direction
• direction counts

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• net force
• sum of the forces
• The amount of force not canceled out.
• examples of forces
• gravity, weight, friction, electro-magnetic
• is inertia a force?
• How can a property of matter tied to mass be a
  force? Did inertia ever cause anything to
  change?
• use arrows to represent how big the force is and
  the direction

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• If a net force is applied on an object the object
  will
• What????
• Change its speed or direction
• Which means it will
• accelerate

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• Is there a relationship between the net force and
  the acceleration?
• What is the relationship?

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• If forces get bigger, what happens to the
  acceleration of a given mass?
• If forces get smaller, what happens to the
  acceleration of a given mass?
• If mass get bigger, and force stays the same,
  what happens to the acceleration?
• If mass get smaller, and force stays the same,
  what happens to the acceleration?

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• Net Force increases (or decreases)
• Acceleration increases (or decreases)
• a ? Fnet
• Mass increases (or decreases)
• Acceleration decreases (or increases)
• a ? 1/mass

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Newtons 2nd Law in words

• The acceleration produced by a _______ on a body
  is directly proportional to the size of the
  _______, in the same ______ as the net force and
  inversely proportional to the _____ of the body.

NET FORCE
NET FORCE
DIRECTION
MASS
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What is a Newton?

• Fnet ma
• unit analysis

(kg)
(m/sec2)
Which means 1 N will accelerate 1 kg, 1 m/sec2
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Weight

• Weight is a measure of the force of gravity on a
  body
• weight always acts straight down

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Acceleration
So the acceleration due to gravity
W mg
So weight must
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• To get gravity to pull harder on an object you
• Increase mass
• So bigger force on more mass means
• Same acceleration due to gravity for all objects!

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• Students often say
• Gravity pulls on everything the same.
• NWB!!!
• Gravity pulls harder on heavy thingsthat is why
  they are heavier!
• Students should say
• Gravity accelerates everything the same (here
  near the surface of the Earth where the
  gravitational field is relatively uniform).

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Which block accelerates the most?
100 N
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Friction
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• friction is a force that opposes motion
• caused by
• peaks and valleys of surfaces falling into each
  other, or in short - roughness
• electrostatic interactions at the surfaces

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Friction

• Friction is a force that depends on
• the surfaces sliding past each other and
• how hard the surfaces are squezz together.

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• How do you quantify the surfaces?
• with coefficient of friction
• a unitless number that describes the interaction
  of the two surfaces
• Symbol m (mu)
• How do you quantify the squeeze?
• With the normal force (Fnorm)
• the normal force is the upward force,
  perpendicular to the surface
• Its the supporting force

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• If m increases, Ff
• If m decreases, Ff
• Ff ? m
• Force Normal increases, Ff
• Force Normal decreases, Ff
• Ff ? Fnorm

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So put the two things together and form an
equation. Ff mFnorm
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• m is different for when you are trying to get
  things to slide and when it is already sliding
• ms is the coefficient of static friction
  (when objects
  are at rest)
• mk is the coefficient of kinetic friction
  (when
  objects are sliding)

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ms gt mk which means it is harder to get it
sliding than to keep it sliding 0 lt m lt 1 m can
be greater than 1. When it is greater than 1, it
is easier lift it than to slide it!
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In this case Fnorm weight
In this case Fnorm only part of the weight
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Suzie or Arnold?
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• Suppose you were unjustly convicted of a capital
  offence in some small country.
• If you were standing in front of a firing squad,
  and they gave you one request, what would your
  last request be?

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What is the tension in the string?
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How hard can you hit a piece of paper?
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Questions

• Does anything contain force?
• What do you see happening when a person leans
  against the wall?
• How do we move?

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Questions

• Whats happening when a person leans against a
  wall?
• How do we move?
• How do birds fly?
• How do fish swim?
• How do rockets soar?

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Answers

• Nothing contains force
• a force is an interaction between two objects
• forces always come in pairs
• formula
• A push on B, than B push on A

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Answers

• Whats happening when a person leans against a
  wall?
• How do we move?
• How do birds fly?
• How do fish swim?
• How do rockets soar?

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Suzie or Arnold?
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Newtons 3rd Law

• For every action there is an equal and opposite
  reaction.

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• If you see the Earth pulling a bowling ball down
• What is the reaction force?
• The bowling ball pulling the Earth up!

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If a horse pulls on a cart as hard as the cart
pulls on the horse, how does the cart even move?
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Which team always wins in a tug-of-war?
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If foot kicks soccer ball, soccer ball kicks
foot. Those forces are equal and oppositecancel
each other out and the soccer ball stays put.
Right?
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Why is the acceleration of the bullet large?
Why is the acceleration of the bullet forward?
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• Forces are equal, ______of the forces can be
  different.

effects
(Thats Newtons 2nd Law)
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• Forces are equal, and are opposite, but
  ____________ cancel each other out
  since they act on ______________ objects!

DONT
DIFFERENT
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• A massless rope is strung over a frictionless
  pulley. A monkey holds on to the rope on one
  side and a mirror having the same weight of the
  monkey is attached to the other side of the rope.
  The monkey, scared by his own reflection, wants
  to get away from the image. To get away from the
  mirror, should the monkey
• Climb up the rope
• Climb down the rope
• Just let go and fall