Motion: Why Newtons Laws

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-Motion Why?-Newtons Laws

• Physics
• Mrs. Coyle

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Part I Newtons First Law
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The Early Days

• Greece
• Aristotle (4th Century BC)
• Natural and violent motion

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Italy

• Galileo Galilei (1564 AD)
• Is a force necessary in order to keep an
• object moving?

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Demos
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Galileos Experiment with Ball

• A force is not required to keep the ball moving
  on a horizontal plane.
• In the absence of friction, the ball would move
  perpetually.

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Newtons First Law

• Isaac Newton -England (1561)
• 1st Law Law of Inertia
• An object at rest or moving at constant
• velocity will continue to be at rest or at
• constant velocity, unless acted upon by a
• net force.

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Inertia---Mass

• Inertia the resistance of an object to a change
  in its state of motion
• Mass amount of matter in an object
• a measure of inertia
• unit kg
• scalar

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Seatbelts!!!!!!!

• (http//www.glenbrook.k12.il.us/gbssci/phys/mmedia
  /newtlaws/cci.html)

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An airplane drops a flare. Which is the path of
the flare?
v
v
A
C
B
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Part II- Newtons 2nd Law
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Force a push or a pull

• Vector
• Unit Newton N

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The 4 Fundamental Forces

• Gravitational (attractive force between masses)
• Electromagnetic (caused by to electric and
  magnetic fields)
• Strong Nuclear Force (holds nuclear particles
  together)
• Weak Nuclear Force (involved in radioactive decay
  of nuclei)

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Friction

• a force acting when two surfaces rub
• against each other.
• Friction opposes motion.
• Is caused by the irregularities in surfaces.

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Newtons 2nd Law

• Fma

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Normal Force

• a force acting perpendicular to the interface
  of contact between two objects.

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Weight

• the gravitational force with which the
  earth pulls an object
• vector
• unit N

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Free Body Diagram

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Part III- Newtons 2nd Law and Kinematics
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Example 1

• A 40kg box is starting from rest and accelerates
  to 2m/s in 3s.
• a)Find the acceleration.
• b)Find the net force acting on the object.

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Example 2

• A 2kg rock falls through a liquid for 3m. It
  initial velocity was 0.5m/s and its final
  velocity was zero.
• a)Find the acceleration.
• b)Find the net force acting on the object.

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Example 3

• A 1500kg car traveling at 5m/s hits a muddy
  barrier and is stopped by a net force of 2000N.
• Find the cars net acceleration.
• Find the distance the car traveled while being
  stopped by the barrier.

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Example 4

• A 0.1kg arrow is pulled back and then after
  being at rest is released by a bow. During the
  release the arrow traveled 0.3m. Its final
  release speed was 12m/s.
• Find the acceleration given to the arrow by the
  bow.
• Find the force with which the bow pushed the
  arrow.

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Part IV- Newtons 3rd Law
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Newtons 3rd Law

• When an object exerts a force on a second object
  (action), the second objects exerts an equal and
  opposite force on the first object (reaction).
• To every action there is an equal and opposite
  reaction.
• Forces occur in pairs.

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What is the reaction force to

• Kicking a soccer ball?
• The windshield of a bus hitting a fly?
• The swimmer pulling the water?

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Can you hit a piece of paper with a large force
with your hand?
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What are the reaction forces to the each of the
forces shown?
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Example 1

• Snoopy has a mass of 15kg and kicks a 0.4kg ball
  with a force of 15N.
• a)Draw the free body diagram showing the forces
  acting on each object.
• b)With what force does the ball kick Snoopy and
  why?
• c)What was the acceleration of the ball?
• d)What was the acceleration of Snoopy?Ans b)
  15N, c) 37.5m/s2, d) 1m/s2
•  

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Example 2

• Two blocks, one 8 times as massive as the other,
  are connected by a compressed spring (sitting on
  a frictionless surface). When the spring is
  released the blocks fly apart.
• Draw the free body diagram showing the forces
  acting on each object.
• What is the acceleration of the lighter block
  relative to the heavier one?

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Example 3

• Two skaters, a girl of mass 45kg and a 65 kg boy
  push off of each other. The boy accelerates at
  2m/s2. Find the girls acceleration.

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Example 4

• A student of mass 55kg is standing on the surface
  of the earth.
• a)What is the force that the earth pulls the
  student with?
• b)What is the force with which the student pulls
  the earth?
• c)What is the acceleration of the earth because
  of the pull of the student?