Newton's Laws

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Newton's Laws

• Physical Science

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Newtons First Law

• An object at rest tends to stay at rest and an
  object in motion tends to stay in motion with the
  same speed and in the same direction unless acted
  upon by an unbalanced force

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Newtons First Law

• There are two parts to this statement one which
  predict the behavior of stationary objects and
  one that predicts the behavior of moving objects

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Newtons First Law

• The state of motion of an object is maintained as
  long as the object is not acted upon by an
  unbalanced force.
• Blood rushes from your head to your feet when
  riding on a descending elevator which suddenly
  stops.
• To dislodge ketchup from the bottom of a ketchup
  bottle, the bottle is often turned upside down,
  thrust downward at a high speed and then abruptly
  halted.
• While riding a skateboard (or wagon or bicycle),
  you fly forward off the board when hitting a
  curb, a rock or another object which abruptly
  halts the motion of the skateboard.

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Inertia

• It is the natural tendency of objects to resist
  changes in their state of motion. Objects want to
  keep on doing what theyre doing.
• Inertia is the resistance an object has to a
  change in its state of motion.

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Inertia
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Inertia

• Newton's first law of motion declares that a
  force is not needed to keep an object in motion.
  In the absence of a frictional force, the book
  would continue in motion with the same speed and
  in the same direction forever!

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Inertia

• The more mass an object has, the more inertia it
  has the more tendency it has to resist changes
  in its state of motion.

Due to the large mass of the books, the force of
the hammer is sufficiently resisted (inertia).
This is demonstrated by the fact that the blow of
the hammer is not felt by the person.
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Newtons Second Law

• The acceleration of an object is dependent the
  net force acting upon the object and the mass of
  the object.
• F m x a
• As the mass of the object increases, its
  acceleration will decrease.
• As the net force increases, so will the object's
  acceleration.

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Newtons Third Law

• For every action, there is an equal and opposite
  reaction
• A fish uses its fins to push water backwards. In
  turn, the water reacts by pushing the fish
  forwards, propelling the fish through the water.

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Problems

• Imagine a place in the cosmos far from all
  gravitational and frictional influences. Suppose
  that an astronaut in that place throws a rock.
  The rock will
• a. Gradually stop.
• b. Continue in motion in the same direction at
  constant speed.
• Answer According to Newton's first law, the
  rock will continue in motion in the same
  direction at constant speed.
•  

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Problems

• Mrs. Thun spends most Sunday afternoons at rest
  on the sofa, watching movies and consuming large
  quantities of deep fried foods. What effect (if
  any) does this practice have upon her inertia?
  Explain.
• Answer Mrs. Thuns inertia will increase! Mrs.
  Thun will increase her mass if he makes a habit
  of this and if her mass increases, then her
  inertia increases.

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Problems

• A gun recoils when it is fired. The recoil is the
  result of action-reaction force pairs. As the
  gases from the gunpowder explosion expand, the
  gun pushes the bullet forwards and the bullet
  pushes the gun backwards. The acceleration of the
  recoiling gun is...
• a. greater than the acceleration of the bullet.
  
• b. smaller than the acceleration of the bullet.
• c. the same size as the acceleration of the
  bullet.
• Answer The force on the gun equals the force on
  the bullet. Yet, acceleration depends on both
  force and mass. The bullet has a greater
  acceleration due to the fact that it has a
  smaller mass. Remember acceleration and mass are
  inversely proportional. 

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Problems

• A net force of 15 N causes a mass to accelerate
  at a rate of 5 m/s2. Determine the mass.
• Answer Use F m x a
• F 15 N and A 5m/s/s.
• So (15 N) (m)(5m/s/s)
• m 15N / 5m/s/s 3 kg