Forces and the Laws of Motion

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Forces and the Laws of Motion

• Chapter 4
• Section 1

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Changes in Motion FORCE

• Force is the cause of an acceleration or the
  change in an objects velocity
• Force can cause objects to
• Start Moving
• Stop Moving
• Change Direction
• Notice that each of these cases involves a change
  in velocity (this is acceleration).

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Changes in Motion FORCE
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The SI unit of Force is the Newton named after
Sir Isaac Newton.

• The symbol for Newton is N.
• Force is measured in Newton's.
• Force is a Vector Quantity.
• Force is equal to Mass times Acceleration.
• 1 N is equal to 1 kg x 1 m/s2

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Forces can act through contact or at a distance.

• Contact Force force from the physical contact
  of two objects
• Ex catch a ball, its motion stops
• Pull on a spring, it stretches
• Field Force does not involve physical contact
  between two objects.
• Ex the force of gravity
• Electrical charges attraction/repulsion

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Free-body Diagrams

• Free body diagrams isolate one object and the
  forces acting on it.
• Shows only the forces acting on the object of
  interest.
• All forces are assumed to act on a single point
  at the center of the object.

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Section Review pg. 128

• Work on problems 1-6

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Newtons First Law

• Chapter 4 Section 4-2

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Newtons First Law of Motion

• Also referred to as the Law of Inertia.
• Inertia is the tendency of an object to maintain
  its state of motion.
• Mass is a measurement of inertia.
• States that an object at rest stays at rest and
  an object in motion stays in motion with constant
  velocity (constant speed in a straight line)
  unless the object experiences a net external
  force.
• When the net external force of an object is zero,
  its acceleration is zero.

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Acceleration is determined by net external force.

• The Net External Force is the vector sum of all
  the forces acting on an object.
• We find X components and Y components.
• Net External Force is the Resultant Force.
• Objects that are at rest or moving with constant
  velocity are said to be in equilibrium.
• Equilibrium means there is no change in a bodys
  motion.

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Lets Practice

• Net External Force Practice 4A
• Pg. 133

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Chapter 4 Section 3

• Newtons Second and Third Laws

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Newtons 2nd Law relates force, mass, and
acceleration.

• Net External Force Mass X Acceleration
• In equation form we have ?F ma
• ?F represents the vector sum of all external
  forces acting on an object.

• States Acceleration is produced when a Force
  acts on a Mass. The greater the mass of the
  object you are trying to accelerate, the greater
  the amount of force needed to do so.

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Overview of Net External Force
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Overview of Net External Force
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Check for Understanding

• What acceleration will result when a 12-N net
  force is applied to a 3-kg object?
• Answer
• ?F ma ? 12N 3kg x a
• 12N / 3kg a
• 4 m/s2 a

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Try this one on your own

• A net force of 16 N causes a mass to accelerate
  at the rate of 5 m/s2. Determine the mass.
• Answer
• ?F ma ? 16N m x 5 m/s2
• 16N / 5m/s2 m
• 3.2 kg m

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Sample 4B

• Pg. 137

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Newtons Third Law

• Forces always exist in pairs.
• For every action, there is an equal and opposite
  reaction.
• Action Ghosts hands force exerted on wall.
• Reaction Walls force exerted on Ghosts hand.

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Check for Understanding

• If a book is on a table what are the action and
  reaction forces involved?
• Answer
• Action Force the book exerts on table.
• Reaction Force the table exerts on the book.

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Newtons 3rd Law Examples