Forces by ajizai


SC Standards

PS 5.7
  Explain the motion of objects on the basis on
  Newton’s three laws of motion: inertia; the
  relationship among force, mass, and
  acceleration; and action and reaction forces.

PS 5.9
  Explain the relationship between mass and
  weight by using the formula Fw = mg.
What is a Force?
• Force – push or pull one body
  exerts on another
  – SI Unit of force: Newton (N)
• Net force – the combining of forces
  when two or more forces act on an
• Balanced forces – forces that are
  equal in size and opposite in
  – Forces are acting on the same
  – Do not cause the motion of the
    object to change
• Unbalanced forces – forces that
  are not equal and/ or opposite in
  – Net force causes object to move in
    direction of force
Check it out…
Common Forces
• Gravity – the attraction two objects
  have for one another
• Air resistance – opposing force an
  object feels when traveling
  through air
• Friction – resistance to motion
  caused by two objects in contact
  with one another.
Inertia and Mass
• Inertia – the tendency of an object
  to resist changes in its motion
  – The greater the mass of an object
    the greater its inertia

• Newton’s First Law of Motion –
  ‘Stuff keeps doin’ what its doin’
  until somethin’ else messes with

• Force, mass,
  and acceleration
  are all
  – For any object,
    the greater the
    force applied to
    it, the greater its
    acceleration will
Learning Checkpoint

• If two equal forces act on an
  object in opposite directions,
  what is the acceleration?

• According to balanced forces,
  the acceleration of the object
  would be zero.
Learning Checkpoint

• Calculate the force on an
  object that has a mass of
  12 kg and an acceleration
  of 4 m/s/s.

• F = mx a
• F = 12 kg x 4 m/s/s
• F = 48 kg x m/s/s = 48 N
Learning Checkpoint

What is the acceleration of a
200 kg object with 3,000 N of net
force acting on it?

a = 3,000 kg x m/s/s / 200 kg
a = 15 m/s/s
Newton’s Third Law

• When one object exerts a force
  on a second object, the second
  one exerts a force on the first
  that is equal in size and
  opposite in direction.
  – “To every action, there is an equal
    and opposite reaction.”
  – Example – when you jump on a
    trampoline the trampoline exerts the
    same force on you in the opposite
How are these not balanced forces?

  • Because
    the forces
    are acting
    on two
Learning Checkpoint

• When a force is exerted on
  a box, an equal and
  opposite force is exerted by
  the box. These forces are
  known as _____ forces.

• These forces are action-
  reaction forces.
Learning Checkpoint

• What is the difference
  between weight and mass?

• Mass is a measure of the
  amount of matter in an object,
  while weight is a measure of
  the force of gravity on the
  object’s matter.

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