NEWTON'S LAWS OF MOTION

1
NEWTON'S LAWS OF MOTION

• There are three of them.
• They explain the motion of an object as resulting
  from the forces acting on the object.

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What is a force?

• An interaction between TWO objects.
• For example, pushes and pulls are forces.
• We must be careful to think about a force as
  acting on one object
• from (or due to ) another object.

3
Adding Forces

• Forces are vectors (They have both magnitude and
  direction) and so add as follows
• In one dimension, note direction using a or
  sign then add like scalar quantities (regular
  numbers with no direction associated with them)
• Examples

3 N
3 N
6 N
0 N
3 N
-3 N
4
Newtons First Law

• Consider a body on which no net force acts. If
  the body is at rest, it will remain at rest. If
  the body is moving with constant velocity, it
  will continue to do so.

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• Consider a body on which no net force acts
• An important word here is NET. It means total
  or sum of all (forces).
• It is not that no force at all can act on the
  body. It is just that all the forces must add to
  zero (cancel each other out).

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• Under this condition (no net force acting on the
  body)
• If the body is at rest, it will remain at rest.
• If the body is moving with constant velocity, it
  will continue to do so.
• What if the body is moving with a velocity which
  is not constant? Why isnt this discussed?

7
Newtons Second Law in One Dimension

• Commonly shortened to Fma. Correctly, it is
• Only forces which act on that object affect the
  acceleration of the object.
• Forces exert by the object on another object do
  not.

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Using Newtons 2nd Law to Solve Problems

• Identify all forces acting on the object
• -Pushes or Pulls -Frictional forces
  -Tension in a string -Gravitational Force (or
  weight mg where g is 9.8 m/s2)- Normal
  forces (one object touching another).
• Draw a Freebody Diagram-draw the object, show
  all forces acting on that object as vectors
  pointing in the correct direction. Show the
  direction of the acceleration.
• Chose a coordinate system.
• Translate the freebody diagram into an algebraic
  expression based on Newtons second law.

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Consider an elevator moving downward and speeding
up with an acceleration of 2 m/s2. The mass of
the elevator is 100 kg. Ignore air
resistance.What is the tension in the cable?

• Identify Forces Tension in cable, weight of the
  elevator
• Draw freebody diagram
• Chose coordinate system Let up be the y
  direction and down y. Then
• Translate the FBD into an algebraic expression.
  T-W m(-a) so
• T-(100 kg)(9.8 m/s2) (100 kg)(-2 m/s2)

v
Note No negative sign
10
Newtons Third Law

• Whenever one object (object A) exerts a force on
  another object (object B), the second object
  exerts a force back on the first object.
• These forces are ALWAYS equal in magnitude (but
  they point in opposite directions).
• Such forces are called Newtons third law force
  pairs.
• Not all forces that are equal and opposite are
  third law force pairs.
• The forces are on different bodies, so do not add
  to zero.