Physics 108 lecture 4: Forces and Newtons Laws

1
Physics 108 lecture 4 Forces and Newtons
Laws

• Forces make things happen. They move mountains,
  cars, your pencil, and subatomic particles.
  Below is a classification of common terms for
  forces.

Types of forces

• gravitational  attraction between any objects
  having mass
• strong nuclear  holds together protons and
  neutrons in the nucleus
• weak nuclear  governs radioactive decay
• electric  repulsion of like charges or
  attraction of opposite charges
• magnetic  repulsion of like poles or attraction
  of opposite poles
• friction  resists sliding motion across surfaces

• tension  acts through ropes, pulleys, and
  cables
• centripetal  keeps an object in circular motion
  from flying off into space
• centrifugal  not a real force, but the outward
  motion felt during circular motion
• push  acts to move objects apart
• pull  acts to move objects together
• normal  force perpendicular to a surface that
  prevents objects from falling through it

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Newton's First Law of Motion (the law of inertia)

• Objects will remain either at rest or in uniform
  motion in a straight line unless acted on by an
  external, unbalanced force.
• This law means objects keep on doing what they
  are doing if undisturbed.
•         An object at rest has a _____________
  velocity of zero.        An object in uniform
  motion has a ______________, non-zero velocity.
•         An ______________force is applied from
  outside an object or system.           An
  ______________force on an object is one that is
  not opposed             by an equal and opposite
  force acting on that same object.
•         Force, two equivalent descriptions     
             A force is a quantity that is capable
  of producing a change in motion.               
  A force is a quantity that is capable of
  producing an acceleration.
• Inertia - the ______________ of an object to
  changes in its motion.             Mass is a
  measure of an object's inertia.Note  Momentum
  is NOT the same as inertia.  Momentum comes from
  the combination of an object's mass and
  velocity.  Inertia comes only from mass.

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Newton's Second Law of Motion (the force law)

• The acceleration of an object is directly
  proportional to the force applied toit and
  inversely proportional to its mass.
• This law means increasing forces on objects give
  increasing accelerationswhile increasing masses
  give decreasing accelerations.
• Newton's second law of motion states two
  facts    1)  The acceleration produced by an
  unbalanced force is directly proportional        
      to the magnitude of the that force  (more
  force --gt more acceleration).    2)  The
  acceleration produced by an unbalanced force is
  inversely proportional            to the mass of
  the object being accelerated (more mass --gt less
  acceleration).
• FORCE             force(mass)x(acceleration)   
            F ma               
  or                        a F/m        The
  unit of force is the Newton N 1 N 1 kg
  x 1 m/s2 1 kgm/s2
•         The force applied to an object due to
  gravity is expressed as its ____________.
• WEIGHT        weight (mass) x (acceleration of
  gravity)                            W mg
• Mass and weight are related, but they are not the
  same.  Because the acceleration of gravity is
  different on the earth and on the moon,an
  object's weight will be different on the earth or
  on the moon.

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Example (force) You are able to push a portable
bed with a patient lying on it with a force of
130N. After 5 seconds the velocity of the bed is
6.5m/s. What is the mass of the patient and bed
together?
Given                    Path                  
Want           Conversions/Equations
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Example (weight)  What is the weight of the
patient and bed in the example above?
Given                    Path                  
Want           Conversions/Equations
6
Example (force) Which is bigger, the force
needed to accelerate a 1550kg truck at 1.2m/s2
(would give you 0 to 60mph in 22s) or the force
needed to lift the 300.kg Africa Stone in the
Worlds Strongest Man competition?
Given                    Path                  
Want           Conversions/Equations
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Newton's Third Law of Motion (the force pair law)

• Whenever one object exerts a force on a second
  object, the second objectexerts an equal and
  opposite force on the first object.
• This law means that forces always come in pairs.
• F1 -F2
• or
• m1a1 -m2a2
• The opposing forces in Newton's third law are
  called _________________________ pairs.

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Example  Two 85kg employees stock boxes on
rolling ladders of mass 70.kg. One throws a 10kg
box with acceleration 5.0m/s2 towards the other,
but did not lock the ladder wheels. a) How fast
will the employee/ladder accelerate backwards?
b) If the ladder moves 2m/s after the throw and
slows down due to friction at 1m/s2, how far will
it move before it comes to rest?
Given                    Path                  
Want           Conversions/Equations
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Force Components
Forces are sometimes applied at angles other than
the direction of motion. This leads to the need
for calculating force components using
trigonometry.