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Chapter 4

• Newton's Second Law
• of Motion

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FORCE CAUSES ACCELERATION
F
a

• The combination of forces that act on an object
  is the net force.
• (Only the net force is shown in the figures on
  this slide.)

F
a
F
a
The acceleration of an object is directly
proportional to the net force.
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2. FRICTION

• Friction opposes the motion between surfaces in
  contact with one another.
• When there is a tendency for movement between two
  surfaces and yet there is no motion, the friction
  is static friction.
• Static friction has an upper limit.
• When there is motion between the two surfaces,
  the friction is sliding (kinetic) friction.

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FA
FA
FA
FA
FA
F
F
F
F
On the verge of slipping
Sliding
Maximum Static Friction
Friction, F
Sliding (Kinetic) Friction
Applied Force, FA
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• Static and kinetic friction
• (In stopping a car, shoot for static friction.)

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3. MASS AND WEIGHT

• Every object possesses inertia (mass).
• Inertia is the sluggishness of an object to
  changes in its state of motion.
• Mass - a measure of the inertia of an object
• (Units - kg and

• slug)
• A slug weighs

• 32 lb.
• Inertia deals with how hard it is to start and
  stop an object.

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• Weight - force that of gravity on an object
• (Units - N and lb)
• A Newton is approximately the weight of a small
  apple.
• 1 kg weighs 2.2 lb or 9.81N.
• Galileo introduced inertia.
• Newton grasped its significance.

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4. MASS RESISTS ACCELERATION

• The acceleration of an object not only depends on
  the force applied to an object but it also
  depends on the mass of the object.

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a
F
Consider the same net force applied to different
mass objects.
F
a

The acceleration is inversely proportional to the
mass of the object.
F
a
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5. NEWTONS SECOND LAW OF MOTION
or

• The force in each of these equations represents
  the vector sum of all of the forces acting on the
  object of mass m.
• Units of force - N and lb

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Weight revisited

• Consider an object in free fall
• a g 9.81m/s2
• The force on the object is the force of gravity.
• F ma
• Weight is the force of gravity on an object
• W mg

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If the net force is in the same direction as the
velocity, then the speed of the object increases.
If the net force is in the opposite direction as
the velocity, then the speed of the object
decreases.

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If the net force is perpendicular to the
velocity, the direction of the velocity changes.

• Acceleration is always in the direction of the
  net force.

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Net Force

• Net means the total force acting on an object.
  It is the vector summation of all the forces
  acting on the object.

Net Force
Story time
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THE WICKED KING AND THE BEAUTIFUL PRINCESS

• Once upon a time there was a terribly wicked King
  who had a daughter who was very beautiful. This
  daughter was in love with a handsome prince who
  didnt take Physics 1410, and before long the
  prince and the princess had become inseparable.
  The King, however, was wicked and did not believe
  in happiness and so he had his daughter locked up
  in a prison at the top of a tall tower.

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• The prince learned of this and was determined to
  rescue the one he loved, so he started out for
  the tower where the unhappy princess sat
  imprisoned.
• When he arrived at the base of the tower the
  prince looked up and noticed that there was a
  wooden beam protruding from the top of the
  structure. He immediately contrived a method to
  use this to reach his princess.

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• He attached a sturdy basket to one end of a very
  long rope and to the other end he tied a stone.
  Then with a mighty heave he threw the stone
  across the top of the beam so that the rope was
  looped across the beam. The prince had thus
  constructed a simple pulley. He then stepped
  into the basket and proceeded to hoist himself up.

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• In due time the prince reached the top and was
  rewarded with a long embrace by the Kings
  daughter. The prince could not return the
  embrace, nor could he begin his work to release
  the princess, since letting go of the rope would
  cause the basket to fall. So he began searching
  for a way to fix the rope to the tower wall.

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Luck seemed to be smiling on the young man
because close by he discovered a metal hook
imbedded in the stone wall. The prince tugged on
the hook with one hand (the other hand holding
the rope tightly), and finding it secure, he
proceeded to tie the rope to the hook.
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• But the instant he did that, the supporting beam
  broke and the basket, together with the poor
  prince, came crashing to the ground.
• Why?
• What had happened was this. The King, who was
  very wicked, also happened to have had Physics
  1410 (no connection between the two), and he had
  originally designed the beam to support the
  weight of the prince and the weight of the
  basket, but no more.

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• During the time the unsuspecting prince was
  hoisting himself up, the total load on the beam
  was simply his weight plus the weight of the
  basket.
• But as soon as one end of the rope was hooked
  onto the tower, the situation changed
  drastically. Now the weight of the prince plus
  the weight of the basket all exerted a force on
  one end of the rope while the tower, via the
  hook, pulled down on the other end with an equal
  and opposite force. The total force on the beam
  was now twice the original weight.

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6. WHEN ACCELERATION IS g FREE FALL

• Only force of gravity is acting on object
• All objects accelerate the same.
• a  g or F/m g .
• Thus two objects of different mass dropped from
  the same height will hit the ground simultaneously

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7. WHEN ACCELERATION IS LESS THAN g NONFREE FALL

• Consider the net force acting on the object.
• The force due to air resistance depends on the
  size and speed of the object.

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Terminal Velocity
Net Force
Acceleration g
Velocity 0 but motion is about to begin
mg
F
0v increasing downward
mg
F
v still increasing downward just not as rapidly
as before
0mg
F
Acceleration 0
Terminal velocity is constant
mg
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• Example
• Light and heavy parachutists

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Chapter 4 Review Questions
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• Which is a better way to get a feeling for the
  mass of an object?
• (a) hold it in your hand
• (b) shake it back and forth

(b) shake it back and forth
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Which of the following represents a situation
when the acceleration of an object is not in the
same direction as the net force acting on the
object.

• (a) a bowling ball swinging by a cord attached to
  the ceiling
• (b) a car speeding up along a straight line on
  the highway
• (c) a book sliding to a stop on the top of a
  table
• (d) All of the above
• (e) None of the above

(e) None of the above
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What is the magnitude of the acceleration of a
2 kg object when the net force on the object is
10 N?

• (a) 10 m/s2
• (b) 20 m/s2
• (c) 5 m/s2

(c) 5 m/s2
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If an object weighs 10 lb, what must the air
resistance force be if the object is falling and
has reached terminal velocity?

• (a) 10 lb
• (b) 32 lb
• (c) there is no way of telling without knowing
  what the value of the terminal velocity is

(a) 10 lb