Chapter 5:Newton

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Chapter 5Newtons Second Law of Motion

• Force and Acceleration

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Introduction

• Most of the motion we see undergoes change
• Recall
• Forces cause acceleration

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5.1 Force Causes Acceleration

• Acceleration depends on net force
• Increase net force to increase acceleration
• Acceleration is directly proportional to net
  force
• Double net force ? doubles acceleration
• Triple net force ? triples acceleration

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5.1 Example

• Consider an object at rest, such as a hockey
  puck on ice
• -Apply a force, it starts moving
• -Force caused the acceleration
• -Moves at constant velocity once the force is no
  longer applied

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Concept Check

• Suppose a pilot announce that the plane is flying
  at a constant 900 km/h and the thrust of the
  engines is a constant 80,000N. What is the
  acceleration of the airplane?
• What is the combined force of air resistance that
  acts on the planes outside surface?

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5.2 Mass Resists Acceleration

• For a given force, the acceleration produced is
  inversely proportional to the mass
• Same force, twice the mass ? half the
  acceleration
• Same force, triple the mass ? 1/3 the
  acceleration

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5.2 Example

• Push on an empty shopping cart
• Push equally hard on a heavily loaded shopping
  cart.
• Which on has the smaller acceleration? Why?
• Shows that acceleration depends on mass

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5.3 Newtons Second Law

• The acceleration produced by a net force on an
  object is directly proportional to the magnitude
  of the net force, is in the same direction as the
  net force, and is inversely proportional to the
  mass of the object

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Concept Check

• How much force must a 30,000 kg jet plane develop
  to achieve an acceleration of 1.5 m/s2.
• What acceleration is produced by a force of 2,000
  N applied to a 1,000 kg car?
• What if the force was 4,000 N instead of 2,000 N?

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5.4 Friction

• Friction acts on materials that are in contact
  with one another and always opposes motion.
• When one object slides against another, it must
  rise over the irregular bumps or else scrape them
  off.
• Depends on the kinds of material in contact and
  how much the surfaces are pressed together

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• Friction occurs in liquids and gasses as well
• Objects will move at a constant velocity when
• Force pushing it just balances the force of
  friction
• Force of air resistance balances the objects
  weight

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Concept Check

• Two forces act on a book resting on a table its
  weight and the support force from the table. Does
  a force of friction act as well?
• Suppose a high-flying jet cruises with a constant
  velocity when the thrust from its engines is a
  constant 80,000 N. What is the acceleration of
  the jet? What is the force of air resistance
  acting on the jet?

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5.5 Applying Force - Pressure

• Pressure is the amount of force per unit of area
• P F/A
• Measured in pascals (Pa)

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Concept Check

• In attempting to do the bed of nails
  demonstration, would it be wise to begin with a
  few nails and work upward to more nails?
• The massiveness of the cement block plays an
  important role in this demonstration. Which
  provides more safety, a less massive block or a
  more massive one?

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5.6 Free Fall Explained

• The dilemma
• Heavy things should fall faster since they have a
  greater force (weight) acting on them
• Heavy things should fall slower since they have
  more inertia (mass) and are more resistant to
  change
• The solution
• They fall at the same rate since the force and
  mass offset each other

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Concept Check

• What is the weight of a 1 kg stone?
• What is the weight of a 10 kg stone?
• For the next question use
• a F/m weight/mass
• During free-fall, what is the acceleration of the
  1 kg stone? The 10 kg stone?
• SEE, SAME ACCELERATION!!! ?

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5.7 Falling and Air Resistance

• When the air resistance on an object equals its
  weight, the net force is zero and no further
  acceleration occurs
• Object reaches terminal velocity
• It takes longer for more massive objects to reach
  terminal velocity
• Body orientation also makes a difference in
  terminal velocity.

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Concept Check

• When the force of air resistance on a falling
  object is equal to the objectss weight, what
  will be the net force on the object?
• What will the acceleration be?
• Does this mean the falling object come to an
  abrupt halt (i.e., that zero acceleration means
  zero velocity)?
• If a heavy person and a light person open their
  parachutes together at the same altitude and each
  wears the same size parachute, who will reach the
  ground first?

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Concept Check

• A sky-diver jumps from a high-altitude balloon.
  As she falls faster and faster through the air,
  does air resistance increase, decrease, or remain
  the same?
• Does the net force on her increase, decrease, or
  remain the same?
• As she falls faster and faster, does her
  acceleration increase, decrease, or remain the
  same?