Reading Quiz - Newton

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Reading Quiz - Newtons Laws

• 1. Which of these laws is not one of Newtons
  laws?
• ___ 1. Action equals reaction.
• ___ 2. F ma.
• ___ 3. All objects fall with equal acceleration.
• ___ 4. Objects at rest stay at rest, etc.

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• 2. The law of inertia
• ___ 1. is not covered in the reading assignment.
• ___ 2. expresses the tendency of bodies to
• maintain their state of motion.
• ___ 3. is Newtons 3rd law.

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• 3. Is the normal force on a body always equal to
  its weight?
• ___ 1. yes
• ___ 2. no
• ___ 3. not covered in the reading assignment

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Newtons First Law

• Force - a vector quantity a push, pull or
  influence that affects the motion of an object.
• Inertia property of an object that resists
  changes in the state of motion.
• Newtons First Law Every object continues in its
  state of rest, or of uniform motion in a straight
  line, unless it is compelled to change that state
  by forces impressed upon it.

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Conceptual Questions

• 1) Beaker and Tablecloth video demo
• 2) What happens as a result of the downward
  motion and sudden stop of a hammer when its end
  (not the head) is slammed against a table?
• 3) The inertial ball video demo
• Common theme INERTIA

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Newtons Second Law

• Mass quantitative measure of the inertia of an
  object (kilograms).
• Newtons Second Law The acceleration of an
  object is directly proportional to the net force
  acting on it and is inversely proportional to its
  mass. The direction of the acceleration is in the
  direction of the net force acting on the
  object. or

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• Note this is a vector equation meaning
• Force an action capable of accelerating an
  object. Allows one to quantify forces - the
  Newton is the unit for forces. One Newton is that
  force that would impart an acceleration of 1
  to an object of mass one kilogram.

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Quantitative Problems

• 1) A person stands on a bathroom scale in a
  motionless elevator. When the elevator begins to
  move, the scale briefly reads only 0.75 of the
  persons regular weight. Calculate the
  acceleration of the elevator, and find the
  direction of the acceleration.
• 2) Starting from rest, a 4.0 kg body reaches a
  speed of 8.0 m/s in 2.0 s. What is the net force
  acting on the body? What can you say about the
  force?

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Newtons Third Law

• A force exerted on an object is always exerted by
  another object.
• Newtons Third Law Whenever one object exerts a
  force on a second object, the second exerts an
  equal and opposite force on the first - action
  and reaction.
• Note The pair of action and reaction forces
  always act on different objects!

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• Weight - the force exerted on an object by the
  gravitational attraction of the
  earth downwards
• Normal force - when objects are in contact,
  forces are exerted by each object on the other
  (nature of forces?). The contact force that is
  perpendicular to the surface of contact is called
  the normal force.
• Note Weight and normal force are not
  action-reaction pairs!

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Quantitative Problems

• 1) A 10.0 kg box is placed on top of a 20.0 kg
  box that is resting on top of a table. Determine
  the normal force that the table exerts on the
  20.0 kg box, and the normal force that the 20.0
  kg box exerts on the 10.0 kg box.
• 2) A 1.0 kg block and a 3.0 kg block are in
  contact on a frictionless horizontal surface. A
  10.0 N force is applied to the first block. Find
  the force this block exerts on the second block.
  If the same force is now applied to the second
  block, what force is exerted by it on the first
  block?

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Applications

• Draw a sketch of the problem.
• Free-body diagram - for each body (hence the word
  free), draw a diagram with all forces acting on
  that body. Apply Newtons 3rd law where possible.
• Do not include forces exerted by this body on
  other bodies (why?).
• Apply Newtons 2nd law to each body - note vector
  nature.
• Solve the (simultaneous) equations for the
  unknowns.

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Conceptual Questions

• 1) A car is towing a trailer at constant speed on
  a level road. The force the trailer exerts on the
  car
• ____ a) is less than the force the car exerts on
  the trailer
• ____ b) is equal to the force the car exerts on
  the trailer
• ____c) is equal to the force the road exerts on
  the trailer
• ____d) is equal to the force the trailer exerts
  on the road

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• 2) A 250 lb man holding a 30 lb bag of potatoes
  is standing on a scale in an amusement park. He
  heaves the bag straight up into the air, and
  before it leaves his hands, a card pops out of
  the slot with his weight and fortune. It reads
• ____a) 250 lb
• ____b) 280 lb
• ____c) less than 250 lb
• ____d) more than 250 lb

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• 3) A skydiver is observed to have a terminal
  speed of 55 m/s in a prone position and 80 m/s in
  a vertical position. Which of the following can
  be concluded from this observation?
• ____a) The force of gravity on the skydiver is
  less in the prone position
• ____b) The force of air resistance on the
  skydiver is proportional to the speed of the body
• ____c) The force of air resistance is greater at
  55 m/s in the prone position than at 80 m/s in
  the vertical position
• ____d) The force of air resistance at 55 m/s in
  the prone position is the same as at 80 m/s in
  the vertical position

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Quantitative Problem

• A 100 kg basketball player can leap straight
  up in the air to a height of 80 cm. How? Let us
  analyze this in the following way
• a) The player bends his legs until the upper part
  of his body has dropped by 60 cm, then he begins
  his jump. Explain in words what the player is
  doing from the time he begins his jump until his
  feet leave the ground.
• b) Draw separate free body diagrams for the
  player and for the floor as he is jumping, but
  before his feet leave the ground.

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• c) Is there a net force on the player as he jumps
  (before his feet leave the ground)? How can that
  be? Explain.
• d) What is the speed with which he leaves the
  ground?
• e) What was his acceleration, assumed to be a
  constant, as he jumped?
• f) Suppose he jumps while standing on metric
  bathroom scales that read in N. What do they
  read
• i) Before he jumps?
• ii) As he is jumping?
• iii) After his feet leave the ground?