Preview

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Preview
Section 1 Newton's Second Section 5 Extra
questions
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Net Force - the Sum of the Forces

• This car is moving with a constant velocity.
• Fforward road pushing the tires
• Fresistance force caused by friction and air
• Forces are balanced
• Velocity is constant because the net force (Fnet)
  is zero.

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Equilibrium

• The state in which the net force is zero.
• All forces are balanced.
• Object is at rest or travels with constant
  velocity.
• In the diagram, the bob on the fishing line is in
  equilibrium.
• The forces cancel each other.
• If either force changes, acceleration will occur.

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Classroom Practice Problem

• An agricultural student is designing a support
  system to keep a tree upright. Two wires have
  been attached to the tree and placed at right
  angles to each other (parallel to the ground).
  One wire exerts a force of 30.0 N and the other
  exerts a force of 40.0 N. Determine where to
  place a third wire and how much force it should
  exert so that the net force on the tree is zero.
• Answer 50.0 N at 143 from the 40.0 N force

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Newtons Second Law

• Increasing the force will increase the
  acceleration.
• Which produces a greater acceleration on a 3-kg
  model airplane, a force of 5 N or a force of 7 N?
• Answer the 7 N force
• Increasing the mass will decrease the
  acceleration.
• A force of 5 N is exerted on two model airplanes,
  one with a mass of 3 kg and one with a mass of 4
  kg. Which has a greater acceleration?
• Answer the 3 kg airplane

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Newtons Second Law (Equation Form)

• ?F represents the vector sum of all forces acting
  on an object.
• ?F Fnet
• Units for force mass units (kg) ? acceleration
  units (m/s2)
• The units kgm/s2 are also called newtons (N).

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Classroom Practice Problem

• Space-shuttle astronauts experience accelerations
  of about 35 m/s2 during takeoff. What force does
  a 75 kg astronaut experience during an
  acceleration of this magnitude?
• Answer 2600 kgm/s2 or 2600 N

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What do you think?

• How do the quantities weight and mass differ from
  each other?
• Which of the following terms is most closely
  related to the term friction?
• Heat, energy, force, velocity
• Explain the relationship.

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Weight and Mass

• Mass is the amount of matter in an object.
• Kilograms, slugs
• Weight is a measure of the gravitational force on
  an object.
• Newtons, pounds
• Depends on the acceleration of gravity
• Weight mass ? acceleration of gravity
• W mag where ag 9.81 m/s2 on Earth
• Depends on location
• ag varies slightly with location on Earth.
• ag is different on other planets.

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Normal Force

• Force on an object perpendicular to the surface
  (Fn)
• It may equal the weight (Fg), as it does here.
• It does not always equal the weight (Fg), as in
  the second example.
• Fn mg cos ?

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Static Friction

• Force that prevents motion
• Abbreviated Fs

• How does the applied force (F) compare to the
  frictional force (Fs)?
• Would Fs change if F was reduced? If so, how?
• If F is increased significantly, will Fs change?
  If so, how?
• Are there any limits on the value for Fs?

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Kinetic Friction

• Force between surfaces that opposes movement
• Abbreviated Fk
• Does not depend on the speed

• Using the picture, describe the motion you would
  observe.
• The jug will accelerate.
• How could the person push the jug at a constant
  speed?
• Reduce F so it equals Fk.

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Friction
Click below to watch the Visual Concept.
Visual Concept
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Calculating the Force of Friction (Ff)

• Ff is directly proportional to Fn (normal force).
• Coefficient of friction (?)
• Determined by the nature of the two surfaces
• ?s is for static friction.
• ?k is for kinetic friction.
• ?s gt ?k

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Typical Coefficients of Friction

• Values for ? have no units and are approximate.

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Everyday Forces
Click below to watch the Visual Concept.
Visual Concept
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Classroom Practice Problem

• A 24 kg crate initially at rest on a horizontal
  floor requires a 75 N horizontal force to set it
  in motion. Find the coefficient of static
  friction between the crate and the floor.
• Draw a free-body diagram and use it to find
• the weight
• the normal force (Fn)
• the force of friction (Ff)
• Find the coefficient of friction.
• Answer ?s 0.32

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Classroom Practice Problem

• A student attaches a rope to a 20.0 kg box of
  books. He pulls with a force of 90.0 N at an
  angle of 30.0 with the horizontal. The
  coefficient of kinetic friction between the box
  and the sidewalk is 0.500. Find the magnitude of
  the acceleration of the box.

• Start with a free-body diagram.
• Determine the net force.
• Find the acceleration.
• Answer a 0.12 m/s2

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The Four Fundamental Forces

• Electromagnetic
• Caused by interactions between protons and
  electrons
• Produces friction
• Gravitational
• The weakest force
• Strong nuclear force
• The strongest force
• Short range
• Weak nuclear force
• Short range

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Preview

• Multiple Choice
• Short Response
• Extended Response

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Multiple Choice

• Use the passage below to answer questions 12.
• Two blocks of masses m1 and m2 are placed in
  contact with each other on a smooth, horizontal
  surface. Block m1 is on the left of block m2. A
  constant horizontal force F to the right is
  applied to m1.
• 1. What is the acceleration of the two blocks?
• A. C.
• B. D.

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Multiple Choice

• Use the passage below to answer questions 12.
• Two blocks of masses m1 and m2 are placed in
  contact with each other on a smooth, horizontal
  surface. Block m1 is on the left of block m2. A
  constant horizontal force F to the right is
  applied to m1.
• 1. What is the acceleration of the two blocks?
• A. C.
• B. D.

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Multiple Choice, continued

• Use the passage below to answer questions 12.
• Two blocks of masses m1 and m2 are placed in
  contact with each other on a smooth, horizontal
  surface. Block m1 is on the left of block m2. A
  constant horizontal force F to the right is
  applied to m1.
• 2. What is the horizontal force acting on m2?
• F. m1a
• G. m2a
• H. (m1 m2)a
• J. m1m2a

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Multiple Choice, continued

• Use the passage below to answer questions 12.
• Two blocks of masses m1 and m2 are placed in
  contact with each other on a smooth, horizontal
  surface. Block m1 is on the left of block m2. A
  constant horizontal force F to the right is
  applied to m1.
• 2. What is the horizontal force acting on m2?
• F. m1a
• G. m2a
• H. (m1 m2)a
• J. m1m2a

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Multiple Choice, continued

• 3. A crate is pulled to the right with a force
  of 82.0 N, to the left with a force of 115 N,
  upward with a force of 565 N, and downward with a
  force of 236 N. Find the magnitude and direction
  of the net force on the crate.
• A. 3.30 N at 96 counterclockwise from the
  positive x-axis
• B. 3.30 N at 6 counterclockwise from the
  positive x-axis
• C. 3.30 x 102 at 96 counterclockwise from the
  positive x-axis
• D. 3.30 x 102 at 6 counterclockwise from the
  positive x-axis

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Multiple Choice, continued

• 3. A crate is pulled to the right with a force
  of 82.0 N, to the left with a force of 115 N,
  upward with a force of 565 N, and downward with a
  force of 236 N. Find the magnitude and direction
  of the net force on the crate.
• A. 3.30 N at 96 counterclockwise from the
  positive x-axis
• B. 3.30 N at 6 counterclockwise from the
  positive x-axis
• C. 3.30 x 102 at 96 counterclockwise from the
  positive x-axis
• D. 3.30 x 102 at 6 counterclockwise from the
  positive x-axis

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Multiple Choice, continued

• 5. A freight train has a mass of 1.5 x 107 kg.
  If the locomotive can exert a constant pull of
  7.5 x 105 N, how long would it take to increase
  the speed of the train from rest to 85 km/h?
  (Disregard friction.)
• A. 4.7 x 102s
• B. 4.7s
• C. 5.0 x 10-2s
• D. 5.0 x 104s

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Multiple Choice, continued

• 5. A freight train has a mass of 1.5 x 107 kg.
  If the locomotive can exert a constant pull of
  7.5 x 105 N, how long would it take to increase
  the speed of the train from rest to 85 km/h?
  (Disregard friction.)
• A. 4.7 x 102s
• B. 4.7s
• C. 5.0 x 10-2s
• D. 5.0 x 104s

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Multiple Choice, continued

• Use the passage below to answer questions 67.
• A truck driver slams on the brakes and skids
  
• to a stop through a displacement Dx.

• 6.
• A. Dx/4
• B. Dx
• C. 2Dx
• D. 4Dx

If the trucks mass doubles, find the trucks
skidding distance in terms of Dx. (Hint
Increasing the mass increases the normal force.)
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Short Response

• Base your answers to questions 1012 on the
• information below.
• A 3.00 kg ball is dropped from rest from the
  
• roof of a building 176.4 m high.While the
  ball
• is falling, a horizontal wind exerts a
  constant
• force of 12.0 N on the ball.
• 10. How long does the ball take to hit the
  ground?

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Short Response

• Base your answers to questions 1012 on the
• information below.
• A 3.00 kg ball is dropped from rest from the
  
• roof of a building 176.4 m high.While the
  ball
• is falling, a horizontal wind exerts a
  constant
• force of 12.0 N on the ball.
• 10. How long does the ball take to hit the
  ground?
• Answer 6.00 s

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Short Response, continued
Base your answers to questions 1012 on
the information below. A 3.00 kg ball is
dropped from rest from the roof of a
building 176.4 m high.While the ball is
falling, a horizontal wind exerts a constant
force of 12.0 N on the ball. 11. How far from
the building does the ball hit the ground?
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Short Response, continued

• Base your answers to questions 1012 on the
• information below.
• A 3.00 kg ball is dropped from rest from the
  
• roof of a building 176.4 m high.While the
  ball
• is falling, a horizontal wind exerts a
  constant
• force of 12.0 N on the ball.
• 11. How far from the building does the ball hit
  the ground?
• Answer 72.0 m

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Short Response, continued

• Base your answers to questions 1012 on the
• information below.
• A 3.00 kg ball is dropped from rest from the
  
• roof of a building 176.4 m high.While the
  ball
• is falling, a horizontal wind exerts a
  constant
• force of 12.0 N on the ball.
• 12. When the ball hits the ground, what is its
  speed?

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Short Response, continued

• Base your answers to questions 1012 on the
• information below.
• A 3.00 kg ball is dropped from rest from the
  
• roof of a building 176.4 m high.While the
  ball
• is falling, a horizontal wind exerts a
  constant
• force of 12.0 N on the ball.
• 12. When the ball hits the ground, what is its
  speed?
• Answer 63.6 m/s

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Extended Response

• 16. A student pulls a rope attached to a 10.0 kg
  wooden
• sled and moves the sled across dry snow. The
  student
• pulls with a force of 15.0 N at an angle of
  45.0º.
• If mk between the sled and the snow is 0.040,
  what
• is the sleds acceleration? Show your work.

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Extended Response

• 16. A student pulls a rope attached to a 10.0 kg
  wooden
• sled and moves the sled across dry snow. The
  student
• pulls with a force of 15.0 N at an angle of
  45.0º.
• If mk between the sled and the snow is 0.040,
  what
• is the sleds acceleration? Show your work.
• Answer 0.71 m/s2

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Extended Response, continued
17. You can keep a 3 kg book from dropping by
pushing it horizontally against a wall. Draw
force diagrams, and identify all the forces
involved. How do they combine to result in a
zero net force? Will the force you must supply
to hold the book up be different for different
types of walls? Design a series of experiments to
test your answer. Identify exactly which
measurements will be necessary and
what equipment you will need.
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Extended Response, continued

• 17. You can keep a 3 kg book from dropping by
  pushing
• it horizontally against a wall. Draw force
  diagrams,
• and identify all the forces involved. How do
  they
• combine to result in a zero net force? Will the
  force
• you must supply to hold the book up be different
  for
• different types of walls? Design a series of
  experiments to test your answer. Identify exactly
• which measurements will be necessary and what
• equipment you will need.
• Answer Plans should involve measuring forces
  such
• as weight, applied force, normal force, and
  friction.