Transcript AP Physics B Summer Course 2012 2012*AP**B

Ch 10: Free Body Diagrams
and Equilibrium
M Sittig
AP Physics B
Summer Course 2012
2012年AP物理B暑假班
Force
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Free-body diagrams help visualize forces.
What is a force?
A force is an interaction between two
objects (a push or pull).
What is the net force?
Net force is the vector sum of all the forces
acting on an object.
Force
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What forces are acting on this box?
Free-Body Diagram Practice
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A book is at rest on a tabletop. Diagram the forces acting on
the book.
A girl is suspended motionless from the ceiling by two ropes.
Diagram the forces acting on the combination of girl and bar.
An egg is free-falling from a nest in a tree. Neglect air
resistance. Diagram the forces acting on the egg as it is falling.
A flying squirrel is gliding (no wing flaps) from a tree to the
ground at constant velocity. Consider air resistance. Diagram the
forces acting on the squirrel.
A rightward force is applied to a book in order to move it
across a desk with a rightward acceleration. Consider frictional
forces. Neglect air resistance. Diagram the forces acting on the
book.
Free-Body Diagram Practice
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A college student rests a backpack upon his shoulder. The pack
is suspended motionless by one strap from one shoulder.
Diagram the vertical forces acting on the backpack.
A skydiver is descending with a constant velocity. Consider air
resistance. Diagram the forces acting upon the skydiver.
A force is applied to the right to drag a sled across loosely
packed snow with a rightward acceleration. Diagram the forces
acting upon the sled.
A football is moving upwards towards its peak after having
been booted by the punter. Diagram the forces acting upon the
football as it rises upward towards its peak.
A car is coasting to the right and slowing down. Diagram the
forces acting upon the car.
Free-Body Diagram How-To
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free-body-diagrams.swf
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From http://www.wisc-online.com/Objects/ViewObject.aspx?ID=tp1502
What is Equilibrium?
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When the net force on an object equals zero.
An object is either at rest or moving with a
constant velocity
Constant velocity: speed, direction not
changing.
Equilibrium
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Could this object be in equilibrium?
How to Solve Equilibrium Problems
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Draw an appropriate free-body diagram.
Resolve vectors into x- and y-components,
if necessary.
Write an expression for the vector sum of
the left-right vectors. Then write an
expression for the vector sum of the updown vectors. Set each of these
expressions equal to zero.
Solve the resulting equations.
Example Problem
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Two astronauts tug on opposite sides of a
satellite. The first astronaut tugs to the left
with a force of 30 N. With what force does
the second astronaut tug in order to keep
the satellite at rest?
Example Problem
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Two astronauts tug on opposite sides of a
satellite. The first astronaut tugs to the left
with a force of 30 N. With what force does
the second astronaut tug in order to keep
the satellite moving toward him a constant
speed of 20 m/s?
Example Problem
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A man is pushing a box across the floor with
a force of 15 N at an angle of 60° from the
horizontal. If the box is moving at a constant
velocity, what is the size of the friction force
on the box?
What is a normal force?
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A contact force.
A support force.
Acts perpendicular to the surface on which
an object rests.
Not always equal to the weight of the object.
Practice Problem
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5S pg 108 #3
What is tension?
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The force applied by a rope or string.
Acts along the direction of the rope.
Is always a pulling force, at both ends of the
rope.
Example Problem
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Two blocks suspended over a pulley are at
rest. Find the masses of each block.
Example Problems
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5S pg 107 #1
Friction
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A force that resists motion.
Acts in parallel to two surfaces pressed
together.
Acts in the opposite direction to motion or
intended motion.
Static friction: when the object is at rest.
Kinetic friction: when the object is moving.
Friction
F f  FN
Force of
Friction (N)
Coefficient of
Friction
(unitless)
Normal Force
(N)
Force
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Find the force F that the man needs to push
with to keep this box moving across the
floor at a constant velocity.
F
60 °
12 kg
μ = 0.45
Force
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Find the force F that the man needs to push
with to keep this box moving across the
floor at a constant velocity.
F
θ
m
μ
Inclined Planes
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Normal force is perpendicular to the plane.
Smarter to choose axes parallel (∥) and
perpendicular (⊥) to the plane.
Draw a free-body diagram for the block.
Practice Problems
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5S pg 107 #4, 2
Torque
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A force that causes an object to turn, not
move in a straight line.
Equal to the product of ① the distance from
the fulcrum, and ② the component of the
force perpendicular to that distance.
Torque
Torque
  F⊥d
Torque (N·m)
Perpendicular
Force(N)
Distance from
fulcrum to
Force (m)
Torque and Equilibrium
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Balanced torques, not forces.
Turning (torque) can be clockwise or
counterclockwise.
So,
Torquenet  counterclockwise  clockwise  0
Example Problem