AP Physics C IB

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Transcript AP Physics C IB

AP Physics C I.B
Newton’s Laws of Motion
Note: the net force is the sum of
the forces acting on an object,
as well as ma.
An object may accelerate in one
dimension or two. ΣFx = max or
ΣFy = may
Free body diagrams
Ex. Two cases for a 0.20 kg hockey puck on a
frictionless surface. Find the horizontal acceleration
when a) F1 = 4.0 N b) F2 = 2.0 N and F3 = 1.0 N at 30.0º
below the horizontal.
Ex. Three students pull on a ring with the forces shown.
The ring doesn’t move. What is the magnitude and
direction of FB?
Weight and Mass
The normal force – the force a
surface exerts on an object,
perpendicular to the surface.
Tension – forces exerted by
strings, ropes, chains, cables
etc. Since these devices can
only pull on an object and not
push, the force of tension is
always away from the object.
Which string experiences the most tension?
2m
m
Newton’s Third Law – When two
bodies interact, the forces are
always equal in magnitude but
opposite in direction. This is true
whether the objects are
stationary, moving at a constant
velocity or accelerating.
Ex. For the system of blocks shown a) draw a free body
diagram on each block b) find the acceleration of the
hanging block c) and the tension in the rope.
Ex. For the hanging block shown, find the tension in
each rope.
Ex. For the block shown find a) the tension in the rope
and b) the normal force on the block.
Ex. Apparent weight of a 72 kg person in an elevator
given by the scale reading (normal force). Find the
apparent weight when a) the elevator is at rest or
moving at a constant velocity b) accelerating upward at
3.20 m/s2 and c) accelerating downward at 3.20 m/s2.
Ex. For the blocks shown find a) the acceleration of the
blocks and b) the force on block B by block A.
Friction – the book on the table
• The book is pushed and released
• The book is pushed across the table at a
constant velocity
• Pushing a stack of books
The direction of friction
Causes of friction and a couple of
equations
Ex. In 1960, the longest recorded skid marks (290 m) on
a public road, the MI highway in England, were made by
a Jaguar. How fast was the car moving at the moment it
began skidding?
Ex. A woman uses a rope to pull a loaded sled with a
mass of 75 kg along a horizontal patch of ice at a
constant velocity. The coefficient of kinetic friction
between the sled runners and ice is 0.10 and the rope
makes an angle of 42º with the horizontal. What is the
tension in the rope?
Ex. A coin rests on a book. When the book makes an
angle of 13º with the table, the coin just begins to slip.
What is the maximum coefficient of static friction
between the coin and book?
For blocks that are sliding without slipping, treat
them as a system. That is, they accelerate together.
For blocks that are slipping, treat them separately.
They are not accelerating together. Draw an FBD to
identify the frictional force on each block.
Fa
Ex. Will the block slide if the applied force is 12 N, the coefficient
of friction between the block and wall is 0.60 and the weight of the
block is 5.0 N?
Fa = 12.0 N
maa
m
mA = 1.0 kg
mB = 3.0 kg
Ex. Find the force on block B by block A.