Transcript 4.3 Solving Friction Problems

Practice on board static and kinetic p. 174 and 176
Tutorials
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A) applied force and friction force in same directions
so larger net force (person stopping a sliding box,
friction and applied force in same direction)
B) applied force and friction force in opposite
directions, smaller net force (pushing table across flat
floor)
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A 1640-kg lift truck with rubber tires is skidding on
wet concrete with all four wheels locked, the kinetic
coefficient is 0.5
Calculate the acceleration of the truck.
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two-body problems are characterized by a set of
two unknown quantities. Most commonly (though
not always the case), the two unknowns are the
acceleration of the two objects and the force
transmitted between the two objects.
One approach involves a combination of a system
analysis and an individual body analysis. In the
system analysis, the two objects are considered to be
a single object moving (or accelerating) together as
a whole. The mass of the system is the sum of the
mass of the two individual objects. If acceleration is
involved, the acceleration of the system is the same
as that of the individual objects.
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A system analysis is usually performed to
determine the acceleration of the system. The
system analysis is combined with an
individual object analysis.
In the individual object analysis, either one
of the two objects is isolated and considered as
a separate, independent object.
A free-body diagram is used to find
individual forces acting upon the object are
identified and calculated. An individual object
analysis is usually performed in order to determine
the value of any force which acts between the two
objects - for example, contact forces or tension
forces.
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A second approach involves the use of two
separate individual object analyses. In such
an approach, free-body diagrams are
constructed independently for each object
and Newton's second law is used to relate
the individual force values to the mass and
acceleration. Each individual object analysis
generates an equation with an unknown.
The result is a system of two equations with
two unknowns. The system of equations is
solved in order to determine the unknown
values.
A 7.00-kg box is attached to a 3.00kg box by rope 1. The 7.00-kg box is
pulled by rope 2 with a force of 25.0
N.
 Determine the acceleration of the
boxes and the tension in rope 1. The
coefficient of friction between the
ground and the boxes is 0.120.
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The solution here will use the approach of a
system analysis and an individual object
analysis . The free-body diagrams for the
system and for the 3-kg object are shown below.
For the system: Ffrict = μ•Fnorm = 0.120• 98.0 N =
11.76 N
Fnet = 25.0 N - 11.76 N = 13.24 N and msystem =
10.0 kg.
So a = Fnet/m = (13.24 N) / (10.0 kg) = 1.324
m/s2 (round to 1.32 m/s2)
a = 1.32 m/s2 and Frope 1 = 7.50 N
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For the individual object analysis on the
3.00-kg box: m = 3.00 kg and a = 1.324
m/s2 (from above); so the Fnet is m•a or
3.972 N.
This value of Fnet is equal to the force in
the direction of the acceleration (Frope 1)
minus the force that opposes it (Ffrict).
For the 3.00-kg box, Ffrict = μ•Fnorm =
0.120• 29.4 N = 3.528 N.
So Fnet = Frope 1 - Ffrict or
3.972 N = Frope 1 - 3.528 N
Solving for Frope 1 gives 7.50 N.
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Two sleds are tied together with a rope. The
coefficient of static friction between the sleds and
the snow is 0.22. Total mass on sled 1 is 27kg and
on sled 2 is 38kg. An adult pulls on the sleds
A) what is the greatest amount of horizontal force
that can be applied without moving either sled?
B) calculate the magnitude of tension in the rope
between sleds 1 and 2 when the adult applies this
horizontal force. GRASP…
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B) calculate the magnitude of tension in the rope
between sleds 1 and 2 when the adult applies
this horizontal force.
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Two sleds are pulled with force of150N[E]. The
mass o sled 1 is 18.0kg and sled 2 is 12.0kg. The
coefficient of kinetic friction is 0.20 for each sled.
A) calculate the acceleration of the sleds.
B) find the magnitude of the tension in the rope
between the sleds.
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P. 174 practice #1-2
P. 175 practice #1-2
P. 177 practice #1-3
P. 178 questions #1,2,5,7
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