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» A box weighing 100 N is pushed on a
horizontal floor. The coefficient of
kinetic friction is 0.2.
» What acceleration will result if a person
applies a horizontal force of 40 N?
» Don’t even begin to calculate anything
until you have drawn a FBD!!!!!!!
a) Horizontally:
Fnet = Fapplied - Ffriction
Fnet = 40 - (0.2) (100)
= 20 N
Now, we want to find the acceleration using
F = ma
F =ma
20 = 10.2 a where m= W/g
a = 1.96 m/s2
» Forces are often applied by means of
cables or ropes that are used to pull an
object.
» A force, T, is applied to the right end of a
rope attached to a box.
» Each particle in the rope in turn applies
a force to its neighbor.
» As a result the force is
applied to the box.
» How does the tension in the left end
of the rope compare with the tension
in the right end?
» Which rope sustains the greater
tension? (three people pull just as
hard in both cases)
» When we look at problems with
tension, we assume that we are using
a “mass-less” rope.
» No net force is needed to accelerate
a “mass-less” rope, so we can ignore
it (otherwise you need to use
calculus!)
» An elevator has a mass of 1000kg. It
accelerates upward at 3m/s2. What is
the force, FT, exerted by the cable on
the elevator?
FT (tension from the cable)
∑F = FT - FW
= ma
F T – 1000(9.8) = 1000(3)
FT = 12,800 N
FW (weight of elevator)
» An elevator has a mass of 1000kg.
The elevator is at rest. What is the
force, FT, exerted by the cable on the
elevator?
FT (tension from the cable)
∑F = FT - FW
= ma
F T – 1000(9.8) = 0
FT = 9,800 N
FW (weight of elevator)
» The ability of a mass-less rope to
transmit tension undiminished from
one end to the other is NOT affected
when the rope passes around a
pulley
» The ability of a mass-less rope to
transmit tension undiminished from
one end to the other is NOT affected
when the rope passes around a
simple pulley (one end is stationary)
» A window washer on a scaffold
is hoisting the scaffold up the
side of a building by pulling
downward on a rope. The
pulling force is 540N and the
mass of the worker and scaffold
is 155kg. What is the
acceleration of the unit?
» An object is in equilibrium when it has
zero acceleration.
» Zero acceleration means constant
velocity. (0 m/s is also constant velocity)
» Equilibrium means that the forces are
balanced in each direction (x, and y)
» ΣFx = 0
» ΣFy = 0
» Make sure to resolve all vectors into xand y-components.
» ΣFx = 0
» ΣFy = 0
» An object is NOT in equilibrium if it is
accelerating, meaning there is an
unbalanced force(s)
» Make sure to resolve all vectors into xand y-components.
» ΣFx = max
» ΣFy = may
» Use a triangle to find the resultant
acceleration