Download When a net force acts on a body, the body accelerates. Newton`s

When a net force acts on a body, the body accelerates. Newton’s second law
of motion describes the relationship between the net force, the body’s mass,
and the body’s acceleration (F = ma).
Whenever there is motion, there is opposition or resistance to the motion.
Mechanical resistance is opposition to motion of a solid object when it is in
contact with another solid object. This opposition is called friction. A linear
model of friction describes the forces adequately in many situations.
According to this model, the force of friction is proportional to the normal
force between the objects (Fstatic ≤ μsN, and Fkinetic = μkN).
Fluid resistance is opposition to fluid flow. Drag is the force that resists
motion of an object moving through a fluid (or a fluid moving past a
stationary object). Drag increases as speed increases. Resistance causes fluid
•
pressure to drop as fluid flows through a pipe, tube, or duct ( Rfluid = – ΔP /V ).
The resistance depends on the radius of the pipe, the length of the pipe, and
the viscosity of the fluid.
Electrical resistance is opposition to charge flow. Resistance causes a
potential difference, or voltage drop, as current flows through a conductor
(R = ΔV/I). The resistance of a wire depends on the length of the wire, its
cross-sectional area, and the resistivity of its material composition.
Thermal resistance is opposition to heat flow. Heat flows through a wall
from a region of high temperature to a region of low temperature. The
thermal resistance of the wall is the ratio of the temperature drop to the heat
•
flow rate ( Rthermal = – ΔT / Q). Thermal resistance depends on the thickness of
the wall, its cross-sectional area, and the thermal conductivity of its material
composition.
C HAPTER 4
SUMMARY
227