Transcript Nonconservative Forces

Nonconservative Forces
Conservation Law

In the work-energy principle, the work is divided into
conservative and non-conservative forces.
• Kinetic energy DK = Wcon + Wnon

The conservative forces have a related potential
energy.
• Kinetic energy DK = -DU + Wnon
DK + DU = Wnon
Other Forms of Energy

The conservation of energy The work done is always negative.
includes nonconservative
forces.
The work dissipates energy.

These are also called
dissipative forces.
F = -mFN
d
-d
F = mFN
Conservation of Energy

Nonconservative forces do not violate the
conservation of energy.

The energy has taken another form.
• Molecules vibrate more
• This becomes internal energy or heat
Using Friction and Energy
q

The hill is 2.5 km long with a
drop of 800 m.

The skier is 75 kg.

The speed at the finish is
120 km/h.

How much energy was
dissipated by friction?
Friction and Height



Find the total change in
kinetic energy.

Find the total change in
potential energy.

The difference is due to
friction and drag.

DK = ½ mv2 - 0
= ½(75 kg)(130 m/s)2
= 5.4 x 105 J
DU = mgh
= (75 kg)(9.8 m/s2)(-800 m)
= -5.9 x 105 J
Wnon = DK + DU
= -0.5 x 105 J
Friction and Springs

The dissipation of energy due to friction also affects
the mechanical energy in a spring.

If the car stopped, the bumper dissipated the energy.
v
980 kJ initial kinetic energy
x
980 kJ dissipated energy
next