CONSERVATION LAWS:

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Chapter 3

• CONSERVATION LAWS
• MOMENTUM AND ENERGY

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The conservation of momentum is most frequently
used in situations in which objects collide. The
conservation of energy is one of the most
important in science. Energy exists in many
different forms and can be converted from one
form into another. When something is conserved,
it is never lost. It may change from one form to
another over time. But, in the end, considering
all the forms that can be taken, the the amount
at the end is the same as the amount at the
beginning. Momentum and energy are conserved.
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Momentum is defined for particles as the product
of the mass (m) of the object and its velocity
(v). momentum
mv Momentum is a vector quantity having the same
direction as the velocity. Ex. 3.1 Momentum is
conserved in collisions between
objects. Regardless of how complicated the
collision, momentum is always conserved in
collisions between two or more objects.
momentum before momentum after Ex. 3.4
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In the physics definition of work, work is a
process of energy transfer or energy
transformation. When we do work on an object, we
give it energy. When an object has energy, it is
capable of doing work for us.
work (force)(distance) work
Fd For work to occur, distance moved must be in
the same direction as the force. Figure 3.2
3.3 The units of work are N m, and this
combination of units is given the name joule
(J). Energy is defined in terms of work. An
object has energy if that object has the ability
to do work.
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kinetic energy the energy that an object has
because of its motion scalar quantity
KE ½mv2 Ex.
3.7 gravitational potential energy the energy
that an object has because of its location in
space PE mgh
Ex. 3.8 law of conservation of mechanical
energy in the absence of friction, the sum of
the k.e. and the p.e. of a system is constant
KE PE constant Ex. 3.9
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Thermal energy is energy of motion of molecules
or atoms in a substance. Ex. pendulum
swing Power is defined as the amount of energy
converted from one form to another divided by
the time required to make this conversion.
power energy converted
time required The
units of power are J/s, and this combination of
units is referred to as a watt, W. Ex. 3.11
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Machines
A machine is a device used to multiply forces or
simply to change the direction of forces. The
concept that underlies every machine is the
conservation of energy. The ramp (an inclined
plane) reduces the force required to do a given
amount of work (by increasing the
distance). (large force)(small distance)
(small force)(large distance) Fig. 3.11
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A lever is a simple machine made of a bar that
turns about a fixed point. The pivot point of a
lever is the fulcrum.
work input work output Since
work equals force times distance
(force x distance)input (force x
distance)output Fig. 3.14 Angular momentum
refers to spinning or revolving objects.
angular momentum mrv
mass, radius,
velocity Fig. 3.15