Page 210 - Physics

Physics for Secondary Schools

Electromagnetic energy (infrared Kinetic and potential energy
radiation, visible radiation, ultraviolet Potential and kinetic energy are due to
radiation, and so on) is associated with the position and motion of an object,
accelerating electric charge, and therefore respectively. In this section, you will learn
with the changing velocity of that charge about kinetic energy and potential energy.
or oscillating electric and magnetic fi elds. Kinetic energy
Radiant light energy is the most common
form of electromagnetic energy. The sun This is the energy possessed by an
provides radiant energy to the earth as object due to its motion.
shown in Figure 9.9.

Any matter that is moving has kinetic
energy. Both a plane fl ying through the
air and an atom vibrating in a solid have
kinetic energy. Consider a 7-tones lorry
and a 1 tone mini vehicle both travelling
at a speed of 80 km/hr. In case of an
accident, one expects the lorry to cause
more damage than the mini vehicle. This
is because the lorry possesses higher
Figure 9.9: Radiant energy from the sun kinetic energy than the mini vehicle. The
kinetic energy of an object depends on
Sound energy is also a form of energy its mass and speed. For a body of mass
that is transferred in the form of waves m travelling at speed v, its kinetic energy
arising from the vibration of a membrane (KE) is expressed as:
and mechanical oscillations. Sources of 1
sound may be speakers, sound boxes, and KE = 2 mv 2
vibrations from the wings of insects. A
microphone converts sound energy into an The SI unit of kinetic energy is Joule (J)
electrical signal and a loudspeaker converts which is equivalent to kgm /s . When the
2
2
electrical energy into sound energy. Figure velocity of a body increases or decreases,
9.10 shows an example of a loudspeaker. its kinetic energy changes. Consider an

object of mass m initially moving with
speed u and attains the fi nal speed v. The
corresponding change in kinetic energy
( ΔKE) is given as the difference between

the object’s fi nal kinetic energy and initial
kinetic energy. That is:

Figure 9.10: A loudspeaker ΔKE = KE final − KE initial

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Student’s Book Form One

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