Work, Kinetic Energy and Potential Energy
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Chapter 6
Work, Kinetic Energy and Potential
Energy
6.1
The Important Stuff
6.1.1
Kinetic Energy
For an object with mass m and speed v, the kinetic energy is defined as
K =
1
2
mv
2
(6.1)
Kinetic energy is a scalar (it has magnitude but no direction); it is always a positive
number; and it has SI units of kg · m
2
/ s
2
. This new combination of the basic SI units is
known as the joule:
1 joule = 1 J = 1
kg·m
2
s
2
(6.2)
As we will see, the joule is also the unit of work W and potential energy U. Other energy
units often seen are:
1 erg = 1
g·cm
2
s
2
= 10
-7
J
1 eV = 1.60 × 10
-19
J
6.1.2
Work
When an object moves while a force is being exerted on it, then work is being done on the
object by the force.
If an object moves through a displacement d while a constant force F is acting on it, the
force does an amount of work equal to
W = F · d = F d cos
(6.3)
where is the angle between d and F.
Work is also a scalar and has units of 1 N · m. But we can see that this is the same as
the joule, defined in Eq. 6.2.
127
Summary :
Chapter 6 Work, Kinetic Energy and Potential Energy 6.1 The Important Stuff 6.1.1 Kinetic Energy For an object with mass m and speed v, the kinetic energy is defined as K = 1 2 mv 2 (6.1) Kinetic energy is a scalar (it has magnitude but no direction);
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energy,work,kinetic,force,joule,units,object,defined,scalar,but,see,potential,moes