168 CHAPTER 7 KINETIC ENERGY AND WORK
Calculation: We use Eq. 7-47 for each force. For force , at
anglef 1 180 to velocity , we have
P 1 F 1 vcosf 1 (2.0 N)(3.0 m/s) cos 180
6.0 W. (Answer)
This negative result tells us that force is transferring en-
ergyfromthe box at the rate of 6.0 J/s.
For force , at angle f 2 60 to velocity , we have
P 2 F 2 vcosf 2 (4.0 N)(3.0 m/s) cos 60
6.0 W. (Answer)This positive result tells us that force is transferring en-
ergytothe box at the rate of 6.0 J/s.
The net power is the sum of the individual powers
(complete with their algebraic signs):
PnetP 1 P 2
6.0 W6.0 W0, (Answer)
which tells us that the net rate of transfer of energy to
or from the box is zero. Thus, the kinetic energy
of the box is not changing, and so the speed of the box will
remain at 3.0 m/s. With neither the forces and nor the
velocity changing, we see from Eq. 7-48 that P 1 andP 2 are
constant and thus so is Pnet.v:F
:
F 2
:
1(K^12 mv^2 )F
:
2F v::
2F
:
1v:F
:
1Sample Problem 7.09 Power, force, and velocity
Here we calculate an instantaneous work—that is, the rate at
which work is being done at any given instant rather than av-
eraged over a time interval. Figure 7-15 shows constant forces
and acting on a box as the box slides rightward across a
frictionless floor. Force F is horizontal, with magnitude 2.0 N;
:
1F
:
F 2
:
1Additional examples, video, and practice available at WileyPLUSFigure 7-15Two forces and act on a box that slides
rightward across a frictionless floor. The velocity of the box is .v:
F: 1 F: 260°
Frictionless F 1F 2vNegative power.
(This force is
removing energy.)Positive power.
(This force is
supplying energy.)Kinetic Energy The kinetic energyKassociated with the mo-
tion of a particle of mass mand speed v, where vis well below the
speed of light, is
(kinetic energy). (7-1)
Work WorkWis energy transferred to or from an object via a
force acting on the object. Energy transferred to the object is posi-
tive work, and from the object, negative work.
Work Done by a Constant Force The work done on a par-
ticle by a constant force during displacement is
(work, constant force), (7-7, 7-8)
in which fis the constant angle between the directions of and.
Only the component of that is along the displacement can do
work on the object. When two or more forces act on an object,
theirnet workis the sum of the individual works done by the
forces, which is also equal to the work that would be done on the
object by the net force of those forces.Work and Kinetic Energy For a particle, a change Kin the
kinetic energy equals the net work Wdone on the particle:
KKfKiW (work – kinetic energy theorem), (7-10)F
:
netd
:
F
: d:
F
:WFd cos F
:
d
:d
:
F
:K^12 mv^2Review & Summary
in which Kiis the initial kinetic energy of the particle and Kfis the ki-
netic energy after the work is done. Equation 7-10 rearranged gives us
KfKiW. (7-11)Work Done by the Gravitational Force The work Wg
done by the gravitational force on a particle-like object of mass
mas the object moves through a displacement is given by
Wgmgdcosf, (7-12)
in which fis the angle between and.Work Done in Lifting and Lowering an Object The work
Wadone by an applied force as a particle-like object is either lifted
or lowered is related to the work Wgdone by the gravitational
force and the change Kin the object’s kinetic energy by
KKfKiWaWg. (7-15)
IfKfKi, then Eq. 7-15 reduces to
WaWg, (7-16)
which tells us that the applied force transfers as much energy to the
object as the gravitational force transfers from it.d
:
F
:
gdF :
:
gforce is angled upward by 60 to the floor and has magni-
tude 4.0 N. The speed vof the box at a certain instant is 3.0 m/s.
What is the power due to each force acting on the box at that
instant, and what is the net power? Is the net power changing
at that instant?
KEY IDEA
We want an instantaneous power, not an average power
over a time period. Also, we know the box’s velocity (rather
than the work done on it).
F
:
2