Power and Work in Physics
6
.
7
P
o
w
e
r
The idea of
power
incorporates both the concepts of work and time.
Power is work done per unit time.
Average power, P is the average rate at which work
W
is done, and it
is obtained by dividing
W
by the time t required to perform the work:
Power is a scalar. Unit of power: ft.lb/s or watt = W = J/s (SI unit)
Horsepower (hp): 1hp = 550 ft.lb/s or 1hp = 745.7 W
Human Metabolic Rates
Force
versus
Distance
Graph
Work = Area under the Force versus Displacement graph
Work Done by a Variable Force
The work done by a variable force in moving an object is equal to the
area under the graph of F cos
versus s.
W
o
r
k
a
n
d
t
h
e
C
o
m
p
o
u
n
d
B
o
w
Find the
work
that the archer must do in
drawing back the string of the compound bow
in Figure
6.22
from 0 to 0.500 m.
P
r
o
b
l
e
m
7
1
The drawing shows the force-versus-displacement graph for
two different bows. These graphs give the force that an archer
must apply to draw the bowstring. (a) For which bow is more
work required to draw the bow fully from S = 0 to S = 0.5m?
Give your reasoning. (b) Estimate the additional work required
for the bow identified in part (a) compared to the other bow.
Power in physics is defined as the rate at which work is done, incorporating concepts of both work and time. This involves calculating average power, unit of power, human metabolic rates for various activities, force versus distance graphs, work done by variable force, and practical examples like the compound bow. Explore the relationship between force, work, and power through graphical representations and calculations.
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Presentation Transcript
6.7 Power The idea of power incorporates both the concepts of work and time. Power is work done per unit time. Average power, P is the average rate at which work W is done, and it is obtained by dividing W by the time t required to perform the work: Power is a scalar. Unit of power: ft.lb/s or watt = W = J/s (SI unit) Horsepower (hp): 1hp = 550 ft.lb/s or 1hp = 745.7 W
Human Metabolic Rates Activity Rate (watts) Running (15 km/h) 1340 W Skiing 1050 W Biking 530 W Walking (5 km/h) 280 W Sleeping 77 W
Force versus Distance Graph Work = Area under the Force versus Displacement graph
Work Done by a Variable Force The work done by a variable force in moving an object is equal to the area under the graph of F cos versus s.
Work and the Compound Bow Find the work that the archer must do in drawing back the string of the compound bow in Figure 6.22 from 0 to 0.500 m.
Problem 71 The drawing shows the force-versus-displacement graph for two different bows. These graphs give the force that an archer must apply to draw the bowstring. (a) For which bow is more work required to draw the bow fully from S = 0 to S = 0.5m? Give your reasoning. (b) Estimate the additional work required for the bow identified in part (a) compared to the other bow.
