A particle is subject to a force F
x
that varies with
position as shown in Figure P7.15. Find the work done
by the force on the particle as it moves (a) from x = 0 to
x = 5.00 m, (b) from x = 5.00 m to x = 10.0 m, and (c) from
x = 10.0 m to x = 15.0 m. (d) What is the total work done
by the force over the distance x = 0 to x = 15.0 m?
Solution
Conceptualize: A few newtons, always pulling forward, over several meters does work between 10 J and
100 J.
Categorize: We use the graphical representation of the def inition of work. W equals the
area under the force-displacement curve.
Analyze: This def inition is still written W = ∫F
x
dx but it is computed geometrically by
identifying triangles and rectangles on the graph.
(a) For the region 0 ≤ x ≤ 5.00 m W==(( .
3.00 N) 5.00 m)
2
J 750 ■
(b) For the region 5.00 m ≤ x ≤ 10.0 mW = (3.00 N)(5.00 m) = 15.0 J ■
(c) For the region 10.0 m ≤ x ≤ 15.0 m W==(3.00 N) 5.00 m)
2
(
. 750 J ■
(d) For the region 0 ≤ x ≤ 15.0 m W = (7.50 J + 7.50 J + 15.0 J) = 30.0 J ■
Finalize: Make sure you know how to fi nd the area of a triangle. The particle on which this
force acts will have good reason to speed up. Without knowing its mass or original speed we
know its change in kinetic energy is 30 J if this force is the only force doing work on it.