Page 69 - Schaum's Outline of Theory and Problems of Applied Physics

54 LAWS OF MOTION [CHAP. 5

Table 5-1 Mass and Weight in the SI and British Systems

System To find mass m To find weight w
of Units given weight w given mass m

w(N)
2
SI m(kg) = w(N) = [m (kg)](9.8 m/s )
9.8 m/s 2
w(lb)
2
British m(slugs) = w(lb) = [m(slugs)](32 ft/s )
32 ft/s 2
2
(a) w = mg = (50 slugs)(32 ft/s ) = 1600 lb
w 50 lb
(b) m = = = 1.56 slugs
g 32 ft/s 2

SOLVED PROBLEM 5.13
(a) What is the mass of a 160-lb man? (b) With what force is he attracted to the earth? (c) If he jumps
from a diving board, what will his downward acceleration be?
w 160 lb
(a) m = = = 5 slugs
g 32 ft/s 2
(b) The attractive force exerted on the man by the earth is his weight of 160 lb.
2
(c) His downward acceleration is the acceleration of gravity g = 32 ft/s .

SOLVED PROBLEM 5.14
A net force of 75 lb acts on a body of mass 25 slugs which is initially at rest. (a) Find its acceleration.
(b) How fast will the body be moving 12 s later?

F 75 lb
(a) a = = = 3 ft/s 2
m 25 slugs
2
(b) v = at = (3 ft/s )(12 s) = 36 ft/s

SOLVED PROBLEM 5.15
A net force of 150 lb acts on a body whose weight is 96 lb. What is its acceleration?
The mass of the body is

w 96 lb
m = = = 3 slugs
g 32 ft/s 2
Its acceleration is therefore
F 150 lb 2
a = = = 50 ft/s
m 3 slugs

SOLVED PROBLEM 5.16
How much force is needed to bring a 3200-lb car from rest to a velocity of 44 ft/s (30 mi/h) in 8 s?
The car’s mass is

w 3200 lb
m = = = 100 slugs
g 32 ft/s 2