Information for questions 2–55 and
o
2–56:
You want to raise an
1,800–pound motor 4 feet up to a
foundation.
You use two double–sheave
blocks rigged to give a mechanical
advantage of 4 and a windlass that has a
theoretical mechanical advantage of 6.
2–55.
Assuming 100 percent efficiency,
how much work is required to raise
the motor?
1.
1,800 ft-lb
2.
3,600 ft–lb
3.
7,200 ft–lb
4.
10,800 ft–lb
2–56.
Neglecting friction, how much pull
must you exert to raise the motor?
1.
18 lb
2.
36 lb
3.
75 lb
4.
300 lb
2-57.
An effect which friction has on the
mechanical advantage of any machine
is to make the
1.
theoretical mechanical
advantage less than the actual
mechanical advantage
2.
actual mechanical advantage
less than the theoretical
mechanical advantage
3.
actual mechanical advantage
less than one
4.
actual mechanical advantage
more than one
2–58.
Assume that the hammer of a pile
driver weighs 1,000 pounds. The
resistance of the earth is 6,000
pounds.
If the hammer drops 4 feet
to drive a pile, how far into the
earth will the pile be driven?
(Assume an efficiency of 100%.)
1.
2 in
2.
6 in
3.
8 in
4.
10 in
When answering questions 2–59
o
through 2–61, assume that a man
lifts a 600–pound load, using a block and
tackle with a theoretical mechanical
advantage of 6.
He does 6,500 foot–pounds
of work in lifting the load 8 feet.
2-59.
How much work does the man do in
overcoming friction?
1.
215 ft–lb
2.
813 ft–lb
3.
1,700 ft-lb
4.
5,900 ft–lb
2–60.
The total force exerted by the man
in lifting the load is
approximately
1.
35 lb
2.
135 lb
3.
215 lb
4.
406 lb
2–61.
The average amount of force which
the man exerted to overcome
friction is approximately
1.
35 lb
2.
215 lb
3.
237 lb
4.
406 lb
2–62.
The handle of a screwjack must move
through a circular distance of 600
inches to lift a load one inch. If
a force of 10 pounds is required to
lift a load of 1,500 pounds, what
is the efficiency of the jack?
1.
25%
2.
33%
3.
78%
4.
90%
2-63.
A block and tackle has a
theoretical mechanical advantage of
4 but requires a force of 50 pounds
to lift a 160–pound load.
The
efficiency of the block and tackle
is
1.
60%
2.
70%
3.
80%
4.
90%
19
