« ΠροηγούμενηΣυνέχεια »
lengths are 60 inches, and in the first and second 10 inches from weight to prop, the third being 10 inches from weight to power. First, 80 x 10
= 16 lbs. power.
50 Second, 80 x 10
= 13.33 lbs. power.
60 Third, 80 x 60
= 96 lbs. power. :
50 EXAMPLE 4-What power is necessary to raise a weight of 620 lbs. by a lever of the first order, 72 inches long, and the prop placed 12 inches from the weight?
72 12 = 60 inches to power. Then 620 x 12
= 124 lbs.
60 EXAMPLE 5.--A weight of 620 lbs. is to be lifted by a power of 124 lbs. applied to the end of a lever of the first order, 72 inches long; required at what distance from the weight the prop must be placed. 124 x 72
= 12 inches. 620+124 EXAMPLE 6.- A beam 20 feet long, and supported at both ends, bears a weight of 73 cwt. 4 feet 6 inches from one end; required the proportion of weight upon each support. 73 x 4.5
= 16.425 cwt. on the farthest support.
20 And 73 x 15.5
= 56.675 cwt, on the nearest support. 20 EXAMPLE 7.-A weight of 300 lbs. is fixed on the end of a lever 6 feet long; required the power, applied 21 feet from the prop, to raise the weight. 300 x 6
= 720 lbs. power. 2.5
Wheel and Axle.
Here the velocity of the power is to the velocity of the weight as the circumference of the wheel is to the circumference of the axle; hence, divide the velocity of the power by the velocity of the weight, and the quotient is the weight that the power is equal to.
EXAMPLE 1.--A power equal to 30 lbs. is applied to the winch of a crane whose length is 15 inches; the pinion contains 10 teeth, the wheel 120, and the barrel is 9 inches diameter ; required the weight raised.
15 x 2 x 3.1416 = 94.248 circumference of
= 1200 lbs. raised by this crane.
150 = 15 = 10, that is, the velocity of the weight is diminished, while the velocity of the
then, 94.248 x 12 x 10 x 30
= 12000 lbs. raised, the
40 x 6
The Pulley. A single pulley that only turns on its axis, and does not move out of its place, serves only to change the direction of the power, but gives no mechanical advantage. The advantage gained is always as twice
power is the
the number of moveable pulleys, without taking any notice of the fixed pulleys necessary to compose the system of pulleys; hence, divide the weight to be raised by twice the number of moveable pulleys, and the quotient is the power required to raise the weight.
EXAMPLE 1.-What power is requisite to raise 250 lbs. with a pair of four-shieved blocks, the one block moveable and the other fixed ?
8 EXAMPLE 2.—What weight will a power of 120 lbs. raise, when applied to a three and four-shieved block, the three being moveable and the other fixed ? 3 x 2 = 6, and 120 x 6 = 720 lbs. raised.
The Inclined Plane. The advantage gained by the inclined plane is as great as its length exceeds its perpendicular height; hence, when the power acts parallel to the plane, the length of the plane is to the weight as the height of the plane is to the power.
EXAMPLE 1. Required the power capable of moving a weight of 300 lbs. up an inclined plane 50 feet long and 16 feet high.
As 50 : 16 :: 300 : 96 lbs. power. That is, 300 x 16 = 4800 = 50 = 96. Example 2.-A power of 120 lbs. with a velocity of 50 feet per minute, is to be applied to move a weight up an inclined plane at the rate of 30 feet per minute; the plane is 25 feet long and 8 feet high; required the weight that the power is equal to.
120 x 50 = 6000, and 30 x 8 = 240; then As 240: 25 :: 6000 : 625 lbs.
The Wedge. As the wedge is seldom used without being driven, the force of the blow is not easily ascertained; of course, in practice it is not worth taking into account, with respect to calculation.
The Screw. The advantage gained by the screw is as much as the circumference of a circle, described by the lever or handle, exceeds the interval or distance between the spirals of the screw; hence, as the circumference of the circle described by the handle is to the pitch of the screw, so is the weight to the power.
ExamPLE.—What power is necessary to raise a weight of 6000 lbs., the length of the lever being 20 inches, and the screw $ pitch.
20 x 2 = 40 x 3.1416 = 125.6 inches, then,
As 125.6 : .75 :: 6000 : 35.8 lbs. power required. N.B.—There are few machines but what, on account of the friction of the parts against one another, will require a third part more power to work them, when loaded, than is requisite to constitute a balance between power and weight. The following Table shows the estimated power of man or horse as applied to machinery.
at the rate of 220 feet
Libs, Avr. at the rate
of 1 foot per minute.
A man is supposed to be capable of lifting or carrying
27.273 or 6000 A man is supposed to be capable of turning the winch of a crane with a force equal to 28.637 or 6300
When the united efforts of two men are applied to the winch of a crane, the handles being at right angles, each man exerts a force equal to
33.499 or 7350 A man is supposed to exert a power in pumping equal to
17.335 or 3814 In ringing, a man exerts a force equal to
38.955 or 8570 And in rowing..
40.955 or 9010 The power of a horse, equal 150
OF FALLING BODIES.
In bodies falling freely by their own weight, their velocities are as the times, and the spaces, as the square
of the times; therefore, if the times be as the numbers, ....
1, 2, 3, 4, &c., The velocities will be also
1, 2, 3, 4, &c., The spaces passed through
1, 4, 9, 16, &c. And the spaces for each time, as the odd numbers,
1, 3, 5, 7, &c. It has been ascertained by experiment that a body falling freely from rest, will descend through 161 feet in the first second of time, and will then have acquired a velocity which being continued uniformly, will carry it through 32 32 feet in the next second, consequently, if the first series of numbers be expressed in seconds.
1" 21 3. &c. Velocities in feet will be......321 64,4 964 &c. Spaces in the whole times....161 644 144] &c. And the spaces for each second 161 487
To find the Velocity a falling Body will acquire in
any given time. Rule.-Multiply the time in seconds by 32.166, and the product will be the velocity acquired in feet EXAMPLE.—Required the velocity in 7 seconds.
32.166 x 7 = 225.162 feet, velocity acquired. To find the Velocity a Body will acquire by falling
from any given height. Rule.-Multiply the space in feet by 64.33, and the square root of the product will be the velocity acquired in feet per second.