January 16, 2013

Force and Motion : Frequently Asked Questions (F.A.Qs)

1. What is motion?
Change of position.

2. What is velocity ?

Rate of motion ; it may be uniform or variable.

3. What is acceleration ?

Rate of change of velocity ; i. e,, the change of velocity
per unit of time.

4. What is force ?

Any cause that tends to produce any change of motion.

5. What is momentum?
Quantity of motion.

6. How is it measured?

By the product of the number of units of mass into the
number of units of velocity.

7. What is the unit of momentum called ?

It has no specific name. We may compare the momenta
of two moving bodies by the ratio between the two
measuring products as above explained. The momentum
of a body having a mass of 40 pounds and a velocity of
15 feet per second is twice as ereat as that of a body
having a mass of 10 pounds and a velocity of 30 feet per

8. What is the first law of motion ?

Every body continues in its state of rest or of uniform
motion, in a straight line^ unless compelled to change
that state by some external force.

9. From what does this law result ?
From the inertia of matter.

la What is centrifugal force?

So-called centrifagal force is simply a confasing name for
inertia, or the tendency of matter to obey the first law
of motion.

11. To what special act of obedience is the term applied?

When a body is compelled to move in a curve, it always
tends to pull away from the centre and to move in a
straight line, tangent to the cnrve.

12. Illustrate this tendency.

Mud flying from a carriage wheftl, or water from a grind-

13. Do the mud and the tvater^ after pulling away from their
circular paths^ move in straight lines?
They do not, because they are continually pulled there-
from by the force of gravity.

14. Give the second law of motion.

The effect of a force will be the same whether it acts alone
or jointly with others.

15. What name is given to the effect of two or more forces ^
acting jointly ?

Resultant motion, which will be different from the effect
of any one of the forces acting, and may be looked upon
as the result of a single force called the resultant force.

16. How is the resultant force determined?

By what is known as the composition of forces.

17. State one case of the composition of forces.

When the given forces act in the same direction, the re-
sultant equals their sum.

18. Give an illustration.

If a man rows a boat with a force that alone will produce
a velocity of 4 miles an hour, down a stream that has a
current of 3 miles an hour, the boat will move at the
rate of 7 miles an hour ; 4-1-3=7.

19. State another case under the composition of forces.

When the given forces act in opposite directions, the re-
sultant equals their difference, its direction will be that
of the greater force.

20. Give an illustralion,

Ifthe boat is rowed with the same force as before but
against the same current, it will move up stream at the
rate of one mile an hour ; 4 — 3=1.

21. State another case under the composition of forces.

When the given forces act at an angle with each other, the
resultant may be found by a process known as the
parallelogram of forces.

22. Give an illustration.

If the boat is rowed easterly with the same force as before,
and the same current is flowing northerly, these two
forces may be represented bv two lines, 4 inches and 3
inches long respectively, and meeting at a right angle.
Call the apex of this angle A. Consider these two lines
as two sides of a parallelogram, and draw the other two
sides. Draw a diagonal from A. By measurement,
or mathematically, we may find that this diagonal is 5
inches long. Its length and direction represent the
intensity and direction of the resultant force. The
boat will move in the direction thus indicated, and with
a velocity of 5 miles an hour.

23. What is the third laiv of motion f

Action and reaction are equal and opposite in direction.

24. Give an illustration.

When Columbus made the famous ^%% stand on end, the
action of the ^^'g may have made a dent in the table.
It is certain that the equal and opposite reaction of the
table broke the shell.

25. What is the law of reflected motion f

The angle of incidence equals the angle of reflection.

26. What is the angle of incidence?

The angle included between the path of the moving body
before reflection, and a line drawn perpendicular to the
int of reflection. reflecting surface at the point k

27. What is the angle of reflection f

The angle included between the path of the moving body
after reflection and the perpendicular drawn as above

28. What very common error in this respect?

To think that these angles are included between the two
paths specified and the reflecting surface, instead of be-
tween the paths and the perpendicular to that surface.

29. How are forces measured?

By comparison with some standard called a unit of force.
There are two kinds of units of force, the gravity unit
and the absolute unit

3a What is the gravity unit of force?

It is the weight of any standard unit of mass, as the gram,
kilogram, or pound. When a force may be balanced by
a weight of 100 pounds, we call it a force of 100 pounds.
If a frictionless horizontal piston at the top of a steam
boiler must be loaded at the rate of 100 pounds to the
square inch to keep it in place against the force of the
confined steam, we say that there is a steam pressure of
100 pounds to the square inch.

31. What is the absolute unit of force ?

It is the force that, acting for unit of time upon unit of
mass, will produce unit of acceleration. There are two
such units m common use ; the poundal and the dyne.

32. What is the poundal ?

It is the force that, when applied for one second to one
pound of maiter, produces an acceleration of one foot
per second. It is called the F. P. S. (foot-pound-second)
unit of force.

33. What is the dyne ?

It is the force that, when applied for one second to one
gram of matter, produces an acceleration of one centi-
meter per second. It is called the C. G. S. (centimeter-
gram-second) unit of force.

34. What is the numerical relation between gravity units and ab-

solute units of force ?
At the sea-level at New York City, the force of gravity
gives to a falling (freely moving) body that weighs one

pound (or any other weight) an acceleration of 32.16
Feet; consequently, at New York, a force of one pound
equals 32.16 poundals. The same force produces an
acceleration of 980 centimeters; consequently, at New
York, a force of one gram equals 980 dynes ; a force of
one kilogram equals 980,000 dynes.

35. How may the acceleration be determined?

By dividing the total velocity that the force has produced
by the number of seconds that the force has acted.

36. How is a force measured in absolute units?

By multiplying the number of units of mass moved by the
number that represents the acceleration produced. For
poundals, the units used must be feet, pounds, and
seconds (F. P. S.); for dynes, the units used must be
centimeters, grams, and seconds (C. G. S.).

37. What force is necessary to give a body weighing jo grams
a velocity of 50 centimeters per second ^ by acting upon the
body for two seconds f
30X50-5-2=750, the number of dynes.


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