|We are discussing Newtons and I was wondering how
you measure Newtons. How do you know when you are
pushing 40 N agianst someone. What makes a Newton
|Question Date: 2000-02-10|
The force of 1 newton equals the amount of force
exerted to move 0.2248089 of a pound, so, how many
newtons would equal the force used to move 2
Unfortunately, the currently posted answer number
1 to the Newton question doesn't really answer the
The answer is not correct,
because it is incomplete.The force of 1 Newton is
equivalent to about 0.22481 pounds of force.
There are several problems with this answer. 1)
The answer implies that a pound is a measure of
mass, which it is not. Mass, in the English
system, is measured in slugs, and in the
International System is measured in kilograms; 2)
The answer implies movement, but does not state
how far or how fast. When a given force (1
Newton) is applied to a given mass (1 Kilogram),
the result is an acceleration (1 meter per second
squared) of the mass. When the force is applied
for a given time (1 second) in a given direction,
at the end of that time the mass will have a
velocity (1 meter per second) in that direction,
relative to the initial velocity of the
The answer I posted (Answer #3) was
couched in the correct terms. If what I have
explained is not clear, please let me know, and
I'll try to do a better
How do you measure Newtons?
Newton is a standard unit of force.
you are really measuring is force, rather than
What makes a Newton a
If you are using the International
System of Units, you express the measure of force
in Newtons (named to honor Sir Isaac Newton). A
force of one Newton applied to a mass of one
kilogram will accelerate the mass one meter per
second per second. You may have guessed by now
that in the International System of Units
measures of length are expressed in meters,
measures of mass are expressed in kilograms, and
measures of time are expressed in
Let's get an idea of what we're
talking about by using a technique made famous by
Albert Einstein; a "thought
So, imagine that you are an
ice-hockey player, wearing skates and holding a
hockey stick. Also imagine that you have a
one-liter bottle of water (which is about one
kilogram in mass) standing at rest next to you on
the ice (like a big hockey puck). Imagine that you
slowly start skating while pushing gently on the
water bottle with the stick, always applying the
same force. If you constantly apply one Newton of
force, then after one second the bottle will be
traveling at one meter per second.
how fast that is... If you sit at a table, like I
am at this moment, and place your hands on the
edge much farther apart than your shoulders, that
will be about
one meter. Now slowly say "One
Thousand One" to approximate a second of time
while you move one hand from its starting position
to the other hand. Try that several times until
your hand, moving at a constant speed, just
arrives at the other hand as you finish saying
"One Thousand One". That is approximately one
meter per second.
Back on the ice, you have
been skating for one second, and the bottle is now
moving at one meter per second. If you continue
applying the same, gentle one Newton of force, at
the end of two seconds the bottle will be moving
at two meters per second.
How fast will the
bottle be moving after applying the same force for
How do you know when you
are pushing 40 N against someone?
to know is to measure the force with a calibrated
measuring device, like a bathroom scale.
"Calibrated" simply means that the scale has been
tested with different known forces, and the scale
has been marked for each of the known
Once the device has been calibrated, it
can be used to measure an unknown force.
this every day with your own hands and their sense
of pressure. You know the weight of certain items,
like a pound of butter, and you probably have a
good sense of how it feels to lift those
Now let's go back to our "thought
First, let's imagine doing the
water bottle experiment again, but this time you
apply 40 N to the bottle instead of 1 N. After one
second, how fast will the bottle be moving?
you stopped pushing after one second, about how
much of the length of a football field would the
bottle move every second? (A football field is
about 100 meters long.)
That's one way to get a
sense of 40 N of force, but I don't think I can
skate as fast as one half a football field in one
second. Can you skate that fast?
try another way of understanding 40 N of force.
Imagine putting 40 one-liter bottles of water into
a burlap sack, tying it up tight, and setting it
on the ice next to you. Now push with 40 N of
force for one second. After that one second the
bag should be traveling at one meter per second.
After doing that a few times, you will be
"calibrated" to feel 40 N of force.
Finally, let's use an "astronomical"
example involving you, Earth and the bathroom
scale I mentioned earlier. Find a bathroom scale
that measures your mass in kilograms, and see how
much mass you have, writing it down. Remember to
"kilogram" scale, not the "pound"
scale. Now multiply that mass by 9.8 meters per
second per second (Earth's gravitational
acceleration at the surface). The result is the
amount of force you apply to the scale in Newtons.
It is also the amount of force Earth's gravity
applies to you, keeping you here on it's surface!
So, how does it feel to your feet, to exert that
much force on Earth?
Can you think of
another question you can answer with a "thought
A Newton is the basic unit of force in the MKS
(meter-kilogram-second) unit system. If you
remember F=ma and the acceleration has units of
m/sec^2 you might guess (correctly) that a Newton
is 1 kg*m/sec^2. Or, it is the amount of force to
accelerate a 1 kg object to a velocity of 1 m/sec
in one second. The acceleration of gravity is 9.8
m/sec^2 so the effective force on a 1 kg object
due to gravity is 9.8 Newtons. (Note that for a 10
kg object the effective force is 98 Newtons since
gravity has the peculiar property that it is
directly proportional to rest mass.)
the way, in the CGS system (cm-gm-sec) the unit
is: gm*cm/sec^2 = 1 dyne which is 1/100,000 times
smaller than a Newton.
The relation between
acceleration and force gives the clue about how to
measure a force. Most instruments which measure
mass do so by measuring the effective force of the
object due to gravity. Hence, most mass-measuring
techniques actually measure force... A spring
balance will work as will a bathroom scale. The
trick is to fix the alibration so it reads in
Newtons -- not in pounds or kilograms. A spring
scale displaying 0.5 kg is actually responding to
a force of 4.9 Newtons--A bathroom scale measures
pounds --2.2 pounds per kilogram, it is simple
arithmetic to find the conversion.
to check this is the calculate the centripetal
acceleration of a wirling weight and use a spring
scale to measure the force -- these should
Hello, my name is Arnold Schwarzanegger and I have
decided to give up fame and fortune to become a
humble physics grad student.
your questions about measuring Newtons, I suggest
you visit your nearest Gold's Gym.
can hold up a wimpy 4 kg barbell then you are
pushing 40 N on the barbell. This is because
gravity pulls a thing downward with a force
proportional to its mass, specifically 10 N per
kg. This pushing against gravity is how you can
use physics to build a ripped muscular
Of course, pushing upwards may
pump up your pecs, but if you want to buff your
lats you must pull down, not up! But this is not
too hard a problem to fix by using a pulley to
change the direction of pull from up to down. You
can also use a pulley to change the direction of
pull from up to sideways to work a variety of
other muscle groups in your back and
But all this pulling will not help
you measure your sideways pushing capability. Can
you figure out how to use levers to convert your
horizontal pushing to vertical lifting of weights?
(hint: think leg-press!)
Another way of
measuring force is to use springs, because how
much you stretch or squeeze them is proportional
to how much force you exert. But who would want
to sit around sqeezing some sissy springs when you
could be pumping iron? Sure, you can tune in to a
late-night infomercials and see Chuck Norris
trying to sell you a home workout machine based on
springs, but look at how skinny his arms are!
Besides, no Texas Ranger is a match for the
Terminator or Conan the Barbarian.
this has a been a valuable lesson in the practical
application of physics to physique.
PS I'll be back.
A regular old bathroom scale measures Newtons.
Though the scale may say "pounds" on it, a Newton
is just a metric version of a pound, the way a
meter is a metric version of a foot. Both meters
and feet measure length, and both pounds and
Newtons measure force.
You might say that a
scale measures weight, but weight is just a
special kind of force - it is the force of gravity
pulling on you. You can measure if you are pushing
40 N against someone (or something) if you place a
bathroom scale against them and push against the
scale. As it happens, one pound of force is equal
to four-and-a-half Newtons. So if you push hard
enough to make the scale read 9 pounds, that is
about 40 Newtons.
Newton is the name of the unit of force in the
international system of units or SI.
measure a force, you would use an instrument
called dynamometer (from the Greek dynamos, that
means force, power and metron,
There are many types of
dynamometers, but the one most used in the general
physics lab is the spring dynamometer. A spring
deforms reversibly, and its deformation
(elongation or contraction) is directly
proportional to the force applied. If k is the
spring constant, then F (Newton) = k
(Newton/meter) * x (meter). So, if you pull from
the free end of the dynamometer and the spring
becomes 0.005 m longer (supposing that the
dynamometer's spring constant is 4000 N/m), then
the force applied would be F = 4000 N/m * 0.005 m
= 20 N.
But if you want a simpler way to
have an idea of what a Newton is, here is how.
When you use a scale, you can measure the mass of
a body. Any body on the surface of our planet is
subject to the acceleration of gravity,
abbreviated g and equal to 9.8 m/s^2. But as you
know from the second of Newton's laws, the force
exerted by the Earth on a body, the gravitational
force, equals g times the mass of the
F (Newton)= m (kg) * g (m/s^2) The
units in this expression are fine, because the by
definition 1 N = 1 kg m/s^2.
So, if you
take a mass of 1 kg, and lift it vertically with
your arm, then your arm will be pulling the mass
with a force of F = 1 kg * 9.8 m/s^2 = 9.8 N. If
you want to experience only 1 N, then you need to
lift a mass of 0.102 kg.
I hope this is
clear enough, but do not hesitate to ask again if
you still do not get it.
You are very familiar with one method of measuring
Newtons, you probably just didn't realize it.
Newtons are a unit of force, which is equal to
mass (the amount of material) times the
acceleration that the mass is subject to. When
you stand on a bathroom scale and (if it is a
metric scale) it says 50 kg, the scale may be
telling you your mass, but it has measured your
weight (Newtons) divided by the average
acceleration of gravity on the Earth's surface
(9.8 m/s^2) to get mass.
One way to
picture this is to imagine you are floating in a
swimming pool. Do you seem to weigh less? Has
your mass (the amount of stuff you are made of)
changed just because you jumped into the pool? If
not, what has changed? Now imagine you are in
space and there is no gravity at all. What do you
think you would you measure for your weight using
a bathroom scale?
By the way, as a side
note: Pounds are a unit of weight, not mass. A
unit called a "slug" is the unit of mass in our
funky english system of units. Isn't that
confusing? How come we don't talk about slugs
very much, but kilograms all the time?
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