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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 a Newton?
Question Date: 2000-02-10
Answer 1:

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 pounds?

Answer 2:

Unfortunately, the currently posted answer number 1 to the Newton question doesn't really answer the question.

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 mass.

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 job.

Answer 3:

How do you measure Newtons?

Actually, a Newton is a standard unit of force.
So, what you are really measuring is force, rather than Newtons.

What makes a Newton a Newton?

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 seconds.

Let's get an idea of what we're talking about by using a technique made famous by Albert Einstein; a "thought experiment".

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.

Imagine 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 three seconds?

How do you know when you are pushing 40 N against someone?

One way 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 forces.
Once the device has been calibrated, it can be used to measure an unknown force.
You do 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 items.

Now let's go back to our "thought experiment".

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?
If 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?

Now let's 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 use the
"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 experiment"?

Answer 4:

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.)

Bye 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.

One way to check this is the calculate the centripetal acceleration of a wirling weight and use a spring scale to measure the force -- these should agree.

Answer 5:

Hello, my name is Arnold Schwarzanegger and I have decided to give up fame and fortune to become a humble physics grad student.

To answer your questions about measuring Newtons, I suggest you visit your nearest Gold's Gym.

If you 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 physique.

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 arms!

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.

I hope this has a been a valuable lesson in the practical application of physics to physique.


PS I'll be back.

Answer 6:

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.

Answer 7:

Newton is the name of the unit of force in the international system of units or SI.

To measure a force, you would use an instrument called dynamometer (from the Greek dynamos, that means force, power and metron, measure).

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 body.

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.

Answer 8:

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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