Physics question

@James2580,

If you don't want to go through the calculation formula, there are some online converters like this one,
http://www.airgunhome.com/pages/calculators.html

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@James2580,

James2580 wrote:

a hand gun fires a 7.50 g bullet at a speed of 377 m/s.
a. Calculate the bullet’s kinetic energy.

E=[1/2]*Mass*[VEL^2] in comparable units IE to get joules convert 7.5 grams to 7.5*10^-1000 kilograms

1 Reply

@BillRM,

LOL would anyone like to confess to being such a fan of myself that they had taken the time to mark down a posting of mine dealing with a simple physics problem?

The world seems full of very very strange people indeed.

1 Reply

@BillRM,

I didn't downthumb your answer. But, it is incorrect.

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@maxdancona,

maxdancona wrote:

I didn't downthumb your answer. But, it is incorrect.

How is it incorrect? As the formula for kinetic energy is well known as E=1/2 *MASS*VEL^2 and even those it been fifty years since my days in a college physic class I am 99.99 percent sure it is the correct formula.

The mass of the bullet was given in grams and either the vel needed to be change from meters per second to cms per second to get the energy unit of erg or the mass needed to be change from grams to kilograms to get the energy unit of joules.

In any way please feel free to show me my error or errors.

Thanks...........

Footnote we once loss a very costly Mars probe due to a confusion of units as the builders of the probe hardware used the English units for mass and vels and the navigational programmers used the meteic units.

Quote:

http://articles.latimes.com/1999/oct/01/news/mn-17288

NASA lost its $125-million Mars Climate Orbiter because spacecraft engineers failed to convert from English to metric measurements when exchanging vital data before the craft was launched, space agency officials said Thursday.

A navigation team at the Jet Propulsion Laboratory used the metric system of millimeters and meters in its calculations, while Lockheed Martin Astronautics in Denver, which designed and built the spacecraft, provided crucial acceleration data in the English system of inches, feet and pounds.

As a result, JPL engineers mistook acceleration readings measured in English units of pound-seconds for a metric measure of force called newton-seconds.

In a sense, the spacecraft was lost in translation.

"That is so dumb," said John Logsdon, director of George Washington University's space policy institute. "There seems to have emerged over the past couple of years a systematic problem in the space community of insufficient attention to detail."

1 Reply

@BillRM,

Your conversion was wrong. 7.5*10^-1000 is a very small number.

1 Reply

@maxdancona,

maxdancona wrote:

Your conversion was wrong. 7.5*10^-1000 is a very small number.

There is a thousand grams in a kilogram as the name imply so one kilogram have 10^1000 grams or one gram is 1*10^-1000 kilogram and 7.5 grams is 7.5*10^-1000.

To put it in another way one kilogram is equal to 2.2 pounds an one gram is .035 ounce an one pound contain 16 ozs.

Note in the english system pounds are not a unit of mass but of force so it can get confusing but for everyday use 2.2 pounds is roughly one kilograms.

oK you are right as my used of scientific notation is too long in the past so one gram is 10^-3 kilograms.

Everything rust away after 50 years of not using it more then a few times.

1 Reply

@BillRM,

The difference between mass and force is very important in the equation for kinetic energy. In the equation $https://wikimedia.org/api/rest_v1/media/math/render/svg/82c6f8ceda1650271df82f27287811c32c629a68$ the "m" is for mass. You can not use a force for this value of the equation or the units won't work out (and any Physics student is taught how important units are in their first high school Physics course).

In the imperial system (i.e. pounds and feet), the unit of mass is called a "slug". If you were to do this problem in the Imperial System of units, you would calculate the quantity of slugs (it is not correct to call this a "conversion").

1 Reply

@maxdancona,

maxdancona wrote:

The difference between mass and force is very important in the equation for kinetic energy. In the equation $https://wikimedia.org/api/rest_v1/media/math/render/svg/82c6f8ceda1650271df82f27287811c32c629a68$ the "m" is for mass. You can not use a force for this value of the equation or the units won't work out (and any Physics student is taught how important units are in their first high school Physics course).

In the imperial system (i.e. pounds and feet), the unit of mass is called a "slug". If you were to do this problem in the Imperial System of units, you would calculate the quantity of slugs (it is not correct to call this a "conversion").

True however how many readers here would had even hear of the concept of a slug nor when comparing a kilogram to pounds in every day terms and not in physic equations does that concept seem to be needed.

Pounds are units of force not mass and the same as newtons in the metric system.

I still frankly have a hard time thinking in anything but the english system for everyday use such as needing to convert 40 C to fahrenheit to know if I would be hot or not or when I walked the length of my community that happen to be one kilometer I still think of it as walking .62 miles.

Too bad Franklin could not had talk the federal government into going to the metric system so that would now be our native language for measurements.

1 Reply

@BillRM,

The equation for Kinetic Energy using Force is completely different.

Ek = F * d

- If you had a 20N force applied over 3 meters, it would result in 30J of energy.
- If you had a 30lb force applied over 9 feet, it would result in 270 foot-pounds of energy.

It is not a conversion. They are completely different concepts with different equations. The only reason that students haven't heard about slugs is because science in the US is taught using the metric system.

Repeat after me... you can not convert Kilograms into Pounds (any more than you can convert hours into dollars).

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@maxdancona,

Oops... that first example should read;

- If you had a 20N force applied over 3 meters, it would result in 60J of energy.

0 Replies

@maxdancona,

maxdancona wrote:

The equation for Kinetic Energy using Force is completely different.

Ek = F * d

- If you had a 20N force applied over 3 meters, it would result in 30J of energy.
- If you had a 30lb force applied over 9 feet, it would result in 270 foot-pounds of energy.

It is not a conversion. They are completely different concepts with different equations. The only reason that students haven't heard about slugs is because science in the US is taught using the metric system.

Repeat after me... you can not convert Kilograms into Pounds any more than you can convert hours into dollars.

Unless you are doing physic equations you can indeed converted kilograms to pounds on the surface of the earth at least for everyday use.

If you placed a object of kilogram mass on one end of a balance scale and another object of 2.2 pounds weight on the other end of a balance scale they would balance.

Quote:

https://www.merriam-webster.com/dictionary/pounds?utm_campaign=sd&utm_medium=serp&utm_source=jsonld

plural pounds also pound
1 : any of various units of mass and weight; specifically : a unit now in general use among English-speaking peoples equal to 16 avoirdupois ounces or 7000 grains or 0.4536 kilogram — see weights and measures table

1 Reply

@BillRM,

Quote:

Unless you are doing physic equations you can indeed converted kilograms to pounds on the surface of the earth at least for everyday use.

We are doing physics equations.

Quote:

If you placed a object of kilogram mass on one end of a balance scale and another object of 2.2 pounds weight on the other end of a balance scale they would balance.

This is not strictly true (depending on the size, location and accuracy of the balance). It is almost always true in everyday circumstances, but it is not strictly true... and worse this claim muddies the important point.

But again we are doing Physics equations and the distinction between mass and force is quite important to any student of Physics.

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@maxdancona,

Give me a break weight is just the force that a gravity field subject a given object of a given mass to.

An object of a kilogram mass on earth is subject to a force of 2.2 pounds toward the center of the earth an in everyday use on earth a mass of a kilogram can be consider the same as a 2.2 pounds object.

An object weighting in at 2.2 pound in the earth surface gravity field had a mass of one kilogram or .o68 slugs

No one as far as I know used the unit of the slug or newton for that matter to sell material but instead used the kilogram or the pound and freely convert from one to the other.

Until we begin to sell things on the moon or Mars you can indeed just convert from kilograms to pounds and back again.

1 Reply

@BillRM,

Bill. This is a Physics thread. It is called "Physics Question" and the OP is asking a question about a Physics equation.

If you are buying tomatoes, then your "conversion" of mass to weight is perfectly fine. This would be an example of every day use. You don't need to understand Physics to buy tomatoes (which is the point, I suppose).

If you are working with Physics equations, such as calculating the value of Kinetic Energy, then interchanging mass with force is dead wrong. They are not the same at all.

This is a Physics thread.

1 Reply

@maxdancona,

When you are facing a physic problem the easier way to deal with it is to used the metric system not the english system and then it needed to have the results in the english system doing the conversion afterward.

If you need the results of kinetic energy in foot/pounds you find the energy in joules and convert and not deal with units such as slugs for example.

Ie one joules is equal to 0.737562 footpounds.

Not solving the problem by using feet per second and slug and so on.

How many more generations of Americans are we going to cripple by having them thinking in the English system?

Hell as far as physic thinking in the english systems is concern most of us do not think correctly and my guess is not one in a thousand know what the hell is the slug unit of mass happen to be.

1 Reply

@BillRM,

The vast majority of adults don't understand the difference between mass and weight... and in your ordinary life of not solving physics problems, there is really no need. I can take 3 minutes asking questions about falling elevators and satelites to see whether someone understands the abstract question. Outside of fellow science nerds and engineers, very few people I have talk to have any understanding (or real interest) in this difference.

I bet that the same lack of understanding of the Physics concepts exist in Europe (where they use the metric system) as in America. The issue is that these are abstract concepts that are outside our normal everyday existence. The concepts matter. The system of units we use to express time don't matter.

Can anyone in Europe tell me if Newtons are used in everyday life (outside of a scientific context)?

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