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Re: [Phys-l] force or mass



A 2000 lb car at rest accelerates to 100 ft/s in 10 seconds. What was the average force on the car?

F = ma F = 2000lb*10 ft/s^2 = 20,000 lb ft/s^2 Is that what you want as an answer--rather than 625 pounds?

And please, how do I interpret FOOT-POUNDS in all various situations in which it occurs?

OK--the disclaimer is 'the _approximate_ weight of 1 kg (near the surface of the earth) is 2.2 lbs. The point being (in our usage, shared by many), pounds is a force (weight) unit while kilograms is a mass unit. BTW--what about the common usage of kilogram for weight. Are we now going to let that pass too--forget the Newton? What I really want to avoid (especially in gen-ed classes) is lb_sub_f and lb_sub_m!

Sorry, your appeal to authority (ignoring the engineers who use slugs and pounds, I guess) doesn't convince me to change here--only group I have that might be affected are prospective engineers anyway. Only thing I will do, is make the disclaimer that in some circles, the pound is considered a mass, but the common usage of the term most usually refers to a force which is how we (and the engineers) will continue to use it! ;-)


----- Original Message ----- From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Thursday, November 06, 2008 11:32 AM
Subject: Re: [Phys-l] force or mass


On 11/06/2008 06:04 AM, Rick Tarara wrote:
I'll 'stick to my guns' too. Clearly some groups use the pound as mass,
other groups (engineers and intro physics textbook writers) take it as
force. I will continue to go with the engineers and intro books. Students
come to us with the clear knowledge that WEIGHT is measured in pounds.
....

If you want to persuade anybody, you'll need a better
argument (see below). Students "come to us" with innumerable
misconceptions, and it would be very bad policy to
redefine basic physics to match students' naive beliefs.

....
They 'know' pounds to be weight!

Yeah, but:

*) They 'know' that _elastic_ is soft and squishy.

*) They 'know' that red means hot and blue means cold.

*) They 'know' that heating something raises its temperature.

*) They 'know' that the bubbles in boiling water are air bubbles.

*) Etc. etc. etc. etc. etc.

===========

We agree that students "come to us" with misconceptions, and
we must take that into account ... but taking it into account
does not mean letting the inmates run the asylum.

We take each misconception as a starting point, and then move
away from it as quickly as possible.

weight of 1 kilogram is 2.2 lbs,

That is going from bad to worse. Even if/when we use lbf as
a unit of force, it is a unit of _force_ not a unit of "weight".
The weight of a kilogram will be dramatically unequal to 2.2 lbf
on the surface of the moon, or in the weightless environment of
a space station.