To keep this within my ability to understand can we please live in the perfect world of the junior physics class i.e. uniform density, no friction g=10 etc - thanks

I understand that 'weight' is mass x g

So the weight of something is how hard the Earth is 'sucking' that's why things weight differently on Earth and the Moon

But the objects mass remains the same...

Now I'm arguing with somebody (he's an engineer and I'm a pen pusher so I'm guessing he'd be right) on another forum about things like energy and momentum so we are slinging p1v1=p2v2 and e=(mv^2)/2 at each other.

The other chap is telling me the weight of something needs to be converted from pounds weight to pounds force before we apply the equations...

I don't get this, I thought for example 1 litre of water weighed 1kg and also has 1kg of mass.

This came about because I challenged his use of the gravitational constant in an equation to calculate the energy and momentum of an object. When I asked why it was there I got the answer above.

I'm perfectly prepared to admit I'm wrong but can some body shed some light?

And as an aside - does the term weight really get used in physics? Or is it a term used to allow regular folk get a grip on something you guys would call potential energy or something LOL

Even though you "weighed" the potato, 500 grams is its mass (not its weight).

It's the first. Kinetic energy is defined as:
[tex] (1/2)m v^2[/tex]
where m is the mass, not the weight. (The weight of the potato would given by mg = (.5 kg)*(10 m/s^2) = 5 Newtons.)

If you use a balance scale, it will work just the same on the moon. (Since you are just comparing the weights of two masses, and weight and mass are proportional.)