momentum = force * time. This can't be right.

[h4tt3n]

momentum = mass * velocity

 

and since velocity = acceleration * time then

 

momentum = mass * acceleration * time

 

since force = mass * acceleration and therefore acceleration = force / mass then

 

momentum = mass * (force/mass) * time

 

mass cansels out and

 

momentum = force * time

[swansont]

Newton's second law, properly stated, is F = dP/dt

 

So, for a constant force, it is indeed correct

[fredrik]

> momentum = force * time. This can't be right.

 

The power of logical reasoning in a nutshell.

 

You deduce something still refuse to believe it.

 

Lesson learnt: Have faith in deductive power, it really works

 

/Fredrik

[swansont]

> momentum = force * time. This can't be right.

 

The power of logical reasoning in a nutshell.

 

You deduce something still refuse to believe it.

 

Lesson learnt: Have faith in deductive power, it really works

 

/Fredrik

 

Careful, though. d = v*t and v = a*t are both correct but d = at^2 is wrong

 

It's OK to question. Many a wrong, unphysical answer has been proposed because the person blindly believed the math, but made a mistake. Having an instinct for what a reasonable answer is is a good thing. What should have been included is why p = F*t seemed unreasonable.

[fredrik]

> Careful, though. d = v*t and v = a*t are both correct but d = at^2 is wrong

>

> It's OK to question. Many a wrong, unphysical answer has been proposed

> because the person blindly believed the math, but made a mistake. Having

 

Good point.

 

A somewhat misplaced comment from me I see now. I couldn't help beeing a bit amuzed by the title so I jumped

 

I was trying to encourage the faith and understanding in deduction since I think sometimes it can answer questions to which it is hard to have an instinct on. Usually, knowing how to deduce something is more fundamental than just knowing the answer. My comment didn't fit that awfully very well in this particular case considering the sort of overly simplified level of formalism.

 

/Fredrik

[h4tt3n]

Thats a very good discussion.

 

It just really bothers me that since P = m*v is such a fundamental equation, you can express it in a way (the mentioned P = F*t) that allows you to deduce neither mass nor velocity from any of the values given that they (m and v) are both unknown.

[Farsight]

What's the problem, h4tt3n? If you take this a little further, you can find something that tells you the energy of a photon is hf and its momentum is hf/c, so:

 

energy E = hf

 

momentum P = hf/c

 

To get from mass m to momentum P=mv you have to multiply by velocity, which is c. And to get from momentum P to energy E you have to multiply by velocity c again. So to get from mass to energy you have to multiply by c squared. Which gives you:

 

E=mc2

 

Great fun!

[h4tt3n]

Omg Farsight...

 

That is SO insightful!

 

</sarcasm>

 

Now we just need to improve the method to include objects that actually has mass, moves at speeds less than C and doesn't have a frequency!

[swansont]

Thats a very good discussion.

 

It just really bothers me that since P = m*v is such a fundamental equation, you can express it in a way (the mentioned P = F*t) that allows you to deduce neither mass nor velocity from any of the values given that they (m and v) are both unknown.

 

 

If m and v are unknown, you need more information anyway. That's not something restricted to this particular example. One equation with two unknowns does not have a unique solution.

[h4tt3n]

swansont,

 

Yes I understand that, thanks for your replies. Anyway, a situation where F on an object is known, but neither mass nor velocity of the same object, is pretty unlikely to happen. I was just wondering at dP = F*dt since it looks odd on paper.