Science Forums

Real Equation Of Force F = Ma Not Dp/dt

Recommended Posts

This is an extract of "Appendix A" in my manuscript on a new theory which can be accessed through the link in my profile if someone would be interested.

The real Equation of Force is F = ma

Today's Physics is stating that the Equation of Force is F = dp/dt.
We will analyze the equation of motion of rockets to see that the real Equation of Force is:

F = ma

A rocket has variable mass in its trajectory and it's important to see its motion equation.
Let m be its variable mass at any instant in its movement composed by the mass of the rocket plus the mass of its contained fuel.
I have made a search in the internet about rocket motion equations and all the sites agree in the equation:
F = m(dv/dt) = –ve(dm/dt) where ve is the speed of the fuel expelled relative to the rocket.

One web site:
http://www.braeunig.us/space/propuls.htm

They all agree that the force acting on the rocket is due to the expelled mass and is F = –ve(dm/dt) and that the equation of motion is F = m(dv/dt) = ma.
I assume the equation have been completely verified experimentally with enough precision from a long time ago.
It is evident that it is used the equation:
F = ma
for the force and not:
F = dp/dt

... ... …

This indicates that today's Physics is wrong stating the Equation of Force as F = dp/dt.

The right equation for force is F = ma even when mass varies.

Note that the natural derivation of the famous equation E = mc2 by Relativity Theory has no sense since it is based in the wrong relation F = dp/dt (http://www.emc2-explained.info/Emc2/Deriving.htm#.XVBNlvZFyM8)
Relativity Theory becomes a wrong theory since it is based on a wrong law.

Edited by martillo
Share on other sites

It must be noted that by definition:

p = mv

Then:

dp/dt = mdv/dt + vdm/dt = ma + vdm/dt

While F = ma (demonstrated by rockets' dynamics)

dp/dt = F + vdm/dt

It follows that F = dp/dt only if dm/dt = 0 meaning constant mass only

Share on other sites

F = ma, hum so what happens when acceleration changes F/a, does your mass suddenly change or perhaps are you accelerating an inertia. Your hypothesis is wrong, very wrong !!.

Share on other sites
• 2 weeks later...

This is an extract of "Appendix A" in my manuscript on a new theory which can be accessed through the link in my profile if someone would be interested.

The real Equation of Force is F = ma

Today's Physics is stating that the Equation of Force is F = dp/dt.

We will analyze the equation of motion of rockets to see that the real Equation of Force is:

F = ma

A rocket has variable mass in its trajectory and it's important to see its motion equation.

Let m be its variable mass at any instant in its movement composed by the mass of the rocket plus the mass of its contained fuel.

I have made a search in the internet about rocket motion equations and all the sites agree in the equation:

F = m(dv/dt) = –ve(dm/dt) where ve is the speed of the fuel expelled relative to the rocket.

One web site:

http://www.braeunig.us/space/propuls.htm

They all agree that the force acting on the rocket is due to the expelled mass and is F = –ve(dm/dt) and that the equation of motion is F = m(dv/dt) = ma.

I assume the equation have been completely verified experimentally with enough precision from a long time ago.

It is evident that it is used the equation:

F = ma

for the force and not:

F = dp/dt

... ... …

This indicates that today's Physics is wrong stating the Equation of Force as F = dp/dt.

The right equation for force is F = ma even when mass varies.

Note that the natural derivation of the famous equation E = mc2 by Relativity Theory has no sense since it is based in the wrong relation F = dp/dt (http://www.emc2-explained.info/Emc2/Deriving.htm#.XVBNlvZFyM8)

Relativity Theory becomes a wrong theory since it is based on a wrong law.

Why not use $\frac{dp}{dt}$? I've used the same relativistic description for force a number of times, most widely time I used it was for the new luminosity calculations of moving black holes.

Share on other sites
• 7 months later...
Dear friends, you must repeat elementary mathematics and physics because:

F = m a = m dv/dt = d(mv)/dt = dp/dt,  so

F = m a,  and  F = dp/dt   are one and the same equation !!!
Share on other sites

Dear friends, you must repeat elementary mathematics and physics because:
F = m a = m dv/dt = d(mv)/dt = dp/dt,  so
F = m a,  and  F = dp/dt   are one and the same equation !!!

Not always. In special relativity, p = γmv, where γ = 1/√(1 - (v²/c²) ).

But F = dp/dt is always true, whether in Newtonian or relativistic situations.

So, dear friend, it is you that needs to learn a bit more.

Share on other sites
Dear friend, The basic principle of Relativity is that in the Universe, the same laws of physics always apply, no matter what inertial system we observe! Therefore, there are no "relativistic" or "Newtonian" situations in the Universe. The Special theory of relativity is much simpler than you think! If you, for (y), have "relativistically" corrected the moment in the numerator then for the same amount (y) you had to correct the time in the denominator, and again it will be:

F = m a = m dv/dt = d(mv)/dt = ydp/ydt = dp/dt

So don't waste your precious time on charlatan theories of "science popularizers" and/or fake "physics professors"! Life is short! Be healthy!

Share on other sites

Dear friends, you must repeat elementary mathematics and physics because:
F = m a = m dv/dt = d(mv)/dt = dp/dt,  so
F = m a,  and  F = dp/dt   are one and the same equation !!!

No.

d(mv)/dt = dm/dt v + mdv/dt if you assume that dm/dt=0 then they are the same, but otherwise they aren't the same.

Share on other sites
Dear Alon,

You would certainly be right if this was a post about math and the derivation of the product of two independent functions! But this is a post about Physics! In Physics, mass and velocity are not two independent functions, but the opposite! Mass and velocity are the parameters (variables) of one unique function, which we call Moment! That's why the Force is just a simple derivation of Moment by Time!

But suppose your derivation is "physically correct"? So what in Physics would be the first member of your equation and what would you call it? And how much would that member be, in most cases in Physics, which is m = constant? (well, you've already answered that!)

But again, if we obey Newton and his "Law of Force" which states that each "force of action" is opposed by a symmetric "reaction force" (Fa = Fr), then it means that each Force, indeed, has two identical members F = Fa + Fr , so due to Fr = Fa, the Force is F = 2Fa! Maybe this is this "one and the same formula" that we are discussing here:

F = 2 m a = 2 m d2x/dt2 = 2 m dv/dt = 2 d(mv)/dt = 2 dp/dt   !!!

Alon, thank you so much for your interesting remark! Maybe someone learns something from this!
Share on other sites
• 2 weeks later...

Not always. In special relativity, p = γmv, where γ = 1/√(1 - (v²/c²) ).

But F = dp/dt is always true, whether in Newtonian or relativistic situations.

So, dear friend, it is you that needs to learn a bit more.

This is true.

Share on other sites
• 1 month later...

No.

d(mv)/dt = dm/dt v + mdv/dt if you assume that dm/dt=0 then they are the same, but otherwise they aren't the same.

Seems you got the point…

Share on other sites
Posted (edited)

I took a look at my profile…

I wonder how someone with just 152 posts can get the bad negative -1831 likes!

Amazing!!!

What else could be expected from the mainstream to someone proposing, not a new theory, but a totally new Physics debunking Relativity, Quantum Physics and even the Standard Model of basic particles at once, isn't it?

I understand…  :cool:

Edited by martillo
Share on other sites

F = ma, hum so what happens when acceleration changes F/a, does your mass suddenly change or perhaps are you accelerating an inertia. Your hypothesis is wrong, very wrong !!.

Doesn't it, Flummoxed?  Doesn't acceleration make a mass lighter?  Or does it only seem to make it lighter?  Once it gets going, it goes with  less energy behind it.  Hmmm???

Share on other sites

Doesn't it, Flummoxed?  Doesn't acceleration make a mass lighter?  Or does it only seem to make it lighter?  Once it gets going, it goes with  less energy behind it.  Hmmm???

Er yes Mass is related to acceleration and gravity. However it is the inertia of the object that resists the change in movement. Inertia is proportional to mass when accelerated. In space with no gravity, it is the inertia of the object that requires force to change its movement, not the objects mass. A mass only becomes apparent when an object is accelerated, and as the acceleration increases the apparent mass increases, the inertia remains the same.

Share on other sites

Er yes Mass is related to acceleration and gravity. However it is the inertia of the object that resists the change in movement. Inertia is proportional to mass when accelerated. In space with no gravity, it is the inertia of the object that requires force to change its movement, not the objects mass. A mass only becomes apparent when an object is accelerated, and as the acceleration increases the apparent mass increases, the inertia remains the same.

All right.  I should not have jumped in here when I did but that underscored part (my underscoring) is confusing me.  I'll back out and think on it.  Sorry for the interruption.  Carry on.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.