Question about calculating force...

[Guest Tyson_626]

Is there any environment or situation where it is possible to calculate force from ONLY acceleration?

 

I know Force = Mass x Acceleration, but what if you only have any acceleration and deceleration?

[bloodhound]

u answered urself f= mass times acceleration, so if u dont know the mass, u dont know the force applied on the particle. corrent me if i am wrong

[Guest Tyson_626]

You're right of course...hmm...I was just wondering if there was any situation where acceleration and deceleration alone would determine force, but it doesn't make sense that there would be.

 

What can you determine from just acceleration?

[Martin]

I believe that I can calculate the force between two stars

which form a binary pair.

 

to make it simple suppose i observe that both stars have the same constant speed v

(they are in circular orbit)

 

 

I should be able to calculate the force of attraction between them

only knowing v. How about this?

 

4v⁴/G

 

[math] F = \frac{4v^4}{G}[/math]

 

should we check it?

 

anyway Tyson asks if there is some situation where one can calculate force from acceleration alone. And I believe the answer is yes. here is a case where one can calculate force from speed alone

 

of course mass is part of the picture, but the actual calculation never

bothers to extract it

 

I need to check this, might be making a dumb error. and I have to

go right now. but will come back and check and correct mistakes if necessary

 

------------------------

 

I'm back let's check it

 

you are observing a binary star system, which is stationary wrt you, it isnt going anywhere, and you see the two stars have constant velocity, for simplicity it is the same velocity v for each star

 

Let's say the velocity is 10,000 meters per second

 

I claim that you can calculate the force between the two stars by raising that to the fourth power, dividing by G, and multiplying by 4----that will give the force in Newtons.

 

In metric units G is 6.67 E-11

so the newtons is going to be 4E16 divided by 6.67E-11

very roughly to order of magnitude E27 newtons.

 

I think that is right---you can calculate the force just from the speed in that situation

[timo]

I´d rather wonder where you use forces in modern/theoretical physics. Only part that uses forces that´d come to my mind right now would be Newtonian Physics which is a bit outdated (about 100 years).

About calculatong the force without knowing the mass: Yes, you could construct problems like "I know the momentum p(t) of my particle for all times t. What is the force working on it?" => F = d/dt p.

 

I wrote a paragraph about "F=ma is an axion in Newtonian Physics so it´s allways true while in this scope". But then I decided to look it up in my very good handbook of physics and it said:

2. Newtonian axiom: F = d/dt p (allready used that in the previous paragraph). If the mass is constant, then: F = d(mv)/dt = m * dv/dt = ma.

[albertlee]

You're right of course...hmm...I was just wondering if there was any situation where acceleration and deceleration alone would determine force' date=' but it doesn't make sense that there would be.

 

What can you determine from just acceleration?[/quote']

 

 

Tyson,on earth, gravitational acceleration is constant, like one thing, the varriable is the mass, which is like the one which changes the force.......

 

by this way, gravitational acceleration is always 10m/s^2, which means any mass of object drop at same height would fall at same speed and hit the ground at the same time, again constant acceleration, but different force.

[Martin]

Tyson' date='on earth, gravitational acceleration is constant, like one thing, the varriable is the mass, which is like the one which changes the force.......

 

by this way, gravitational acceleration is always 10m/s^2, which means any mass of object drop at same height would fall at same speed and hit the ground at the same time, again constant acceleration, but different force.[/quote']

 

hi albertlee,

you know Tyson much better than I do (as a newcomer) so you may

understand his question better

but I would be surprised if he were only interested in vertical force

 

on the earths surface one can have different horizontal accelerations and horizontal forces, like in physics class with the air track

 

so not everything is 10m/s^2

 

---------

 

hello Tyson,

 

its not a bad type of question to ask:

 

what can we calculate from observing accelerations and decelerations, even if we dont know mass?

 

can we calculate mass from observing something about motion----like e.g. acceleration?

 

if you are still around, and look at this thread, maybe you should keep thinking about it a bit more

[Guest Tyson_626]

I am really glad I found these forums. I am learning a lot! I am a bit worried about sounding stupid since my physics knowledge is a little lacking. I will think more about my questions and I really appreciate everyone's answers!

[albertlee]

Martin, I know there are all diferent accelerations....

 

but since Tyson was asking about any situation that acceleration can determine the force, so I tell that a situation that gravitational acceleration is a key constant component of force...

 

Albert