Question

Is it correct to say that the radial-force must be proportional to m without doing any kind of experiment because f=ma ?

Answer 1

If by radial force you mean 'torque' (rotational force), different equations are used than F=m*a.

But if you mean conventional force (in Newtons), then yes. If you know two of the values (eg the force value in Newtons, or the value for mass in kg per second or the value for acceleration in m per second per second) then you do not need to perform an experiment. Two of the values are all you need.

Answer 2

I did not mean a torque. I was thinking about a simple thing like a ball at the end of a string how rotates around the other end of the string or sth. like that.

Answer 3

A ball at the end of a string is simply a weight. If it is static (if it is not moving), then the only acceleration applied to it will be gravity (9.81 m/s/s). But once you begin spinning the ball on the end of the string and try calculating all relevant forces, you are then into torque territory (because any rotational force...any spinning...is a torque). The correct SI units for torque are N.m.

Answer 4

But if I just want to calculate the radial-acceleration or the radial-force than I do not need a torque do I?