Understanding F=ma: Constant Speed & Acceleration

[thetexan]

TL;DR Summary

Newton’s 1st law

I’ve been going back and reviewing my basic physics and I have a nagging question?

F=ma

aid acceleration in terms of units such as meters per seconds squared. for example, 20 meters per second per second. This indicates an increasing speed.

so,

let’s take an asteroid moving through space at a constant speed. hoe do I determine the kinetic force it has? Is it F=ma? If so a is measured as an increasing accelerating speed. But the asteroid has a constant speed that is not accelerating.

even weight, the vertically downward force is measured as a force with a as acceleration. I guess I don’t understand how a constant speed is used as an acceleration.

tex

 

[jrmichler]

A few points:
1) An asteroid moving through space at constant speed in a straight line, by definition, has zero force acting on it.
2) There is no such thing as "kinetic force".
3) A moving object has kinetic energy. Energy and force are two different things.
4) If a moving object is accelerating, there is an external force on it. That force is not "kinetic force".
5) (Repeat) There is no such thing as "kinetic force".

 

[Dale]

Summary:: Newton’s 1st law

an asteroid moving through space at a constant speed. hoe do I determine the kinetic force it has?

As @jrmichler said there is no such thing as kinetic force. An asteroid moving at constant velocity has 0 net force acting on it.

 

[jack action]

kinetic force

AS others have said, there is no such thing. But there is kinetic energy, which is equal to half the mass times the speed squared of the object (½mv²).

even weight, the vertically downward force is measured as a force with a as acceleration.

The weight is due to gravity where $F = G\frac{m_1 m_2}{r^2}$. Considering only that gravitational force, if the masses are free to move with respect to each other, say $m_1$ moves with respect to $m_2$, its acceleration can be found with:
$$m_1 a = G\frac{m_1 m_2}{r^2}$$
Or:
$$a = G\frac{m_2}{r^2}$$

 

[kuruman]

It is a misconception to believe that an object moving at constant velocity, such as your asteroid, needs a force to keep it going. That misconception arises naturally in humans. One observes that a book sitting on a table will not move if left alone. One also observes that if one's hand starts pushing, the book will start moving at constant velocity and stop if the hand is removed. All that is true but one's conclusion should not be "therefore, in order to move at constant velocity, the book needs an external force to act on it." I believe this external force is what you call the "kinetic force".

The truth is that if one performed the same experiment in gravity-free space, the book would keep on moving forever with the speed it had just at the moment the hand is removed. The difference is friction that exists between book and table on Earth but not in free space. When the book moves at constant velocity, the correct conclusion is that "there is zero net force acting on it". This is true in both cases as I explain below.

On the table, there are two opposing forces that cancel each other, friction and the pushing force; the net force is zero as long as the hand is pushing. When the hand is removed, the book will move (I think) by an imperceptibly short distance before it stops. In free space, the book is increasing its speed for as long as the hand is pushing; it keeps the speed it has at the moment the hand is removed and the net force becomes zero.

 

[Lnewqban]

Summary:: Newton’s 1st law

I’ve been going back and reviewing my basic physics and I have a nagging question?

F=ma

aid acceleration in terms of units such as meters per seconds squared. for example, 20 meters per second per second. This indicates an increasing speed...

A less confusing way of understanding this is observing the acceleration of a mass as a result, an effect.
The force applied upon that mass for certain period of time (named impulse) is the cause of the change in the velocity of the object (named acceleration).

$Acceleration=Force/Mass$

Hence, for a fixed amount of mass, the application of a stronger force will cause a more dramatic acceleration or change of velocity, but only during the time the application of that force lasts.
Once that external force is removed, the body remains with the velocity and direction it had at the instant the force was removed.

A bullet inside a rifle is only accelerating during the time the propellant gasses are pushing it out of the barrel.
Once it leaves the barrel, it only free-flies with the gained kinetic energy (fast by steadily slowing down due to friction against air, except when accelerated downwards by gravity).

The asteroid moving through space at a constant speed does not have any force that we could discuss, not only because it is not accelerating, but because is not interacting or colliding with another object (except any gavitational remote interaction).

What your asteroid has is accumulated momentum or kinetic energy, which means it has potential to destroy anybody it collides with.
That collision means that the second body will try to suddenly stop your asteroid, inducing a time during which a high rate of deceleration will occurr.

Going back to our re-arranged equation, you can see that the inverse is also true for a fixed amount of mass:
A more dramatic deceleration or change of velocity will cause the application of a stronger force (destructive in this case), but only during the time that dramatic deceleration lasts.

Please, see:
https://en.m.wikipedia.org/wiki/Impulse_(physics)

 

[thetexan]

In other words

any net force will naturally produce a change in velocity (acceleration). Any change of velocity is the result of a net force. So any object with constant velocity has no net force applied to it. Such as an airplane in unaccelerated straight and level flight...the thrust equal drag and lift equals weight. If any of those were unequal there would be an acceleration in one of those directions.

so F=ma is not really the equation for determining the kenetic energy of the asteroid. So what is the equation for determining the kinetic energy of a moving body?

tex

 

[Dale]

so F=ma is not really the equation for determining the kenetic energy of the asteroid. So what is the equation for determining the kinetic energy of a moving body?

$$KE=\frac{1}{2}mv^2$$

 

[A.T.]

So what is the equation for determining the kinetic energy of a moving body?

You searched and found no answer to this question?

 

[davenn]

So what is the equation for determining the kinetic energy of a moving body?

you were given the answer to that in post #4

AS others have said, there is no such thing. But there is kinetic energy, which is equal to half the mass times the speed squared of the object (½mv²).