K basically this is F = Ma right?

Isn't this law impractical for any situation outside of a vacuum? If I was pushing a toy car with a constant force 10N in order to make it accelerate, resistive forces would build up to oppose the resultant force and give it uniform motion. Therefore I would have to apply more force (20N) to get it to accelerate again. But resistive forces keep building up, so it's like I need an insane amount of force because the acceleration is never going to be constant.

It's always going to keep decreasing unless I add more force.

Freefall I'm falling at 9.81ms-2. Resistive forces build up, so my 9.81m-2 is not constant anymore and decreases to 0 until I reach terminal velocity. The only way I can picture a constant acceleration is in a vacuum where there are no resistive forces and I can apply a force and get a permanent/constant acceleration out of it

So is f=ma wrong? Or am I deluded? Where have I gone wrong if I am?

Evil dead

Why does nobody want to answer my question? What have I done?

Unknown008

Well, it takes some time for me to wake up and look at your question

Joke apart;

F=ma is valid

F is the net force on the object. If any resistive forces are concerned, then the net force will decrease, decreasing the acceleration, hence to constant velocity.

Evil dead

But doesn't f=ma cater for a constant acceleration? You just said that if any resistive forces are concerned (which is mostly the case on earth) then net force will decrease and therefore acceleration will decrease.

Newtons 2nd law is not: dF=Mda where 'd' is delta. Is it not meant a uniformly accelerating object is acted on by a resultant force? This is never the case this is why I opened this thread. :/

ebaines

Hello ED. The F and a in F=ma are actually vectors - meaning they have both magnitude and direction - and the equation should properly be written as , meaning that you have to add up all the forces acting on the object to find the total magnitide and direction of . So F=ma leads to a result of constant acceleration ONLY if the sum of the forces acting on the object is constant.

In the example of the falling object, the force of gravity acting on it in the downward direction is resisted by a force of air resistance acting on it in the upward direction. The magnitiude of the air resistance increases as the object's velocity increases. At first air resistance is nil, and the object falls with an aceleration that is , or . But as the velocity increases you need to consider the upward force of air resistance:



As long as is less than the object's weight, it continues to accelerate as it falls, although at a smaller rate than before. At some point the magnitude of the air resistance equals the magnitude of the object's weight, so that , and the object no longer continues to accelerate. At that point it has reached its "terminal veocity."

Hope this explanation helps.

Evil dead

Thank you, that explained it.

laura_123

Hey, Here is a break up of Newton's 2nd Law:

--A force is the name given to whatever causes motion.
--The most familiar force is weight, the downward force on an object due to gravity. We can therefore measure force in grams or kilograms, units of weight, and loosely define force as "anything that can be matched by weight" (e.g. the tension of a spring).
--Forces can be opposed or unopposed.
--In the absence of opposing forces, if no force acts on an object at rest or moving at constant speed, it continues to do so indefinitely (Newton's first law).
--In the absence of opposing forces, if a force does act on an object at rest or moving at constant speed, it accelerates in the direction of the force.
--The acceleration of such an object is limited by its own resistance to motion, which Newton named its inertia.
--If air resistance can be ignored, a light object falls just as fast as one twice as heavy. Newton proposed that the reason was that although the force of gravity on the heavier object (its weight) was twice as large, so was its inertia.
SOURCE:Newton?s Laws of Motion | Newton?s Laws of Motion Online | Download Newton?s Laws of Motion | Sciences - Examville.com Online