Understanding Resultant Forces: Exploring Constant Speed and Zero Acceleration

[Guidestone]

So force equals mass times acceleration. What happens then if we pushed a box horizontally without fiction at constant speed? The box keeps moving but acceleration equals zero? Is there still force?
Thank you!

 

[DaveC426913]

No force is required to keep the box moving.
But the box has momentum. This momentum will be applied as a force if the box encounters an object.

 

[A.T.]

So force equals mass times acceleration.

No. The net force equals mass times acceleration.

What happens then if we pushed a box horizontally without fiction at constant speed?

If the net force is not zero, velocity cannot be constant. The speed can be constant if you have a centripetal net force, perpendicular to velocity.

 

[zanick]

No. The net force equals mass times acceleration.If the net force is not zero, velocity cannot be constant. The speed can be constant if you have a centripetal net force, perpendicular to velocity.

B
But i think he is not thinking that if there is constant speed (velocity) then there is no net force. any net force will result in acceleration (a=F/m)

 

[ZapperZ]

So force equals mass times acceleration. What happens then if we pushed a box horizontally without fiction at constant speed? The box keeps moving but acceleration equals zero? Is there still force?
Thank you!

Unless there is another equal but opposite force other than friction, then this will never happen. If velocity is a constant, there is no net force. If there is a net force, then velocity is not a constant.

Zz.

 

[lychette]

there is NO RESULTANT force...ther could be 2 (at least) forces acting...There is no resultant

 

[DaveC426913]

there is NO RESULTANT force...ther could be 2 (at least) forces acting...There is no resultant

The term normally used is net force.

 

[lychette]

I am more familiar with RESULTANT... in my textbooks. But the meaning is the same. Makes no difference to the physics

 

[lychette]

I have always heard of the resultant of vectors... I suppose it is the net ?? resultant is a good word, should not be dismissed.

 

[davenn]

I have always heard of the resultant of vectors... I suppose it is the net ?? resultant is a good word, should not be dismissed.

have to agree with Dave and Zz
net is the normally used term and it's only 3 letters to type

 

[DaveC426913]

have to agree with Dave and Zz
net is the normally used term and it's only 3 letters to type

Perhaps, although "normally" does not exclude the possibility that lychette's term is not common and accepted where he teaches. I don't know where, but I'm willing to grant that we may be guilty of provincialism. Or whatever the inverse of provincialism is.

 

[lychette]

have to agree with Dave and Zz
net is the normally used term and it's only 3 letters to type

are you sure the word is 'net' and not 'nett'.(4 letters...does it matter!)... what exactly do you mean by 'normally'...do you mean ; here on physics forums or in textbooks ? .. Is there any confusion over 'resultant'... what is used in 'your' textbooks.
Do you ever refer to the 'net' of 3 vectors... I would say the 'resultant' of 3 vectors... in reality the difference in meaning is zero
Thinking about it,... the 'net' of 3 vectors sounds plain wrong...it has to be 'resultant' ?

 

[DaveC426913]

... what exactly do you mean by 'normally'...do you mean ; here on physics forums or in textbooks

Common in the science world which mostly uses English as a common language.

There's nothing wrong with the word resultant; it's just unfamiliar to many. We often forget that PF is global, not exclusively Western.

 

[lychette]

surely one aim here should be to ensure that we all understand the same physics language in a communication. In this thread the difference between 'resultant' and 'net' (nett?)is unimportant...some reference to textbooks should be made...( i think it is part of the membership here) I would refer to 'Nelkon and Parker' as a start..in fact it came from a contribution by ''Davnn'' pointing out that only 3 (4) letters needed to be typed!..nothing to do with physics knowledge... (is this a p;hysics or an english language forum?)...pathetic...sorry, subjective opinion
A physics forum should be absolutely clear about the terms used in communications, I have seen pd confused with emf in recent posts amongst other sloppy usage.
I will continue to contribute to these forums but it seems to be a process of finding fault rather than imparting knowledge.

 

[FactChecker]

The situation in the OP is similar to pushing a car on a level surface. Once you get it going, you are mostly just keeping up with it and only pushing a little to overcome the friction that tries to slow it down. Essentially no pushing is necessary, once it is rolling at the desired speed.

 

[Mister T]

So force equals mass times acceleration. What happens then if we pushed a box horizontally without fiction at constant speed?

The push would have to be zero!

The test for negligible friction is to see what happens when you stop pushing. If the object continues to move at a steady speed across a horizontal surface with no push, friction is negligible. Of course, friction can never be eliminated, but it can be reduced to the point that the measuring devices being used detect no significant change in speed during the time it's being measured.

As to the discussion of resultant versus net force. In the US, introductory college and university level textbooks use resultant to refer to any vector sum. In the special case of force vectors the phrase "net force" is used to refer to the vector sum of the forces acting on an object. You could also call it the resultant force, but that's not as common. If referring to the sum of velocity vectors, or displacement vectors, or any other vector quantity, the term resultant could be used, but the term net would not.

In grocery stores we commonly see "net weight" used on product labels. By law, it refers to what physicists would call the mass of the package contents, after subtracting off the mass of the packaging material.

 

[lychette]

In Physics 'net' and 'resultant' are essentially the same. I, and many others, prefer the word 'resultant' when referring to forces because it is a specific science term. 'net' is a more everyday term and can cause confusion, especially students learning the basics.
But now we are talking about semantics not physics...pity the poor student.
I must admit that a site I greatly value...'hyperphysics'.. does use the word 'net' in its articles about Newtons laws... I prefer 'resultant'..it is a great point for discussions for those of us who are not 'learners'.

 

[lychette]

The situation in the OP is similar to pushing a car on a level surface. Once you get it going, you are mostly just keeping up with it and only pushing a little to overcome the friction that tries to slow it down. Essentially no pushing is necessary, once it is rolling at the desired speed.

'no pushing is necessary' ..I think you mean that no resultant/net force is nessary...A push is required if there is friction.

 

[FactChecker]

'no pushing is necessary' ..I think you mean that no resultant/net force is nessary...A push is required if there is friction.

Yes. "only pushing a little to overcome the friction"

 

[lychette]

Yes. "only pushing a little to overcome the friction"

to equal friction ?

 

[Mister T]

I must admit that a site I greatly value...'hyperphysics'.. does use the word 'net' in its articles about Newtons laws... I prefer 'resultant'..it is a great point for discussions for those of us who are not 'learners'.

Regardless, you will find that introductory textbooks in the US do not have that usage, so if you happen to be a learner in the US it's something you must confront. My preference would be to use "sum of forces" rather than an adjective such as net or resultant. Students tend to think that the resultant force or the net force is a force, when of course it in general isn't.

 

[lychette]

Regardless, you will find that introductory textbooks in the US do not have that usage, so if you happen to be a learner in the US it's something you must confront. My preference would be to use "sum of forces" rather than an adjective such as net or resultant. Students tend to think that the resultant force or the net force is a force, when of course it in general isn't.

I completely agree with you !...sum of forces is perfect...once they use a term such as 'net' or 'tesultant' they see it as a force in its own right and they want to show it as such on a diagram of forces ! something I instil into my students is that reultant (net) forces are never shown on a free body diagram.

 

[FactChecker]

to equal friction ?

Yes.

 

[Mister T]

I completely agree with you !...sum of forces is perfect...once they use a term such as 'net' or 'tesultant' they see it as a force in its own right and they want to show it as such on a diagram of forces ! something I instil into my students is that reultant (net) forces are never shown on a free body diagram.

The same argument applies to the term "centripetal force". Again, not a force in it's own right. It usually means "sum of forces points towards center".