Definition of Force confused about how it includes direction

[alkaspeltzar]

In early physics, i learned a force was simply a push or pull, talked about as a number of Newtons or pounds, such as "the force is 5lbs".

Then you learn that a force is really a vector quantity, having both the direction and strength/force, so it should be "5lbs to the east or 15lbs downard".

Yet today, after all my physics, forces are still talked about and calculated in book ignoring direction. When asked to find a force, we simply care if it is "5lbs or 15 lbs" and direction is almost assumed.

So that is my questions, how can we do that? Is it just for most practical applications we can generally think of forces as push/pull without direction?

I assume this since i see this is how my classmates think about it too. Just looking for others inputs. Thanks

 

[sophiecentaur]

To be of any use, knowledge about a force absolutely has to have direction information - in the same way as displacement. It's no use telling someone "the Town Centre is 4km away" if ask you where it is and there's no point in telling someone the force on a spanner unless you tell them your doing the nut up or slackening it.
However, often, the direction is implied in the background information. The apple Weighs 1Newton implies a downward force. When a drag car is accelerating, you imply that the force from the back wheels is pushing the car forward.

Unless your Physics is at a very elementary level I doubt that problems involving forces do not imply a direction. By A level (Maths and Physics), problems involve forces in all possible directions which are added together to find a 'resultant'.

 

[SteamKing]

It depends on what sort of forces you are talking about. If the force, for example, represents weight, then the understood direction of the force is toward the center of the earth. If the force represents a tension or compression, then the direction is understood to be the same as that object which is under tension or compression. This is easy to visualize if we have a rope or a solid bar; it becomes less so if this is not the case.

 

[arildno]

In some cases, it might not be very interesting to actually describe the direction of the force. either 1. because it is obvious, or 2. that the direction of the force changes all the time, whereas the magnitude of the force remains constant.

But, just because some aspect happens to be uninteresting, does not mean the aspect is nonexistent.

 

[russ_watters]

To me the issue is about grammar. It is sometimes permissible to leave words out of a sentence if their existence is implied. Same for the direction of a force.

 

[sophiecentaur]

Right.

 

[Popper]

In early physics, i learned a force was simply a push or pull, talked about as a number of Newtons or pounds, such as "the force is 5lbs".

When force is described in that way they are trying to pass on to you the "feeling" of what a force is. If you have ever had to push a car off the street after it ran out of gas then you've exerted a force on the car durint the time the car was accelerating. You get the feeling of it from the sensation that well all get when you feel our muslces contracting. I believe this is how Newton described force in his book The Principia. Sometimes it's said that if an object is accelerating that there is a force acting on the object. But this assumes a particular frame of reference. That frame is called an inertial frame of reference. But an inertial frame of reference is a frame in which objects that are motion remain in motion or at rest unless acted upon by a force. That's the famous circular logic in Newton's laws.

It's best to think of an inertial frame as a region of space far removed from any other object other than what you're investigating and when a few test objects are placed in that region of space (the objects being relatively small in mass) are moving at constant velocity then that frame is called an inertial frame of reference.

Next you need to define momentum. Momentum p is defined as the vector p = mv where m = inertial mass and v = velocity vector. Then force is defined as the vector F such that F = dp/dt = time rate of change of momentum. It has both a magnitude and direction.

Then you learn that a force is really a vector quantity, having both the direction and strength/force, so it should be "5lbs to the east or 15lbs downard".

Yet today, after all my physics, forces are still talked about and calculated in book ignoring direction. When asked to find a force, we simply care if it is "5lbs or 15 lbs" and direction is almost assumed.

The magnitude and direction is not being ignored. One is assuming it implicitly, i.e. we know that it's there, we're just not stating it outright.

So that is my questions, how can we do that? Is it just for most practical applications we can generally think of forces as push/pull without direction?

No. It's extremely important to take magnitude and direction into account because when multiple forces are acting on a particle the resulting force is the vector sum of all the forces acting on the particle.

If the force, for example, represents weight, then the understood direction of the force is toward the center of the earth.

I'd like to comment on this. A few objections have been made against this viewpoint. One argued that magnitude of the weight is the magnitude of the force required to support the field at rest in a static gravitational field. The direction of the weight being opposite of the direction of the supporting force. Strangely enough, most physics textbooks, incorrectly I add, state that weight = mg where m = passive gravitational mass and g = acceleration due to gravity. IF that were true then a particle in freefall has weight. That means that an astronaut orbiting the Earth in his capsule has weight and in fact is not weightless. I hold that this is the wrong viewpoint.

If the force represents a tension or compression, then the direction is understood to be the same as that object which is under tension or compression. This is easy to visualize if we have a rope or a solid bar; it becomes less so if this is not the case.

To add to this I would say that a force is a pulling force is the force acts to stretch the body and a push acts to compress the body.

 

[Doc Al]

I'd like to comment on this. A few objections have been made against this viewpoint. One argued that magnitude of the weight is the magnitude of the force required to support the field at rest in a static gravitational field. The direction of the weight being opposite of the direction of the supporting force. Strangely enough, most physics textbooks, incorrectly I add, state that weight = mg where m = passive gravitational mass and g = acceleration due to gravity. IF that were true then a particle in freefall has weight. That means that an astronaut orbiting the Earth in his capsule has weight and in fact is not weightless. I hold that this is the wrong viewpoint.

"Weight" is usually defined (in most common textbooks) as the gravitational force on an object. By this definition, of course an astronaut has weight. (He'd better, else what is holding him in orbit?) And of course a particle in freefall has weight. A perfectly reasonable definition.

What you are calling "weight" is usually referred to as apparent weight.

 

[CWatters]

Yet today, after all my physics, forces are still talked about and calculated in book ignoring direction. When asked to find a force, we simply care if it is "5lbs or 15 lbs" and direction is almost assumed.

So that is my questions, how can we do that? Is it just for most practical applications we can generally think of forces as push/pull without direction?

NO. In most problems you need to know the direction.

Can you give us an example from your book which ignores direction?

 

[alkaspeltzar]

Sure, my textbook will say, using F=MA, find the force given mass 5kg and acceleration 5m/s'2...there is no talk of direction. Answer will also have no direction stated.

Or another example would be my book starts out with showing a free body diagram of forces, with directions, but all the calcs are done with scalar compnents of the vector, and these scalar values are still referred to as "the force"

Are we just suppose to know they are talking about magnitude and are they just being sloppy with english?

 

[sophiecentaur]

What level is your course? If you give it some time, you should find that your worries get resolved as other (vector based) units are introduced. I must say, the order of your topics could be better! ;-)

 

[alkaspeltzar]

I agree the book seems to jump around and make generalizations...is it normal in high school physics to worry about the magnitude of force, still call it force? Seems like that is what they are doing, only giving us taste of more to come.

 

[sophiecentaur]

The problem you guys have these days in your Science education is that 'they' keep trying to make it constantly 'interesting' for you by giving you loads of little bits in a random order (your "tastes"). It's fine for the majority of students who will never actually become Scientists but I think it messes up people like you - who are bothered enough to get involved with PF and other groups. I can't remember. (way way back) anything being taught to me 'out of order' and we were always made to get basic things really well sorted out before moving on to something more advanced. (Like, for instance, we did not consider electrons in our treatment of electricity until we were really easy with all the conventional 'sums' about current, volts, etc..). I found it all so interesting that I never considered that it would be OK to fast track the order of learning.
As for your worry about what to call a 'Force', until you start to deal with vectors in a serious way, you just need to assume that the direction of the forces that you are considering will always be 'obvious'. Forces, either 'in the direction of' or 'against' will always be along a straight line and will sometimes cancel out (you must have done that sort of thing already).

PS Why are you carrying on two parallel conversations about basically the same topic on two different threads? If you aren't careful, you - or somebody else will miss something.

 

[alkaspeltzar]

Right, so sometimes direction of a force is obvious or unimportant, so we keep it basic. This is what you were saying?
Sorry for multiple posts, wasnt intentional.
thanks for the help

 

[Popper]

"Weight" is usually defined (in most common textbooks) as the gravitational force on an object.

Please recall what I posted above, i.e. the comment of mine which you were responding to

Strangely enough, most physics textbooks, incorrectly I add, state that weight = mg where m = passive gravitational mass and g = acceleration due to gravity.

By this definition, of course an astronaut has weight.

Please recall again what I posted above and what you were responding to

That means that an astronaut orbiting the Earth in his capsule has weight and in fact is not weightless

Do you mind if I ask you a question? If not then why are you repeating what I had already posted? Perhaps it was merely an oversight on your part? Just curious for which I believe are obvious reasons.

And of course a particle in freefall has weight. A perfectly
reasonable definition.

Once last time, please recall what I wrote

I hold that this is the wrong viewpoint.

Which means that I disagree with your opinion on how weight is defined.

What you are calling "weight" is usually referred to as apparent weight.

Yep. Way ahead of ya. I already understood that. In fact in the article I was referring the author writes

Our weight (let us not muddy up the issue by calling it “apparent weight”) increases when we are in an elevator...

Since you seem to want to consider this point more carefully I’ll fill you in regarding my comment A few objections have been made against this viewpoint. as far as my sources. I was referring to at least two examples which came to mind which are both in The American Journal of Physics. One is from

The equivalence principle and the question of weight, Kenneth Nordtvedt Jr., Am. J. Phys. 43(3), March 1975

The other is a Letter to the Editor written by A.P. French and found in Am. J.Phys. 63 (2) Feb. 1995, page 105

I've attached the French article for those who want to read it. However I'm not much for chatting over definitions myself. I'll at least state once what I prefer for the record.