force = mass x acceleration

[spunnery]

if force = mass x acceleration

 

could any body explain the force on a body moving @ constant speed?

 

since acceleration = 0 ; force = 0 ???????

[swansont]

if force = mass x acceleration

 

could any body explain the force on a body moving @ constant speed?

 

since acceleration = 0 ; force = 0 ???????

 

 

Yes, the net force is zero. You can have forces present, but they must sum (as vectors) to zero.

[spunnery]

Hai swansont

how if such a body with zero net force,hits ur body? if vector sum was Zero,from where the force which is pushing u comes from?

[m4rc]

If something is moving at a constant velocity then the sum of all the forces acting on it will be zero.

 

If something collides with your body, the object won't stay at the same velocity in this situation. The object will slow down (thus have an acceleration). The magnitude of the force will be equal to the product of the mass of the object and its acceleration.

[Klaynos]

If you change the mass of a system F=ma is not valid, you need to use [math]F=\frac {dp}{dt}[/math]

[spunnery]

hai M4rc

does the body moves from one place to another?

does it done a work in moving so ?

how can a work done without a force.???

[swansont]

hai M4rc

does the body moves from one place to another?

does it done a work in moving so ?

how can a work done without a force.???

 

No force means no work. But as m4rc has discussed, this does not hold during a collision, since there will be a force. Klaynos has mentioned the proper equation, and this ties in to Newton's third law; during the collision, each body acts on the other with equal magnitude forces, but opposite in direction. Momentum will be conserved, and this is the relevant quantity to calculate. Under some circumstances, kinetic energy will be conserved as well, but this does not hold universally.

[Sisyphus]

Maybe an example would help.

 

You're floating in space. A rock is flying towards you at 500mph. Both of your velocities are constant, so there's no forces at work. The rock hits you. Now there's a great deal of force, as the rock exerts force on you (accelerates you in the direction of its movement) and you exert on the rock (you "deccelerate" the rock). If the rock has come to a stop, it's done on YOU the full amount of work needed to accelerate it from 0 to 500. More aptly, it's done the work on the part of you the rock hit, which will be violently accelerated with respect to the rest of your body. Most likely, more than the tensile strength of your body can withstand, and it will break free, and the rock will pass through you, having slowed down by as much work as was needed to rip that hole in you.... yay physics!

[Phi for All]

Hai swansont

how if such a body with zero net force,hits ur body? if vector sum was Zero,from where the force which is pushing u comes from?

I'm officially asking you to please STOP using text-message abbreviations. This is not your phone or a messenger service and it's important to communicate your ideas and questions in a manner most will understand.

 

This is for your benefit as well as ours. You will get more responses if people don't have to take so much time deciphering what you mean. Thanks in advance for your understanding.

[Heretic]

I think spunnery just has velocity and acceleration confused.

 

Velocity (generally speaking) is the speed at which an object travels. Acceleration is the change in velocity.

 

So if an object is moving at a certain velocity and maintains that velocity then there is no force acting on that object.

[Bignose]

Maybe an example would help.

 

You're floating in space. A rock is flying towards you at 500mph. Both of your velocities are constant, so there's no forces at work. The rock hits you. Now there's a great deal of force, as the rock exerts force on you (accelerates you in the direction of its movement) and you exert on the rock (you "deccelerate" the rock). If the rock has come to a stop, it's done on YOU the full amount of work needed to accelerate it from 0 to 500. More aptly, it's done the work on the part of you the rock hit, which will be violently accelerated with respect to the rest of your body. Most likely, more than the tensile strength of your body can withstand, and it will break free, and the rock will pass through you, having slowed down by as much work as was needed to rip that hole in you.... yay physics!

 

This is a good example. Maybe an oppostie example would help too:

 

Consider driving in a car along a flat, straight stretch of highway. You want to drive at exactly the speed limit, say 70 mph, and set the cruise control for that speed. Are there forces on the car? Sure, friction between the road and tires, air resistance, and of course, the engine applying a force to keep the car going. But, if the cruise control is working properly, speed will be maintained at exactly 70 mph. No change in velocity means no change in acceleration, means no net forces on the the car. But, there are definately forces acting on the car.

[spunnery]

swansont,

Of course,there is a force in collision.My concern is ,if the net force on a body moving with uniform velocity is zero,then howcomes a work is done (moving from one place to another) without a force acting on it?

[swansont]

swansont,

Of course,there is a force in collision.My concern is ,if the net force on a body moving with uniform velocity is zero,then howcomes a work is done (moving from one place to another) without a force acting on it?

 

No work is done. [math]W = \Delta KE =\int F.ds[/math]

[spunnery]

No work is done. [math]W = \Delta KE =\int F.ds[/math]

what is the defenition of work done.Is it the distance travelled by a body due to the influence of a force(may not be correct grammer or correct defenition).So if the body move from one place to another means there is a work done,which you are denying

[swansont]

what is the defenition of work done.Is it the distance travelled by a body due to the influence of a force(may not be correct grammer or correct defenition).So if the body move from one place to another means there is a work done,which you are denying

 

Work is the energy transfer by a force acting through a displacement. No force means no work. (No displacement means no work as well)

 

One body may travel to another place at constant velocity, which means no work is done in doing so. There may be forces that cancel, meaning there is work done by each of these forces, but the sum will be zero, as in the example Bignose gave.

[spunnery]

Work is the energy transfer by a force acting through a displacement. No force means no work. (No displacement means no work as well)

 

One body may travel to another place at constant velocity, which means no work is done in doing so. There may be forces that cancel, meaning there is work done by each of these forces, but the sum will be zero, as in the example Bignose gave.

No displacement means no work!.doesn't it means if displacement is there ,work is done there?

here bodyin the question is displaced from one place to another and you are telling me no work is done.Could you please explain it little more?

take the body to outer space where there is no gravitational field.so no other forces will be acting on the body(to eliminate all other forces and stick to the point)

[Bignose]

OK, to space then where we will just assume all gravity is zero. A comet passes by. Assume also that the comet is not losing any mass. It is definately moving, but it's movement is due entirely to inertia. Lots of displacement, but no forces, no change in kinetic energy, ergo no work.

 

If you stop the comet, or the comet hits a rock or something like that, then a force will be applied and then work. But, an object in motion tends to stay in motion, and unless some force is applied to the comet, it will continue to move at a constant speed forever. Look at the work equation again. dW=F*dl. It is a product on the righthand side. If either F or dl equals zero, the prodcut will be zero. If there is no net forces, F=0, then work equals zero.

[swansont]

No displacement means no work!.doesn't it means if displacement is there ,work is done there?

here bodyin the question is displaced from one place to another and you are telling me no work is done.Could you please explain it little more?

take the body to outer space where there is no gravitational field.so no other forces will be acting on the body(to eliminate all other forces and stick to the point)

 

W=Fs

 

F = force

s = displacement

 

If either is zero, there is no work. The kinetic energy does not change.

 

A body in motion remains in uniform motion (at a constant velocity) unless acted upon by a net external force. From Newton's first law.

[spunnery]

W=Fs

 

F = force

s = displacement

 

If either is zero, there is no work. The kinetic energy does not change.

 

A body in motion remains in uniform motion (at a constant velocity) unless acted upon by a net external force. From Newton's first law.

I agree with you swansont,But really i want you to come to this point.

Now if i give a new formula to force,F1 = mass x velocity (vector quantity).and the defenition of force in original equation(F2= m x a) as the additional force per second required to change the velocity ,can't we change our understanding of inertia(it is not a new theory-it is only a different view).

 

i will explain it further

A body of mass 'm' is travelling at a constant velocity 'v'.

now as per our new formula ,force on the body is F1 = m x v;

We want to stop this body in 't' seconds(to bring the body at rest with respect to inertial frame),

For this,we have to bring 'v' to zero in 't' seconds.so the acceleration required is

a= (0 - v)/t = -v/t.

so the force per second required to stop the body is F2= m x a.

this is true for any change in velocity(in magnitude & direction).

 

what is different in this angle of view?

 

force is a vector quantity (unit is kg*m/sec).

 

Now we can say ,A force acting on the body will remain same until and unless another force is acted on the body.(can be used to explain the cause of inertia?????)

[Sisyphus]

Why are you using the word force there for two different things? mv is momentum, not force.

[spunnery]

Why are you using the word force there for two different things? mv is momentum, not force.

sorry i am not concerned about the words or terms, i am interested in the logic behind my explanation

[Sisyphus]

But it IS important. You say "force on the body is F1 = m x v." But that's just the body's momentum, and is nothing acting on the body. And you use the two different meanings interchangably in your conclusion, rendering it meaningless.

[swansont]

But it IS important. You say "force on the body is F1 = m x v." But that's just the body's momentum, and is nothing acting on the body. And you use the two different meanings interchangably in your conclusion, rendering it meaningless.

 

This cannot be emphasized enough.

 

The terminology is already established. Learn it. Use it.

[Bignose]

spunnery, sorry if this seems like piling on, but the exact usage of terms that are defined unambigously demand preciseness. It is not like the regular English language (or other spoken word), where there are subtleties, and slight changes of meaning, and words that mean one thing in one context and mean another thing in another context.

 

Physics and mathematics demand very precise and correct use of the terms. You want to talk with other physicists and people who understand this material, right? It is just like learning a language, but a language with very little use for ambiguity. A force is defined exactly how it is, you cannot just change what it means. So, defining F1 as you have, and calling it a force is, quite simply, meaningless. [math]Force \ne mv[/math].

 

So, in order to convey your ideas with other people who are knowledgeable, you need to learn the correct terminology. Yes, force is a vector quantity, but so is momentum. Just because two quantities are both vector quantities does not imply anything special, like equality. Why not use the position vector then? By that same logic, position and force should be equal. As should acceleration, and rotation, and the derivative of acceleration, etc.

 

To respond to your "logic behind my explanation," quite simply in light of what I and the others have said above, there is no logic at all. What you have written is pretty close to having zero meaning. Since the very first equation your attempt to redefine force, is flawed, everything you dervied from that equation is wrong. It's like building a home on a sinkhole. It doesn't matter how well-built the first floor is, the ground the house is sitting on is sinking and everything will fall down.

 

p.s. As an aside, even mathematics and physcs aren't immune from some language-ambiguity creeping in, there are two different meanings of the word homogeneous in mathematics, for example. I am sure there are others. That said, both definitions of homogeneous are very exactly defined, and it is unlikely to confuse one with the other. Unlike your attempt to redefine force and then use the correct formula later, where confusion is rampant.

[Farsight]

Hai swansont

how if such a body with zero net force,hits ur body? if vector sum was Zero,from where the force which is pushing u comes from?

 

It's inside the moving body, spunnery. If you let the handbrake off a car and push it, you're exerting force on it to get it going. If you push it for say five seconds you're giving it some amount of "momentum". Then you stop pushing, and the force has gone into the car. There's no force acting on it any more, but it keeps on going because it's got that momentum. If you run round and get in front of it, the force it hits you with depends on how fast you stop it. Ummm. Don't try this at home.

 

The easiest way to think about what's going on here is to imagine that every atom of the car is tracing a circular path. When you pushed the car you bent these little circles into spirals, and every atom of the car moves forward, and keeps on moving forward. You have to push the spirals back into circles to stop the car moving.

 

Like the other guys said, do pay attention to the terminology.