What is Momentum? - Explanation Needed. (1 Viewer)

[zeropoint]

simply put: (and dont hit me if i am wrong)

inertia: a mass's ability to resist movement
momentum: force of a moving object

Try to avoid using other scientific words in the description of these quantities.

For example, your definition of inertia is acceptable in my opinion, but it would be better to use the word matter.

Your definition of momentum is definitely wrong. The word force has a precise scientific meaning and should not be confused with momentum. The whole point of the principle of inertia is that an object unaffected by an external force will maintain its momentum. If the object has a force, then it will accelerate and therefore change its momentum.

What is momentum then? Well it's the quantity of motion equal to the product of mass and velocity, the rate of change of which is equal to the net force according to Newton's second law. That's the most concise definition I can think of.

Hope this helps.

James

 

[mitsui]

so is it kinda like impulse?

not a force, a measurement of the motion in a given amount of time??

????????

 

[wanton-wonton]

simply put: (and dont hit me if i am wrong)

inertia: a mass's ability to resist movement
momentum: force of a moving object

It's been a while since I did physics, but are you sure momentum is a force??

p = mv.

The units should be kg m/s. Well anyway, if it isn't a force, then don't say force. But I really can't remember if it's a force now.

 

[mitsui]

It's been a while since I did physics, but are you sure momentum is a force??

p = mv.

The units should be kg m/s. Well anyway, if it isn't a force, then don't say force. But I really can't remember if it's a force now.

actualli i am not realli sure, now i think about it, it is more like a measurement of a motion made by a moving matter. (if that makes sense...hmm :mad1: )

 

[zeropoint]

actualli i am not realli sure, now i think about it, it is more like a measurement of a motion made by a moving matter. (if that makes sense...hmm :mad1: )

First, impulse is the change in momentum that you get from an external force in a given amount of time.

As for your definition of momentum, I don't understand the point of the phrase "made by a moving matter"? I guess what you're saying is true, but it's not really part of the definition of momentum. If you're trying to answer a question such as "What is momentum", then I would suggest the following three key points

Definition: Quantity of motion directly proportional to mass and velocity.

Characteristics:
1. Law of inertia: an object subject to no external force maintains constant momentum.
2. Newton's second law: The rate of change of momentum is directly proportional to the net force.

Can you explain how characteristic 1 follows directly from 2?

James

 

[zeropoint]

It's been a while since I did physics, but are you sure momentum is a force??

p = mv.

The units should be kg m/s. Well anyway, if it isn't a force, then don't say force. But I really can't remember if it's a force now.

Momentum is definitely not a force (see 5 posts up for explanation).

 

[A l]

If you are asked to define momentum in an exam, writing the product of mass and velocity is enough to gain you full marks.
Momentum isn't exactly a force. Momentum relies on constant velocity, whereas force relies on acceleration. It might be plausible to say that momentum (especially impulse, noting Impulse = Ft) makes up a component of force. Note the different formulas: F = ma and p = mv.

 

[zeropoint]

If you are asked to define momentum in an exam, writing the product of mass and velocity is enough to gain you full marks.

The fact is, you're much more likely to be asked to explain momentum in relation to rocket propulsion, since this shows more understanding than simply stating a definition.

Momentum isn't exactly a force. Momentum relies on constant velocity, whereas force relies on acceleration.

To say momentum relies on constant velocity is misleading. Any moving object has momentum, whether it is accelerating or not.

It might be plausible to say that momentum (especially impulse, noting Impulse = Ft) makes up a component of force. Note the different formulas: F = ma and p = mv.

Better to say that force is the rate of change of momentum, note that F = ma is what you get when you take the rate of change of p = mv.

James

 

[A l]

To say momentum relies on constant velocity is misleading. Any moving object has momentum, whether it is accelerating or not.

Any moving object with a constant velocity has momentum (by this I mean a constant momentum). Any object accelerating has a change in momentum. Distinguish between momentum and change in momentum.
The rate of change in momentum is force. However, momentum itself (i.e. constant momentum) is not force. Constant momentum relies on constant velocity and at constant velocity means there are no forces acting on the object. Change in momentum relies on a change in velocity (hence acceleration). When there is a change in velocity there is a force acting on the object.

 

[vizman]

Any moving object with a constant velocity has momentum (by this I mean a constant momentum). Any object accelerating has a change in momentum. Distinguish between momentum and change in momentum.

change in momentum is just impluse isn't it?

 

[A l]

change in momentum is just impluse isn't it?

Yes I suppose so, but I chose the words "change in momentum" to illustrate my point and distinguish between constant momentum and its rate of change. In my opinion, impulse is a fancy word for change in momentum.

 

[vizman]

Yes I suppose so, but I chose the words "change in momentum" to illustrate my point and distinguish between constant momentum and its rate of change. In my opinion, impulse is a fancy word for change in momentum.

yehp fair enough. its better in that context

 

[zeropoint]

Any moving object with a constant velocity has momentum (by this I mean a constant momentum). Any object accelerating has a change in momentum. Distinguish between momentum and change in momentum.
The rate of change in momentum is force. However, momentum itself (i.e. constant momentum) is not force. Constant momentum relies on constant velocity and at constant velocity means there are no forces acting on the object. Change in momentum relies on a change in velocity (hence acceleration). When there is a change in velocity there is a force acting on the object.

Your explanation could be improved significantly.

The first line implicitly suggests that momentum is a characteristic of uniform velocity motion --- it's not. Momentum depends on the instantaneous velocity of the object, not on the rate of change, as I'm sure you are aware.

While it's true that an object moving with constant velocity can have constant momentum, the vice verse is _absolutely_ untrue, which is obvious by inspection of the equation p = m v. Furthermore, an object doesn't have constant (or changing) momentum because it is moving with constant velocity (or it is accelerating). A net force CREATES a change in momentum, this affects the velocity by virtue of the fact that p = m v.

James

 

[A l]

Your explanation could be improved significantly.

The first line implicitly suggests that momentum is a characteristic of uniform velocity motion --- it's not. Momentum depends on the instantaneous velocity of the object, not on the rate of change, as I'm sure you are aware.

While it's true that an object moving with constant velocity can have constant momentum, the vice verse is _absolutely_ untrue, which is obvious by inspection of the equation p = m v. Furthermore, an object doesn't have constant (or changing) momentum because it is moving with constant velocity (or it is accelerating). A net force CREATES a change in momentum, this affects the velocity by virtue of the fact that p = m v.

Well, SORRY you still remember and understand the Preliminary Physics course with technical precision. Congratulate yourself!
Right now at this stage, I have to concentrate on the topic Ideas To Implementation so back to HSC work for me.

 

[zeropoint]

Well, SORRY you still remember and understand the Preliminary Physics course with technical precision. Congratulate yourself!
Right now at this stage, I have to concentrate on the topic Ideas To Implementation so back to HSC work for me.

Hey, don't appologise, I'm merely trying to refine your answers.

 

[airie]

... As confused as ever.

According to the wikipaedia, intertia is 'the tendency of a body to resist acceleration', whereas momentum is 'a measure of the motion of a body equal to the product of its mass and velocity. Also called linear momentum'.

Does this mean that momentum is the 'amount' of motion a body has? And inertia is just a characteristic that every body possesses, which cannot be measured?

 

[zeropoint]

... As confused as ever.

Hmmm, that's not good. What exactly are you confused about in particular? If you tell me soon, I might still have time to explain before I go away.

According to the wikipaedia, intertia is 'the tendency of a body to resist acceleration', whereas momentum is 'a measure of the motion of a body equal to the product of its mass and velocity. Also called linear momentum'.

Does this mean that momentum is the 'amount' of motion a body has? And inertia is just a characteristic that every body possesses, which cannot be measured?

I'm familiar with Wikipedia's definition of inertia, and in my opinion, it is acceptable, but not great.

The motivation for this definition stems from the fact that when you push on an object in order to accelerate it, the object pushes back on you with a so-called "inertial force" caused by the object's "inertia".

A quick check of University Physics 11th edn by Young and Freedman (as good a source as any for these things) suggests the same definition I provided, that is, inertia is the property of matter that causes it to remain in uniform velocity motion unless acted upon by an external force. I suggest you stick to this definition because it is the most commonly used amongst high school teachers and HSC markers.

As for your definition of momentum, yes it's perfectly correct. Although Newton himself used the word "quantity" as opposed to "amount" of motion, so I suggest you follow him It's also important to mention the direct proportionality with mass and velocity.

You can't really measure inertia because it's a propertry rather than a physical quantity, but you can think of it as being quantified in terms of mass.

Hope this helps.

James

 

[airie]

Hey thanks for that. You're a great help