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

[eX-Bhai]

I've must of searched every single physics textbook there is - whether the Jacaranda, Macmillian, Physics in Context, Excel, Macquarie.. and they all say the same thing about momentum.

"Momentum is the product of the mass and velocity of an object. It is given by the formula p = mv" blah blah blah

But I'm not really understanding what momentum really is.. I know we should have learnt this in the prelim year, I was in america then.. so I kinda missed out on a bit =x So if someone could provide a nice explanation on momentum, i'd really appreciate it. Its just that.. everything in the HSC course relates to momentum one way or another.. I just wanna get my hands on top of it so I can grasp the concept of conservation of momentum. Thanks in advanced.

 

[richz]

if u push an object (ie a ball), it moves, once u stop pushin it, the ball continues to move, this movement is called momentum.

 

[insert-username]

I learnt that momentum is the resistance to change of direction of motion of a moving object. To bastardise some terms, it's the "inertia" of a moving object (don't ever use that definition, it's not entirely right, but it makes it simpler to explain). For more information (try next time to use the search function ), see here.


I_F

 

[eX-Bhai]

Oh okay, I tried the search function and it came up with all these random results. So basically, momentum is the energy gained from movement, and when there is a situation of conservation of momentum, the sum of momenta = 0?

 

[A l]

That was the same problem I faced in Year 11. Most people interpret the definition, "the product of mass and velocity" mathematically. It wasn't until my teacher asked me a few questions about the theory behind momentum, did I gain some understanding of it.
Try not to interpret the definition mathematically. Momentum is kind of an abstract thing. Ask yourself the following questions:
*How difficult is it to stop a slow moving light object?
*How difficult is it to stop a fast moving light object?
*How difficult is it to stop a slow moving heavy object?
*How difficult is it to stop a fast moving heavy object?
*How difficult is it to make a heavy stationary object move?
*How difficult is it to make a light stationary object move?
The answers to all these questions all rely on the MASS and the VELOCITY of the object. For example stopping a slow moving light object is relatively easy because the product of its mass and velocity (hence momentum) is low, whereas stopping a fast moving heavy object will be more difficult because the product of its mass and velocity (hence momentum) is high.
The difficulty in stopping or moving a moving object will depend on its mass and velocity, hence it the combination of velocity and mass (i.e. their product) which is momentum. If you understand the above then you'll realise that the definition can be interpreted non-mathematically.
However, when it comes to writing it in the exam, most teachers will accept "the product of mass and velocity".

 

[vizman]

yeh so the equation p = mv tells you a lot. basically that any increases in mass or velocity will increase the momentum, thus making it harder to stop. for instance it is more difficult to stop a car moving at 20ms-1 than at 10ms-1 if mass is kept constant

 

[vizman]

Oh okay, I tried the search function and it came up with all these random results. So basically, momentum is the energy gained from movement, and when there is a situation of conservation of momentum, the sum of momenta = 0?

the conservation of momentum states that momentum is conserved in an isolated system - hence the vector sum of the momentum changes is = to 0 in an isolated system

 

[eX-Bhai]

the conservation of momentum states that momentum is conserved in an isolated system - hence the vector sum of the momentum changes is = to 0 in an isolated system

uh.. is there a more simple way to put it?

 

[vizman]

In interactions between objects, momentum is conserved (in an isolated or closed system). thats basically another version of the one mentioned above

 

[Riviet]

Try not to interpret the definition mathematically. Momentum is kind of an abstract thing. Ask yourself the following questions:
*How difficult is it to stop a slow moving light object?
*How difficult is it to stop a fast moving light object?
*How difficult is it to stop a slow moving heavy object?
*How difficult is it to stop a fast moving heavy object?
*How difficult is it to make a heavy stationary object move?
*How difficult is it to make a light stationary object move?
The answers to all these questions all rely on the MASS and the VELOCITY of the object. For example stopping a slow moving light object is relatively easy because the product of its mass and velocity (hence momentum) is low, whereas stopping a fast moving heavy object will be more difficult because the product of its mass and velocity (hence momentum) is high.
The difficulty in stopping or moving a moving object will depend on its mass and velocity, hence it the combination of velocity and mass (i.e. their product) which is momentum. If you understand the above then you'll realise that the definition can be interpreted non-mathematically.
However, when it comes to writing it in the exam, most teachers will accept "the product of mass and velocity".

A superb explanation of the meaning of momentum. That was how I understood the concept of momentum. Once you clearly understand the idea, you should be able to deduce that the definition of mass x velocity sufficient. If you are asked to explain it, use an example. I like using a fly buzzing into a truck traveling at each other, and explaining how much velocity the fly would have to be travelling to stop the truck, lol.

 

[eX-Bhai]

In interactions between objects, momentum is conserved (in an isolated or closed system). thats basically another version of the one mentioned above

But what do you mean by "an isolated or closed system"?

 

[vizman]

But what do you mean by "an isolated or closed system"?

no external forces acting

 

[eX-Bhai]

no external forces acting

Oh okay, thanks for the explanation... it all kinda makes sense now

 

[airie]

So. What is the difference between inertia and momentum then? I feel like I've been reading about inertia all along Or is the momentum just inertia? Am I missing something?

 

[Riviet]

The principle of inertia is one of the fundamental laws of classical physics which are used to describe the normal motion of matter and how it is affected by applied forces. The concept of inertia is today most commonly defined using Isaac Newton's First Law of Motion, which is often paraphrased as:

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction, unless acted upon by an external force.

The description of inertia presented by Newton's law is still considered the standard for classical physics. However, it has also been refined and expanded over time to reflect developments in understanding of relativity and quantum physics which have led to somewhat different (and more mathematical) interpretations in some of those fields.

In common usage, people may also use the term "inertia" to refer qualitatively to an object's momentum or its "amount of resistance to change in velocity" (its mass), depending on context (e.g. "this object has a lot of inertia"). The term "inertia" is more properly understood as a shorthand for "the principle of inertia as described by Newton in his First Law". It should be emphasised that 'inertia' is a scientific principle, and thus not quantifiable.

The bold bit is what differentiates the two, inertia can't be measured in magnitude, however momentum can be and has units.

 

[airie]

The bold bit is what differentiates the two, inertia can't be measured in magnitude, however momentum can be and has units.

So you mean inertia is just a property of all objects, and the momentum describes the size of such property?

 

[Riviet]

So you mean inertia is just a property of all objects, and the momentum describes the size of such property?

Sought of, but don't say it like that in an exam, it's best to explain each separately; mentioning both in the same sentence can be confusing.

 

[mitsui]

So. What is the difference between inertia and momentum then? I feel like I've been reading about inertia all along Or is the momentum just inertia? Am I missing something?

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

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

 

[zeropoint]

So. What is the difference between inertia and momentum then? I feel like I've been reading about inertia all along Or is the momentum just inertia? Am I missing something?

Inertia is the property of matter that causes it to remain in uniform motion unless acted upon by an external force. It can be thought of quantitatively as mass (SI unit kilogram).

Momentum is quite different. The best way to understand it is as "the quantity of motion" which inertia attempts to preserve. If a body is acted upon by some external force, then the rate of change of the object's momentum is directly proportional to that force (Newton's second law). The higher the inertia of an object, the less responsive it is to changes in momentum brought on by the external force. If you like maths, then think of inertia as as the coefficient, m , in the equation of momentum. Or, equivalently, as the same coefficient in newton's second law.

Hope this helps.

James

 

[zeropoint]

So you mean inertia is just a property of all objects, and the momentum describes the size of such property?

The first part is correct, but not the second. I think it's much more accurate to say that inertia describes the "size" of momentum. Although the vice versa is correct mathematically, it doesn't really relate cause and effect in my opinion.

James