What is momentum?

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I know momentum is mass times velocity, and that it is a conserved quantity, but I can't get the intuition of what momentum is, unlike mass and velocity.
Mass relates to how heavy an object is, and velocity is how fast an object is moving.

One idea I had is that momentum is how hard an object hits me, but I'm not entirely sure if that's an okay thing to say.
 

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  • #2
phinds
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Yeah, momentum is how hard something hits you ... that's not a bad way to think of it.

If it's moving faster, it hits you harder. If it's heavier, it hits you harder. If it's both heavier and faster, you really should get out of the way.
 
  • #3
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You could also think of it as "the quantity of motion" of an object.
 
  • #4
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The easiest way for me to think of it is "mass in motion." Momentum is just how much "mass in motion."
 
  • #5
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I know momentum is mass times velocity, and that it is a conserved quantity, but I can't get the intuition of what momentum is, unlike mass and velocity.
Mass relates to how heavy an object is, and velocity is how fast an object is moving.

One idea I had is that momentum is how hard an object hits me, but I'm not entirely sure if that's an okay thing to say.

Momentum can also be thought of as Kinetic energy.

KE = p2/2m

p = SQRT(2m*KE)

The higher the KE, the higher the momentum.
 
  • #6
Vanadium 50
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The higher the KE, the higher the momentum.

True, for two objects of the same mass.

Momentum can also be thought of as Kinetic energy..

Not true.
 
  • #7
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Well they each have the same unit... the eV :p
 
  • #8
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Well they each have the same unit... the eV :p

Not sure what you mean.The eV is a unit of energy,not momentum.
 
  • #9
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momentum p is indeed mass m x velocity v
p = mv

mass..... is not actually the weight.... weight is mass x gravity..... so mass of something is its weight then deduct any gravity it was weighed in ie earth, moon etc

velocity..... though is not actually speed..... it is a speed and a direction..... 50mph east
 
  • #10
Drakkith
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Kinetic energy scales at 1/2 mv^2. When you double your speed you quandruple your kinetic energy. This does not happen with momentum. It scales linearly at p=mv.
 
  • #11
If 2 objects made of the same material have the same momentum, but different KE, the little one will be harder to stop. If they both have the same KE but different momentum, the big one will be harder to stop.
 
  • #12
sophiecentaur
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Well they each have the same unit... the eV :p

??? Momentum has the Unit Ns (Newton seconds) but KE has the unit J (Joules). Not the same at all.
Momentum is a Vector quantity - it has a direction associated with it. KE has no direction specified because it is a Scalar, not a Vector.

Momentum is conserved in all collisions. Kinetic Energy is not.

But, as an object speeds up, both its momentum and KE increase - so there is a kind of association between them.
 
  • #14
sophiecentaur
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That article claims that "in high energy Physics, electron-volt is often used as a unit of momentum". I really doubt that, unless they are talking in some very isolated context. It sounds so wrong that I can't take that sentence seriously. Wiki can often be wrong and, more often, be written badly or with insufficient editing.
Whatever the article says, it is important to realise that they are two distinct quantities. I can't think anyone would claim otherwise.
 
  • #15
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That article claims that "in high energy Physics, electron-volt is often used as a unit of momentum". I really doubt that, unless they are talking in some very isolated context. It sounds so wrong that I can't take that sentence seriously. Wiki can often be wrong and, more often, be written badly or with insufficient editing.
Whatever the article says, it is important to realise that they are two distinct quantities. I can't think anyone would claim otherwise.

I had the same reaction when I first read it but then the paragraph goes on to describe that momentum can be described by eV/c.I'm guessing that if there are people who use the eV as a unit of momentum then it is implied(though not stated) that c is included as above.If so it seems a bit sloppy to me.
 
  • #16
sophiecentaur
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Sloppy, yes. But you get this sort of thing with terminology within specialised fields. Very confusing for the outsider. (Perhaps that's why it's used?)
 
  • #17
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That article claims that "in high energy Physics, electron-volt is often used as a unit of momentum". I really doubt that, unless they are talking in some very isolated context. It sounds so wrong that I can't take that sentence seriously. Wiki can often be wrong and, more often, be written badly or with insufficient editing.
Whatever the article says, it is important to realise that they are two distinct quantities. I can't think anyone would claim otherwise.

We used natural units in just about every physics class we had. Its pretty common.
 
  • #18
sophiecentaur
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"Natural", meaning what?
 
  • #20
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Good King Hal established the only system of Natural Units of any importance and the Americans, God bless their cotton socks, are still using some of them.
 
  • #21
sophiecentaur
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The system of natural units.

http://en.wikipedia.org/wiki/Natural_units

?? Im not sure where the confusion lies. Have you really not heard of natural units before? Your professors are doing you a disservice!


HAHA. My professors are nearly all dead by now, I reckon.

I still think that a system of units which draws no distinction between Momentum and Energy is asking for trouble.

But I can't find a reference to Momentum in that link. The link itself seems to make good sense. Is there some confusion somewhere in this thread, perhaps?
 
  • #22
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There is a distinction, but they are also intimately related. Both energy and momentum are the constituents of the 4-momentum. You boost between the two and turn your energy into momentum and back. In this sense, I think you can think of momentum as energy (or vise versa).
 
  • #23
sophiecentaur
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What does "boost" mean, please?
 
  • #24
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I think modus is referring to the four vector pμ for a relativistic particle formed from its three spatial momenta and its energy divided by c.

pμ = the four vector {px, py, pz, iE/c}

The scalar product pμpμ is invariant

pμpμ = p2x + p2y+ p2z - E2/c2

= -m02c2 = a constant
 
  • #25
sophiecentaur
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I have yet to see anything to tell me that momentum and energy are the same, though. I know that mass and energy are equivalent but are they also both equivalent to momentum? I don't think so.
 

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