# What is momentum?

1. Dec 31, 2011

### annie122

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.

2. Dec 31, 2011

### phinds

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. Jan 1, 2012

### caleb5040

You could also think of it as "the quantity of motion" of an object.

4. Jan 1, 2012

### Thundagere

The easiest way for me to think of it is "mass in motion." Momentum is just how much "mass in motion."

5. Jan 1, 2012

### Neandethal00

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. Jan 2, 2012

Staff Emeritus
True, for two objects of the same mass.

Not true.

7. Jan 3, 2012

### ModusPwnd

Well they each have the same unit... the eV :p

8. Jan 3, 2012

Not sure what you mean.The eV is a unit of energy,not momentum.

9. Jan 26, 2012

### stephenn

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. Jan 27, 2012

### Drakkith

Staff Emeritus
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. Jan 27, 2012

### TheLil'Turkey

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. Jan 27, 2012

### sophiecentaur

??? 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.

13. Jan 27, 2012

### ModusPwnd

14. Jan 28, 2012

### sophiecentaur

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. Jan 28, 2012

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. Jan 28, 2012

### sophiecentaur

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. Jan 31, 2012

### ModusPwnd

We used natural units in just about every physics class we had. Its pretty common.

18. Jan 31, 2012

### sophiecentaur

"Natural", meaning what?

19. Jan 31, 2012

### ModusPwnd

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!

20. Jan 31, 2012

### Studiot

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.