# Momentum vs inertia

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1. Jan 26, 2016

### Fez98

Hi. So I know that inertia (the resistance of an object to a change in its velocity) is reliant on mass. I also know that momentum is mass times velocity. So momentum is really inertia times velocity. My question is: if that's so, is momentum then not a measure of a moving object's inertia, its resistance to change in speed or direction? Please try to not get into really heavy answers, I haven't taken a single physics class, but this is a personal interest. Thank you :)

2. Jan 26, 2016

Staff Emeritus
That's correct. Inertia and momentum are not the same thing.

3. Jan 26, 2016

### Fez98

Wait, so what IS momentum? If something has more momentum, it's common sense that it's harder to stop, or change direction. Like a truck going at 1 mph vs 120 mph

4. Jan 26, 2016

### BvU

Momentum is amount of movement. Proportional to mass and proportional to speed.

Inertia is 'resistance' against acceleration. Proportional to mass only.

5. Jan 26, 2016

### Fez98

Yes, but something with a lot of momentum is hard to stop or change direction, right? So it seems that more momentum means its harder to change speed or direction

6. Jan 26, 2016

Staff Emeritus
Fez, I don't think I can help you. You have an answer, but you don't seem to like it.

7. Jan 26, 2016

### BvU

I agree. Are you familiar with Newton's laws ? $\vec F = m\,\vec a\ \$ can also be written as $\vec F = {\displaystyle {d\vec p \over dt}}\ \$ (in fact this is even a better form: it also holds true when $m$ is not a constant)

8. Jan 26, 2016

### Fez98

But then what causes my example to happen?

9. Jan 26, 2016

### BvU

No. Changing the speed of a mass of 1 kg with a speed of 1 m/s to a speed of 2 m/s is not 'harder' than changing the speed of a mass of 1 kg from 10 m/s to 11 m/s.

10. Jan 26, 2016

### A.T.

The same force will cause the same (rate of) change in momentum, regardless of speed. But for a faster object you need more change in momentum to bring it to a halt, or to change its direction by some degrees.

11. Jan 26, 2016

### Fez98

Oh, OK, that makes sense, thank you

12. Jan 26, 2016

### PeroK

In fact, momentum is not an absolute quantity of an object but depends on your reference frame. Imagine your truck going at 120mph and you put a wooden barrier in front of it. The barrier would be smashed. But, if you were in another vehicle going along beside the truck at 119 mph, and you put the same wooden barrier out, then the truck would only hit the barrier at 1mph and the barrier would just get a little bump as the truck overtook you.

This ties in with what you've learned that it is just as easy (or difficult!) to slow a truck from 120 mph to 119 mph as it is to slow it from 1mph to 0. And, in a fundamental way, these two scenarios are, in fact, identical. It's only the frame of reference that has changed.

13. Jan 26, 2016

### sophiecentaur

Inertia is not really a concept that's needed when dealing with simple mechanics problems. Mass is a perfectly good quantity that describes 'how hard' it is to accelerate an objet (Newton's Second Law of motion)
The only time that I am aware of 'inertia' is when I really can't be bothered to get up off my backside and do those jobs that need doing around the house. In that context, 'mass' is not the right term to use.

14. Jan 26, 2016

### BvU

Would that be a silverback-side ?

15. Jan 30, 2016

Inertia is a conceptual quantity that is rather unique in physics. It is *not* a physical quantity since it has no units. It's very much related to the physical quantity of mass but is not exactly ≡ same thing .

16. Jan 30, 2016

### sophiecentaur

I think the term manages to confuse rather than to enlighten any argument. It's a sort of semi-official alternative to arm waving when it's introduced into Physics discussions. It allows one to step back from the implied rigour of using the word Mass and to let intuition to sneak in.

17. Jan 30, 2016

### Zachary Smith

It would require more energy though, would it not? force times distance.

18. Jan 30, 2016

### kaustubhan

Hi, i think the question is a very good one. My own findings from articles on the subject are, that originally Johannes Kepler introduced the concept of Inertia as "a property of an object to resist motion", this was at a time when it was thought that "rest" is the default state of things, and forces are needed to create motion. Later on, Copernicus and then Newton proved by the 1st Law that "motion" is the default state of all things in the universe, and in fact net external forces are needed to bring things to "a state of rest". With this , the 2nd Law said F = ma, => a = F/m where mass came to be referred to as "Inertial Mass". The mass became the fundamental property of matter . It defined the tendency of an object to get a "change in velocity per time" , when any Force acts on it. So, if mass is large, a = F/m should be a low acceleration.

The original question was :- is the Momentum a measure of an object's change in Inertia ?

Inertial Mass is not the same as Inertia. Mass is the correct term, and is the right straightforward measurable property. Of course, there are 2 kinds of definitions floating around- a) Gravitational Mass, and b) Inertial Mass. The latter, Inertial Mass, is considered the superior reference today, because it meausures "m" from " a= F/M" and not by using a weighing scale. Example, for a Space Station in orbit around the earth, due to weightlessness, there is no "g", and two different astronauts if put on a "weighing scale" will show the same reading , but we know this not correct. Hence, they use a special seat to apply some external Force and measure "a" , and from that, they measure the "m". This is Inertial Mass and not Gravitational Mass.

Coming to Momentum concept , I think that got really complicated by the Theory of Relativity. Copernicus in fact said that a ship that starts moving can continue moving around the earth unless stopped by an external force. But later, in Einstein's two theories of relativity, the point was "provided we stand in a frame of reference such that the ship is seen to be moving" . What if our frame of reference is such that the ship appears stationary? Then Velocity will appear to be =0 , and the Momentum P = mv =0. Hence Momentum itself is not a good measure, rather Change in Momentum will be a good measure. That bring us back to acceleration , and Newton's 2nd law, a=F/m.

Actually, the ship could be moving with a Momentum from an earth's frame of reference. To overcome this problem, it was agreed by scientists that " change in velocity per unit time " will be more reliable way to find Inertial Mass, as it will get detected by any frame of reference. That would work for normal objects which are not moving at speed of light etc etc .

Let me know if I am making sense.

19. Jan 30, 2016

### sophiecentaur

Energy is frame dependent and that can involve some confusing scenarios.

But what do we mean by "harder" in this quote?:
The momentum change would be the same but not the KE. The Impulse would be Force times time but the the Energy added would be Force times distance.

20. Jan 30, 2016

### PeroK

Good point!

It's worth having a think about this as it's trickier than you might imagine. For Earth-bound motion it's true that the faster you move, the more energy it takes to accelerate. And, the "harder" it is to accelerate. The key factor is the speed relative to the thing that you're using to propel yourself (e.g. friction on a road surface) and the effect of air and other resistance.

But, if you were in a vacuum and you propelled yourself by momentum exchange: throwing or firing things out the back of your spacecraft, then it would make no difference how fast you are travelling with respect to the Earth, say. (To keep it simple, let's consider non-relativistic velocities.)

If you look at post #8, it's interesting to consider what is the same in the two scenarios and what is different: you travelling at 119 mph and the truck at 120 mph; and you at rest and the truck travelling at 1mph. On the one hand, there is no such thing as absolute motion, on the other hand, they are clearly not the same.