# Inertia and momentum

1. Nov 22, 2006

### student85

Are they the same thing?
momentum = p = mv

2. Nov 22, 2006

### cesiumfrog

Inertia is more like m

3. Nov 22, 2006

### lotrgreengrapes7926

Interia has no numerical value unlike momentum. It's just a property of an object based on its mass.

4. Nov 23, 2006

### student85

Ok, that's what I thought. But then, how would you define momentum? (in words)

5. Nov 23, 2006

### Virtual R

"momentum is the measure of inertia "
and inertia is the property of the system or the object to resist a change in its state i.e either the state of motion or of rest

6. Nov 23, 2006

### sneez

Inertia is unchanging whereas momentum can be changing. (for all classical approx.)

momentum is concerned with moving things.

7. Nov 23, 2006

### Staff: Mentor

Inertia is related to mass. Momentum is related to mass and speed (as the equation shows).

8. Nov 23, 2006

### student85

Virtual said momentum is the measure of inertia.
Then, velocity must be part of inertia. I think it makes sense since the faster the object is going, the harder it is to change its condition...am I right?

9. Nov 23, 2006

### cesiumfrog

I wouldn't equate inertia with momentum. It's just as hard to stop a fast object as it is, after stopping it, to then speed the object back up again, however the stationary object has infinitely less momentum.

Perhaps a formal definition for inertia (or moment of inertia) should be the force (or torque) with which an object resists a change in its velocity (or in its angular velocity). Consequently, the SI unit for inertia would be kg (or, er, kgm^2/rad).

10. Nov 23, 2006

### Staff: Mentor

Virtual was wrong to say that because....
No, you are clearly not right as f=ma doesn't say anything about velocity, does it? If it got harder to change velocity when speed was higher, those terms should appear in that equation.

(caveat: relativity not needed here)