# Mass, Inertia and Velocity Question

1. Feb 10, 2010

### jason006

I know that Inertia is an objects resistance to a change in its velocity and Mass is the amount of Inertia which an object posseses so here is my question:

When an object moves at an increasing velocity, wont the amount of force needed to change its velocity increase? Therefore the object has a greater inertia and therefore a greater mass.

Thanks

P.S If this question sounds a bit stupid its because I recently started Grade 11 at school in January and was just introduced to Momentum, Newtons Laws, Impulse and Inertia

2. Feb 10, 2010

### Staff: Mentor

No. Forgetting about special relativity for the moment, a given force on an object will produce the same acceleration (the rate of change of velocity) regardless of the object's velocity. Mass doesn't increase with speed.

3. Feb 10, 2010

### jason006

But what about a force in the opposite direction than that of the force causing the object to accelerate? Wouldnt that opposite force needed to change the objects velocity be greater if the object was moving with a greater velocity

4. Feb 10, 2010

### Staff: Mentor

No. Why do you think that? Can you give a more specific example of what you mean?

Perhaps you are thinking that it requires more energy to stop a fast moving object than a slower moving object? That's certainly true. But for a given net force, the object's acceleration is the same regardless of its speed.

5. Feb 10, 2010

### jason006

Yes, so if you so more energy is needed to stop a fast moving object then its inertia would be greater wouldnt it?

6. Feb 10, 2010

### bp_psy

No only its momentum.

7. Feb 10, 2010

### Staff: Mentor

No. It's kinetic energy (and momentum) is greater, not its mass. Since the mass remains the same, a given force will produce a given acceleration. So if something is moving faster, you'll have to exert that force for a longer time to stop it compared to if it were moving slower. But the rate at which the velocity changes will be the same.

8. Feb 10, 2010

### jason006

oh yes. thanks a lot