# Homework Help: Mass problem

1. Dec 10, 2009

### DerekP

3. Suppose a meteorite collides with Earth with a force of 6.41*1012N.

A. What is the mass of the meteorite if it's impact speed is 10 km/s so that it has an acceleration of approximatley -1.00*108 m/s2

I know I want to use an equation from Newton's second law but what equation do I use? and how to I change it so I can plug it in(if I need to)?

I figured out I want to use $$\sum_{\rm all}\bold{F}\,=\,m\frac{d(\bold{v})}{dt}\,=\,m \bold{a}$$ (I believe) and I know I need these can anyone help me?

Acceleration = Net force divided by mass

Net force = rate of change of momentum

Net impulse = change of momentum

Last edited: Dec 10, 2009
2. Dec 10, 2009

### denverdoc

Is there a chance that you have more info than needed? Which one of Newton's laws relates mass, force, and acceleration?

3. Dec 11, 2009

### Redbelly98

Staff Emeritus
Newton's second law has only 1 equation. Look in your textbook.

Yes, that equation is Newton's 2nd Law.

Of the various terms listed in the equation (F, m, a etc.), which ones are given in the problem statement? Which one are they asking for?

4. Dec 11, 2009

### dr_k

Derek,

First draw a picture:

http://img709.imageshack.us/img709/7478/fma1.jpg [Broken]

In this picture, the meteorite will apply an external force on the Earth (during the collision), pointing to the right. By Newton's 3rd postulate, the Earth will apply an equal and opposite force on the meteorite to the left. But this makes physical sense, since the meteorite will slow down, and come to rest, so its acceleration vector must point in the opposite direction of its velocity vector.

Isolate the meteorite (in the dashed line circle), and apply Newton's 2nd postulate. As others were pointing out, in this thread, you don't need to know the meteorite's speed to answer the question posed. Hope this helps.

If we zoom in on the collision, which lasts $$\Delta t$$ you'll see that a distance is required to bring the meteorite to rest. So there's going to be a crater. Hopefully, it's a small crater!

http://img693.imageshack.us/img693/7120/fma2z.jpg [Broken]

Last edited by a moderator: May 4, 2017