# Variation on momentum q

1. Aug 9, 2011

### physsurveyor

If a 1kg object is moving at 3m/s in a positive direction, and a 12N force is applied in the negative direction, what is the velocity immediately after 2s?

I'm fairly sure this will be a variation of relevent momentum equations, and/or mixed with kinematics, yet I'm not seeing the correct application. I lean towards answers c or b.

Obviously mo will be conserved.

conservation of mo: m1Vi= (m1+m2)vf t=v/a

a. 12m/s
b. 6 m/s
c. 3 m/s
d. 0 m/s

2. Aug 9, 2011

### Modulated

I'm afraid you can't use conservation of momentum here - that only applies in the absence of an external force (in the relevant direction), and here the problem says there's a 12 N force. Do you know any other way you might go about solving this?

3. Aug 9, 2011

### Darth Frodo

What you need is the impulse formula. Whenever time is involved for a force it's almost always Impulse.

Definition: The Impulse (I) imparted onto a body is the change in it's momentum. Impulse is a vector quantity.

Derivation: F = ma

F = m (v-u)/t
F = mv - mu /t
Ft = mv -mu

Unit is obviously Newton seconds.

4. Aug 9, 2011

### physsurveyor

Yes, I do believe this is on the right track, so thanks, but

I believe I'm remiss in something. What is u?

I'm recalling something like.......Ft=mvf-mvi (change in p)

Last edited: Aug 9, 2011
5. Aug 10, 2011

### Darth Frodo

u = initial velocity