# Problem with newtons second law

1. Sep 3, 2010

### Physonic

hello!

here's my problem:
newtons second law implies: F=m*dv/dt+v*dm/dt
first part of the equation says if I act on the object then it's reaction will be acceleration, but the second part says if act on the object it will cause changes in object's mass.
In case where I'm dealing with non relativistic case how applied force can change object's mass?

thank you!

2. Sep 3, 2010

### phyzguy

In the non-relativistic case the applied force doesn't change the object's mass, so the second term is zero.

3. Sep 3, 2010

### Dr Lots-o'watts

A practical example is how the mass of a vehicle decreases as fuel is burnt.

4. Sep 3, 2010

### D H

Staff Emeritus
It's the other way around. F=dp/dt=m*dv/dt+v*dm/dt is one definition of force. This equation is not saying what forces do, it is giving a definition of what a force is in terms of observable effects. Both of those terms on the right hand side are observable effects. There are other definitions of force, by the way. Some prefer F=ma as definitional, for example.