Hi!

I need some help for Newton's 2nd Law of Motion!

My teacher gave me this definition:

It states that the rate of change of momentum is directly proportional to the applied force and it takes place in the direction of the Resultant Force.

Really, I didn't understand it! Do you have another definition which is much easier to understand and can you please give me an example where it can be applied?

Thanks in advance! :)

What the teaches said is this:


\vec F = \frac {\Delta (m \vec v)} {\Delta t}


The force appied (which is a vector, meaning it has both a magnitude and a direction), equals the change in momentum of the object \Delta (mv) per unit time.

If you apply a force to something of constant mass (like a ball), this simplifies to:


\vec F = m \frac {\Delta \vec v} {\Delta t}


What it's saying is that if you push the ball to the left, it will accelerate to the left. The fraction \frac { \Delta v} {\Delta t} is the rate of change of the object's velocity, or in other words - it's acceleration.

Now the more accurate definition is the one the teacher gave - where the applied force causes a change in the object's momentum in the direction of the force. The difference is that this version allows for the possibility of the object's mass changing. This doesn't come up too often in elementary physics classes, but consider what happens to an object that grows lighter over time - like a rocket ship. The rocket's motors apply a certain amount of thrust to the rocket, causing it to accelerate upward. As the rocket burns up its fuel it gets lighter, and the result is that as its mass decreases the rate of change of its velocity increases. So it accelerates at an ever faster rate. This effect is properly captured using the definition:


\vec F = \frac {\Delta (m \vec v)} {\Delta t}


Hope this helps.