Newton's Second Law of Motion

Newton's First Law of Motion States that:

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

I know, there are a lot of big words there, so let's break it down.

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force...

What this first part means is that when the magnitude of the force making the object move increases, the acceleration also increases at the same rate.

When you are swimming, the harder you kick, the faster you go, and the less you kick, the slower you go.

... in the same direction as the net force...

What this middle section of the law means is that the direction the object moves is the same direction as that the force causing the object to move.

Since the kick produces a forward force, you also go forward.

...and inversely proportional to the mass of the object.

What this last part means is that as the object gains mass, the acceleration of the object decreases, and when the object loses mass, the acceleration of the object increases

If your coach makes you swim with your shirt on, and your shirt soaks water and becomes heavier, you will go slower. However, if you decide to defy your coach and take it off in the pool, you will go faster because you are lighter.