Newtons are the standard metric units for the force put forth by an object in motion. According to the famous second law of motion put forth by Issac Newton, who gave the unit its name, the force of an object is directly proportional to its mass and inversely proportional to its acceleration, stated mathematically as F = ma. Thus, if you know an object's force in newtons and the rate of acceleration, you can find its mass.
Things You'll Need
 Paper and pencil

Write the formula for finding an object's force in Newtons, F=ma, at the top of the page. Since you are trying to find the mass (m), you will need to isolate m on one side of the equation.

Divide by a on both sides of the equation, and cancel out the a on the right side. This will leave you with F/a = m, which you can switch to m = F/a to get the variable you're looking for on the left.

Plug the numbers you want to convert to mass into the new equation, m = F/a. As an example, we will use an object with a force of 10 N and an acceleration of 2 meters per second squared.

Divide the newtons by the rate of acceleration, which will give you the mass of the object. The mass will be in kilograms, because a single newton represents the amount of force needed to move one kilogram one meter. For our example, we will divide 10 N by 2 m/s/s, which give us a mass of 5 kg.
References
 Photo Credit Photos.com/Photos.com/Getty Images