The mass of an object or body is a property of its degree of inertia.  Inertia is the resistance of an object to any change in velocity or the application of a force inducing acceleration on the body.


The mass of an object is not dependent on its location or environment but solely on the objects size and material properties.


The mass of an object and its acceleration are inter-related as the product of these two quantities will obtain the net force required to move the object at this acceleration.  It therefore follows that the smaller the mass, the less force is required to move the object at a given acceleration. 


So for an object with a known acceleration and applied net force, the mass of the object can be obtained from the following relationship, Force = mass times acceleration.


Mass is a scalar property which means it is a physical quantity with a defined value but has no direction.  As a scalar quantity, the masses of multiple objects can be added or subtracted.  This means that if you are finding the total net force applied to multiple bodies with different masses and accelerations you can just sum the mass acceleration product for each body.


In the Metric system of units, mass is defined in terms of kilograms, kg, while in the US Customary units it is in terms of (lb-s2)/ft or slugs as it is referred to.


Let’s see an example.


A sledge hammer accidentally falls from a third floor balcony (7 meters above the ground) of a residential structure where a contractor is performing renovation work, it hits the front hood of a car.  The hammer has a mass of 10 kg and the contractor who is performing the work and owns the hammer is liable for the vehicle damages and by a county statute if an object that weighs 100 N (22.5 lbs) or more falls more than 6 meters (20 ft) and causes any harm or damage, then an additional fine for negligence will be assessed.  Will the contractor have to pay the fine?


Using Force = mass times acceleration (F = m x a), where the mass, m = 20 kg and the acceleration due to gravity is 9.80 m/s2, we have F = (10 kg)(9.80 m/s2) =

98 (kg – m)/s2 = 98 N.


So F = 98 N, less than 100 N, and so while the contractor will have to pay for the vehicle damage he will not be assessed the additional fine for negligence.




Serway, R. A., Beichner, R. J., & Jewett, J. W. (2000).  Physics For Scientists and      Engineers, Volume 1 (5th Ed.).  Orlando, FL: Saunders College Publishing.