Net force
Signed net force along the chosen axis.
Physics · Mechanics · Motion & Energy
Newton’s second law calculator
Find net force, mass or acceleration along a chosen axis. Enter the resultant of all external forces, including friction when present.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
02
Results
Mass
Strictly positive inertial mass.
Acceleration
Signed change in velocity per unit time.
Relation and numerical substitution
F = m × a—
F and a are signed components; m must be positive.
01Formulas and symbols
Formulas used
F
F = m × aSigned net force along the chosen axis.m
m = F / aStrictly positive inertial mass.a
a = F / mSigned change in velocity per unit time.02Assumptions and limits
Scope of validity
- F and a are signed components; m must be positive.
03Validation example
Reference numerical case
- m = 10 kg and a = 3 m/s²: F = 10 × 3 = 30 N. Inverse modes give m = 30/3 = 10 kg and a = 30/10 = 3 m/s².
04References
Technical references
FAQ
What is the difference between force and weight?
Weight is one particular force: the gravitational force on a mass, approximately W = mg near Earth. Newton’s second law uses the net force, which is the vector sum of weight and every other external force acting on the body.
Can acceleration be negative in F = ma?
Yes. The sign only indicates direction relative to the chosen positive axis. For a positive mass, net force and acceleration always have the same sign along a given axis.
05How it works
A force changes velocity, not position directly. At fixed mass, doubling the net force doubles acceleration. At fixed force, doubling mass halves acceleration. Signed force and acceleration must refer to the same positive axis.
06Limits of the model
Constant positive mass in an inertial frame, at nonrelativistic speeds. This scalar calculation does not add vector forces or account for changing mass. Zero acceleration and zero force do not determine a unique mass.
07Common mistakes
Use net force, not one applied force when others act. Negative acceleration is not necessarily slowing down: compare its sign with velocity. An inferred mass from opposite force/acceleration signs is unphysical.