Newton's Second Law Calculator
Calculate force, mass, or acceleration using Newton's Second Law.
Enter any two values to solve for the third and see results in both SI and imperial units.
Newton’s Second Law is one of the most fundamental equations in physics. It describes how forces change the motion of objects.
F = m x a
Force (F) in newtons equals mass (m) in kilograms times acceleration (a) in meters per second squared. Rearranged, it solves for any of the three variables:
a = F / m (acceleration from force and mass) m = F / a (mass from force and acceleration)
What makes this law profound is what it says about equilibrium. If F = 0, then a = 0 — constant velocity or rest. Every time you see an object moving at constant speed, all forces are balanced. Every time you see acceleration, there is a net unbalanced force.
Real-world examples:
- A 1,000 kg car accelerating at 3 m/s² requires a net force of 3,000 N from the engine (after subtracting friction and drag).
- Gravity accelerates all objects at 9.81 m/s² (on Earth’s surface). The force is F = mg — this is simply Newton’s Second Law applied to gravitational acceleration.
- A 70 kg skydiver in free fall feels 70 × 9.81 = 687 N (weight). Terminal velocity occurs when drag force = 687 N and net force = 0.
The law applies to the net force. If a 100 N applied force is opposed by 30 N of friction, the net force is 70 N and that is what drives the acceleration.
In imperial units, force is measured in pound-force (lbf), mass in slugs (1 slug = 14.59 kg), or you can use weight in pounds (lb) divided by 32.2 ft/s² for g.