Force Calculator (F=ma)
Calculate force, mass, or acceleration using Newton's Second Law (F = ma). Includes unit conversions to pounds-force and dynes.
About this calculator
This calculator solves Newton's Second Law, F = ma, for whichever of the three quantities you're missing: Force, Mass, or Acceleration. The Solve For selector determines which of the two remaining number fields are treated as the known inputs -- when solving for Mass, the Force and Acceleration fields feed the calculation and the Mass field is ignored; when solving for Acceleration, Force and Mass are the knowns and the Acceleration field is ignored; and when solving for Force (the default), Mass and Acceleration multiply directly and the Force field is ignored. In Force mode, raising either Mass or Acceleration raises the calculated Force proportionally, since F = ma is a simple product with no caps or diminishing returns. In the other two modes, Force is instead taken directly from the Force field: in Mass mode, raising Acceleration has no effect on Force (it only changes the recovered Mass), and in Acceleration mode, raising Mass has no effect on Force (it only changes the recovered Acceleration).
Weight on Earth converts the resulting Mass into a force using standard surface gravity (9.81 m/s²), regardless of which Acceleration value you entered elsewhere -- it always represents what that mass would weigh sitting still on Earth's surface, not the force computed from your chosen acceleration. Force (lbf) and the dyne conversion are pure unit re-expressions of the same Force result (1 N = 0.2248 lbf = 100,000 dyn), so they always move in lockstep with it rather than being computed independently. This is classical Newtonian mechanics only: it has no relativistic correction, assumes a single net force with no friction, drag, or opposing forces to subtract, and Weight on Earth is not adjusted for altitude, latitude, or any other planet's gravity.
Inputs
Results
Force
98.1 N
How to Use This Calculator
- Select the variable to solve for: force, mass, or acceleration.
- Enter the two known values in their respective units (N, kg, m/s2).
- Review the calculated result and the formula F = ma.
- Check weight on Earth as a reference (weight = mass x 9.81 m/s2).
- Use for physics problems, engineering load calculations, or mechanism design.
How the result changes with Mass
| Mass | Force |
|---|---|
| 5 | 49.05 N |
| 7.5 | 73.58 N |
| 15 | 147.15 N |
| 25 | 245.25 N |
What each input means
- Solve For
- The variable to solve for.
- Mass
- Mass of the object.
- Acceleration
- Earth's gravity = 9.81 m/s²
- Force
- Used when solving for mass or acceleration.
How this is calculated
Formula
F = m × a (Newton's Second Law). Force in Newtons equals mass in kilograms multiplied by acceleration in m/s².Worked example, using the default values
- Identify Input Parameters4 parametersSolve For = 0, Mass = 10, Acceleration = 9.81, Force = 98.1 = 4 input(s) provided
- Calculate ForceForce = resultForce98.1 = 98.1
- Calculate MassMass = resultMass10 = 10
- Calculate AccelerationAcceleration = resultAcceleration9.81 = 9.81
Engine last updated . Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Which fields do I fill in to solve for Mass?
Set Solve For to Mass (F), then enter the known Force and Acceleration -- the calculator divides Force by Acceleration to recover Mass. The Mass field itself is ignored in this mode, so whatever value sits in it has no effect on the result; only Force and Acceleration drive the calculation.
Does raising Acceleration always increase the calculated Force?
When Solve For is set to Force (the default mode), yes: increasing Acceleration at any fixed Mass raises Force proportionally across this calculator's full range, since Force is simply Mass times Acceleration with no cap. In the other two modes Force is taken directly from the Force field instead, so Acceleration doesn't drive it there.
Why does Weight on Earth stay the same even if I change Acceleration?
Weight on Earth is the calculated Mass multiplied by a fixed 9.81 m/s² standard gravity figure, not by whatever value you entered in the Acceleration field -- so Acceleration has no path into that particular output. Raising the Mass field always raises Weight on Earth in Force and Acceleration modes, since the Mass field directly drives the calculated Mass in both. In Mass mode, though, the Mass field itself is ignored (see the previous question) -- Weight on Earth instead follows the computed Mass (Force divided by Acceleration), so typing a different number into the Mass field has no effect there. In every mode, it represents what the calculated mass would weigh at rest on Earth's surface, independent of any acceleration scenario you're solving.
Why do Force (lbf) and the Newton figure always change together?
Because they're the same physical result expressed in two units, not two separate calculations -- Force (lbf) is just Force in Newtons multiplied by the fixed conversion factor 0.2248. Anything that moves the Newton result moves the pound-force figure by the identical proportion, so they can never diverge from each other.
Is this calculator valid for very high speeds or astronomical masses?
No -- it applies plain Newtonian mechanics with no relativistic correction, so it should not be trusted near the speed of light, where mass and force behave very differently under special relativity. For everyday engineering and physics problems, ordinary Earth-bound masses and accelerations, it's the correct and standard formula.
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