Force Calculator (F = ma)

Newton’s second law connects force, mass and acceleration: push harder and an object speeds up faster; make it heavier and the same push does less. Enter any two of the three and this calculator finds the third, in any units, and shows the mass’s weight on Earth for comparison.

How to Use It

1. Choose what to Find: Force, Mass or Acceleration.
2. Enter the other two values and pick each one’s unit.
3. Choose the unit for the answer in Answer In.
4. Press Calculate to see the answer and the weight of that mass on Earth.

How It Works

The net force on an object is its mass times its acceleration:

Rearranged, $m = \frac{F}{a}$ and $a = \frac{F}{m}$. One newton (N) is the force that gives 1 kg an acceleration of 1 m/s².

Weight is the same law with gravity as the acceleration: $w = mg$, with $g \approx 9.81 \text{ m/s}^2$ on Earth.

Worked Example

A 1200 kg car speeds up at 3 m/s²:

For comparison, the car’s weight is $1200 \times 9.81 \approx 11{,}770 \text{ N}$.

Tips and Common Mistakes

• Use mass, not weight. Enter kilograms (or grams, pounds) for mass; a weight in newtons is already a force.
• Use the net force. Subtract forces that push the other way, like friction, before using F = ma.
• Check your units. Newtons come from kilograms and m/s²; grams and cm/s² give much smaller numbers.
• Heavier doesn’t fall faster. In free fall, a = g for every mass, because weight grows with mass.
• 1 lbf ≈ 4.448 N. Pick lbf as a unit if your problem is in pounds of force.