Force Calculator
Newton’s second law, F = ma, with the weight of the same mass for comparison.
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Your details
Result
- Force
- 140 N
- In kilonewtons
- 0.14 kN
- Weight of the same mass
- 686.465 N
- As a multiple of gravity
- 0.204 g
70 kg accelerating at 2 m/s²
The force gravity already exerts on it at rest
How the acceleration would feel
70 kg at 2 m/s²…
F = m × a
70 × 2
F = 140 N
Formula — written step by step
70 kg accelerating at 2 m/s²…
F = m × a
70 × 2
F = 140 N
F is force in newtons, m mass in kilograms and a acceleration in metres per second squared. One newton is the force that accelerates one kilogram at one metre per second squared.
A worked example
Mass against weight
- Mass
- 70 kg
- On Earth
- g = 9.80665 m/s²
- On the Moon
- g = 1.625 m/s²
- Weight on Earth = 70 × 9.80665 = 686.5 N
- Weight on the Moon = 70 × 1.625 = 113.8 N
- Mass in both places = 70 kg
Same mass, one sixth the weight
Mass measures how much matter there is and never changes with location. Weight is a force and does. A bathroom scale reads weight but is calibrated to display mass, which is why it would be wrong on the Moon while a balance would still be right.
How to use this calculator
- Enter the mass and pick its unit.
- Enter the acceleration in metres per second squared. For a falling object near the Earth’s surface, that is about 9.81.
- Compare against the weight row, which is the force gravity already applies to that mass at rest.
- The "multiple of gravity" row expresses the acceleration in g, which is how pilots and engineers usually talk about it.
Newton’s second law
Force equals mass times acceleration. Read one way it says a bigger force produces more acceleration; read the other it says a heavier object needs more force to accelerate at the same rate. Both are the same statement.
Force is a vector, so direction matters, and what actually determines acceleration is the net force — the sum of everything acting. An object moving at constant speed has zero net force on it, even when several large forces are in play and cancelling.
Mass and weight are different things
Mass, in kilograms, measures how much matter an object contains and how strongly it resists acceleration. Weight, in newtons, is the force gravity exerts on that mass. Everyday language treats them as interchangeable; physics cannot.
The distinction becomes concrete the moment gravity changes. A 70 kg astronaut has the same 70 kg of mass on the Moon but weighs 114 N instead of 687 N. Their inertia is unchanged, which is why moving quickly in low gravity is harder than films suggest.
Why deceleration is the interesting case
The same formula governs stopping, with a negative acceleration. This is where the numbers become alarming: bringing a 1500 kg car from 15 m/s to rest in a tenth of a second requires 225,000 N, which is why collision engineering is entirely about extending that time.
The g-force row is the intuitive reading. Sustained forces above about 5 g cause most people to lose consciousness; brief impacts of 50 g and more are survivable when properly restrained, because the duration is so short.
Formula & methodology
What this calculator assumes
- Gives the net force. Where several forces act, only their vector sum accelerates the object.
- Assumes constant mass. It does not describe a rocket or anything else shedding mass as it accelerates.
- Newtonian, and stated in an inertial frame.
Sources
Forces worth having a feel for
| Situation | Approximate force |
|---|---|
| Weight of an apple (about 100 g) | 1 N |
| Weight of a 70 kg adult | 687 N |
| Weight of a 1500 kg car | 14.7 kN |
| A firm handshake | 100–200 N |
| Bite force of human molars | 700–1,000 N |
A newton is roughly the weight of a small apple, which is the most useful anchor for judging whether an answer is plausible.
Frequently asked questions
What is a newton?
The force that accelerates one kilogram at one metre per second squared. Roughly the weight of a small apple, which makes it a convenient sanity check.
Is weight the same as mass?
No. Mass is in kilograms and never changes; weight is a force in newtons and depends on local gravity. A 70 kg person weighs 687 N on Earth and 114 N on the Moon.
How do I convert kilograms to newtons?
Multiply by the local gravitational field strength — about 9.81 on Earth. Strictly you are converting a mass into the weight force it experiences, not converting between units.
What if several forces act at once?
Add them as vectors. Only the net force accelerates the object, which is why something moving at constant velocity has zero net force despite the forces present.