Force Calculator
Calculate force, mass, or acceleration using Newton's Second Law F = ma. Supports Newtons, pounds-force, and kilogram-force.
kg
m/s²
N
Force (Newtons)
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Force (lbf) —
Force (kgf) —
Mass (kg) —
Acceleration (m/s²) —
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kg
m/s²
Force (N)
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Force (lbf) —
Force (kgf) —
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kg
m/s²
N
Force (Newtons)
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Force (lbf) —
Force (kgf) —
Force (kN) —
Mass (kg) —
Acceleration (m/s²) —
Pressure (Pa = N/m²) —
Weight at g=9.81 (N) —
How to Use This Calculator
- Select what to solve for: Force, Mass, or Acceleration.
- Enter the two known values.
- Results show the answer in Newtons, pounds-force, and kilogram-force.
Formula
F = m × a | m = F ÷ a | a = F ÷ m
where F = Newtons (N), m = kilograms (kg), a = m/s²
Example
Example: m = 70 kg, a = 9.81 m/s² (gravity) → F = 686.7 N = 154.4 lbf (weight of a 70 kg person).
Frequently Asked Questions
- Newton's Second Law of Motion states that Force = Mass × Acceleration (F = m × a). Force is measured in Newtons (N), mass in kilograms (kg), and acceleration in meters per second squared (m/s²). One Newton is defined as the force needed to accelerate a 1 kg object at 1 m/s². The law can be rearranged: m = F ÷ a (find mass from force and acceleration) or a = F ÷ m (find acceleration from force and mass). This law is fundamental to classical mechanics and applies to all everyday motion — from a car accelerating to a rocket launching.
- One Newton (1 N) is the SI unit of force, defined as the force required to accelerate a 1 kilogram mass at 1 m/s². It is a relatively small force: a medium apple (approximately 100 g) at rest on Earth exerts a downward force of about 1 N. For reference: 1 N ≈ 0.2248 pound-force (lbf), so 1 pound-force ≈ 4.448 N. Larger forces are often expressed in kilonewtons (kN = 1,000 N) in engineering. A typical car engine produces 100–400 kN of peak force, and a rocket engine can produce millions of Newtons.
- Standard gravity on Earth's surface is g = 9.80665 m/s² (approximately 9.81 m/s² in everyday use). This means any freely falling object (ignoring air resistance) accelerates toward Earth at 9.81 m/s² — gaining 9.81 m/s of speed every second. The weight force of any object equals its mass × g: a 70 kg person weighs 70 × 9.81 = 686.7 N on Earth. On the Moon (g ≈ 1.62 m/s²) that same person weighs only 113 N. On Mars (g ≈ 3.72 m/s²) they weigh 260 N. Mass (kg) never changes; weight (N) depends on local gravity.
- Weight (N) = Mass (kg) × g (m/s²), where g = 9.80665 m/s² on Earth's surface. For example, a 50 kg suitcase weighs 50 × 9.81 = 490.5 N. To use this calculator for weight, enter the mass in kg and set acceleration to 9.80665 (or 9.81) m/s². The result in the Force (Newtons) field is the gravitational weight. The calculator also shows the weight in pounds-force (lbf): 490.5 N × 0.2248 = 110.2 lbf. In everyday speech "weight" often means mass (kg), but scientifically, weight is a force measured in Newtons.
- Mass is the quantity of matter in an object, measured in kilograms (kg). It is a scalar quantity that does not change regardless of location — a 70 kg person has 70 kg of mass whether on Earth, the Moon, or in space. Weight is the gravitational force acting on that mass, measured in Newtons (N). Weight = mass × gravitational acceleration (g). Weight changes with location: on Earth (g = 9.81 m/s²), a 70 kg person weighs 686.7 N. On the Moon (g = 1.62 m/s²), the same person weighs only 113.4 N. Scales measure force (weight); balances measure mass.