Impact Energy Calculator
Find the energy delivered when a dropped or moving object hits something, the matching fall height, and the average force from the stopping distance.
🔨 What is the Impact Energy Calculator?
Impact energy is the energy an object carries into a collision, and therefore the energy that the thing it hits, or the object itself, has to absorb. For a falling object it comes from the gravitational potential energy lost during the drop, E = m g h. For a moving object it is the kinetic energy, E = ½ m v². The two are the same number when an object falls freely and nothing else takes energy away, which is why this calculator treats a drop height and a speed as two ways of describing one quantity, measured in joules.
The calculator has three modes. Drop from height takes a mass, a height and a gravity value (Earth, the Moon, Mars or your own) and returns the impact energy, the impact speed in three units, the momentum and the fall time. Moving object takes a mass and a speed in m/s, km/h or mph and returns the kinetic energy and the equivalent drop height h = v² / (2g), the height from which a free fall would reach that speed. Impact force adds the missing ingredient for forces, the distance or time over which the object stops, and returns the average force and the deceleration in multiples of g.
Typical uses include a quick check of how hard a dropped tool, tile or box will land, translating the speed of a vehicle or a ball into an intuitive fall height, comparing how a crumple zone or a bent knee spreads a stop over a longer distance, and teaching the work-energy theorem. The equivalents in the results (lifting 1 kg by a given height, a 1 kg mass moving at a given speed, kilowatt hours) are computed directly from the energy, so they are exact conversions, not statistics.
Two points are often misunderstood. Energy and force are different things: the same energy can be delivered as a modest force over a long stop or as an enormous force over a short one, so the energy alone does not say how damaging an impact is. And the force in the third mode is an average. Real impact forces peak above the average, part of the energy turns into heat and sound, and the stopping time depends on how the materials deform. For the peak and the time dependence, use the impulse and momentum calculator. For the energy of a moving object alone see the kinetic energy calculator, and for long falls where drag matters see the terminal velocity calculator.
📐 Impact Energy Formulas
For a drop, energy conservation says the potential energy m g h is converted into kinetic energy ½ m v², which gives v = √(2 g h) and shows that the mass cancels out of the speed. For the stop, the work-energy theorem states that the net work done on an object equals its change in kinetic energy. A constant average force F acting over a stopping distance d does work F d, and stopping the object removes all of its kinetic energy E, so F d = E. If the deceleration is steady, the average speed during the stop is v/2, so d = v t / 2, and the same force follows from the impulse form F = m v / t. The equivalent drop height is just the energy divided by m g, which is why it depends on speed and gravity but not on mass. In a vertical landing the surface also has to hold up the weight, so the force it exerts is F + m g.
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — A tool dropped from 3 m
2 kg tool dropped from 3 m on Earth (g = 9.80665 m/s², no air resistance)
Example 2 — Car hitting a barrier with a crumple zone
1,500 kg car at 50 km/h stopped by 0.5 m of crumple (illustrative values)
Example 3 — Jumping from 1 m with bent knees
70 kg person lands after a 1 m drop and stops over 0.5 m of knee flexion (illustrative, not a safety limit)
Example 4 — A baseball at 40 m/s
0.145 kg baseball travelling at 40 m/s (illustrative values)
Example 5 — A car at 50 km/h as a fall height
1,500 kg car at 50 km/h, expressed as a drop height
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate impact energy from a drop height?
Multiply mass, gravity and height: E = m g h. A 2 kg tool dropped from 3 m on Earth arrives with E = 2 x 9.80665 x 3 = 58.84 J. This equals the kinetic energy at impact, E = 1/2 m v^2, if air resistance is ignored. The calculator also gives the impact speed, momentum and fall time.
How do you find impact speed from drop height?
Use v = sqrt(2 g h). From 3 m on Earth, v = sqrt(2 x 9.80665 x 3) = 7.67 m/s, about 27.6 km/h. The mass does not appear in the formula, so a heavy and a light object dropped together reach the same speed in a vacuum. Air resistance lowers the real speed for light or bulky objects.
What is the equivalent drop height of a speed?
It is h = v^2 / (2 g), the height from which a free fall would reach that speed. At 50 km/h (13.889 m/s) on Earth it is 9.84 m. The comparison is useful because it expresses a car speed as a fall height, but it only matches energy per kilogram, so the same height means very different energy for different masses.
How do you calculate average impact force?
Use the work-energy theorem: the force times the stopping distance equals the energy removed, so F_avg = E / d. A 1,500 kg car at 50 km/h has 144,676 J. Stopping in 0.5 m of crumple needs an average force of about 289,352 N. The number is an average over the stopping distance, not the peak.
Why does a longer stopping distance reduce the impact force?
The energy to be removed is fixed by mass and speed, and work equals force times distance, so F = E / d. Doubling d halves F. That is the principle behind crumple zones, padded surfaces, airbags and bending your knees on landing. The force per unit of stopping distance is what the chart on the page shows.
Does the mass or the height matter more for impact energy?
Both enter linearly in E = m g h, so doubling either one doubles the energy. Speed behaves differently: it depends only on height, and grows with the square root of height. Doubling the height gives 1.41 times the speed but 2 times the energy, and doubling the speed of a moving object gives 4 times the energy.
How is impact energy different from impact force?
Energy, in joules, is the total amount that has to be absorbed. Force, in newtons, is how hard the object pushes during the stop. The same energy can produce a small force over a long stopping distance or a huge force over a short one. Force needs the stopping distance or time as an extra input, as the third mode of this calculator does.
Does the calculator include air resistance?
No. Drop mode assumes free fall with constant gravity, which is accurate for short drops of dense objects and overestimates speed for long falls or light, bulky objects. Once drag matters, the speed approaches a terminal velocity that is lower than sqrt(2 g h). Use the terminal velocity calculator for that case.
What gravity values does the planet menu use?
Earth uses the standard gravity 9.80665 m/s squared, the Moon 1.62 m/s squared and Mars 3.721 m/s squared. The menu only fills the gravity box, so you can still type any other value. On the Moon the same 2 kg tool dropped from 3 m has energy 9.72 J and arrives at 3.12 m/s.
How many joules is a drop of 1 kg from 1 m?
On Earth, E = 1 x 9.80665 x 1 = 9.80665 J, so roughly 9.8 J. The impact speed is 4.43 m/s. In the other direction, 9.80665 J is also the energy needed to lift a 1 kg mass by 1 m, which is why the calculator reports the height you could lift 1 kg with the impact energy.
Why is the real force higher than the average force?
The average spreads the stop evenly over the distance, but real forces rise, peak and fall, and part of the energy goes to heat, sound and permanent deformation. The peak therefore exceeds the average. The impulse and momentum calculator shows peak factors for simple pulse shapes. Never use the average as a pass or fail limit.