Use this calculator to find impact energy, impact speed, momentum, and optional average impact force from mass, speed, or drop height.
Advanced options
How to use our Impact Energy Calculator
- Choose Calculation mode: use Mass + speed if you know the impact speed, Mass + drop height if the object falls before impact, or Solve for missing if you want the calculator to find one value from the others.
- Enter Mass (selected unit) and pick the matching Mass unit. Mass must be greater than 0.
- If you chose speed mode, enter Impact speed (selected unit) and its Speed unit. If you chose drop mode, enter Drop height (selected unit) and Drop height unit.
- If you chose Solve for missing, select whether you want to find Impact energy, Impact speed, or Mass, then fill in the other visible fields.
- If you want force, set Estimate average impact force? to stopping distance or stopping time, then enter either Stopping distance (selected unit) or Stopping time (seconds). Use the one that matches your problem.
- Open Advanced options only if needed. In drop mode, Gravity preset and Gravity g (m/s^2) can change the result. Number display settings only change how answers are shown, not the physics.
- Click Calculate to see Impact energy, Impact speed, Momentum at impact, and optional Average impact force (if estimated).
- Sanity-check the results: if speed doubles, energy should become about 4 times larger, while momentum only doubles. If your force looks extreme, recheck units and whether you used stopping distance or stopping time correctly.
Definitions
Impact energy: The energy the moving object has right before contact. In this calculator, that is kinetic energy from motion, measured in joules (J). Kinetic energy is one-half of mass times speed squared [1].
Impact speed: The object's speed just before it hits.
Momentum at impact: Mass times speed. Momentum and energy are different ideas, even though both matter in impacts [1].
Average impact force: The average force during the stopping part of the collision. It is not the peak force.
Drop height: The vertical distance the object falls before impact. In drop mode, the calculator assumes the lost gravitational potential energy becomes impact energy.
Stopping distance: How far the object keeps moving while slowing to rest after contact.
Stopping time: How long the object takes to slow to rest after contact.
Common mistakes and quick fixes
Mistake: Entering weight instead of Mass (selected unit) .
Fix: Type the object's mass and choose the correct Mass unit . Do not enter force units like lbf in the mass box.
Mistake: Mixing up Impact speed (selected unit) and Drop height (selected unit) .
Fix: In Calculation mode , choose the setup that matches what you know, then fill only the visible input with the right physical quantity.
Mistake: Using a negative value for Impact speed (selected unit) or Drop height (selected unit) .
Fix: Enter speed and height as 0 or greater. If direction matters in your class, use the magnitude here because this tool reports impact size, not signed direction.
Mistake: Turning on Estimate average impact force? but leaving Stopping distance (selected unit) or Stopping time (seconds) at 0 or blank.
Fix: Enter a value greater than 0 for the force method you selected, or switch Estimate average impact force? back to No.
Mistake: Reading Average impact force (if estimated) as the largest force during the crash.
Fix: Treat it as an average over the stopping distance or stopping time. Use Notes and warnings to check this reminder.
Mistake: In Solve for missing , choosing Solve for one value but forgetting to enter the other needed visible inputs.
Fix: After you pick Solve for , make sure the remaining visible inputs such as Mass (selected unit) and Impact speed (selected unit) are filled with valid numbers.
Limitations & Key Assumptions / Boundary Conditions
- Drop-height results assume free fall with negligible air resistance and no energy loss before impact.
- Average force is only estimated when you provide Stopping distance (selected unit) or Stopping time (seconds). It is not peak force.
- Peak force cannot be found from mass and speed alone. It depends on how the object and target deform during impact.
- The stopping-time force formula assumes the object slows from the shown impact speed to 0 during the entered time.
- The stopping-distance force formula uses energy spread over the entered distance, so a shorter distance gives a larger average force.
- Very light or high-drag objects can fall slower than the free-fall model predicts, so drop mode may overestimate speed and energy.
- Extremely large inputs can exceed normal number limits and trigger an overflow error instead of a result.
Methodology
Core equations
This calculator first converts all visible inputs into SI units: kilograms, meters, seconds, joules, and newtons. Then it applies the formula that matches your selected mode.
E = (1/2) m v^2
Use this for Mass + speed mode and for Solve for missing when energy, speed, or mass is based on kinetic energy. Kinetic energy is measured in joules [1].
E = m g h
This is used in Mass + drop height mode, where the calculator assumes gravitational potential energy becomes impact energy at contact.
v = sqrt(2 g h)
In drop mode, this gives the impact speed just before contact.
p = m v
This gives Momentum at impact. Momentum is tracked separately because an impact can be described by both energy and momentum [2].
Average force options
If you ask for force, the calculator uses one of two average-force estimates.
F_avg = E / d
Use this when Estimate average impact force? is set to stopping distance, where d is the entered stopping distance.
F_avg = (m v) / t
Use this when force is based on stopping time, assuming the object comes to rest by the end of the entered time.
Solve-for mode
When Solve for is set to impact speed, the calculator rearranges the kinetic-energy equation.
v = sqrt(2 E / m)
When Solve for is mass, it rearranges to:
m = 2 E / v^2
When Solve for is impact energy, it returns to the standard kinetic-energy form.
Unit conversions used
The calculator converts common US and metric inputs into SI first, then converts selected outputs back for convenience. This includes lbm to kg, slug to kg, ft to m, mph to m/s, km/h to m/s, joules to ft*lbf, and newtons to lbf.
Mini example
Suppose Mass (selected unit) is 2 kg and Impact speed (selected unit) is 3 m/s.
E = (1/2) x 2 x 3^2 = 9 J
p = 2 x 3 = 6 kg*m/s
If the object stops over 0.5 m, then:
F_avg = 9 / 0.5 = 18 N
This shows why force depends on stopping distance or stopping time, not just on impact energy alone.
Assumptions behind the results
The tool uses idealized equations, so real crashes can differ. Some impact energy may transfer into sound, heat, bouncing, or damage to the target rather than only simple stopping behavior [2]. Peak force is not computed because that needs more detailed material and deformation information.