Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Impact Force

Impact forces acts on falling objects hitting ground, crashing cars and similar

Sponsored Links

crashed car impact force

The dynamic kinetic energy in a moving object, like a falling ball or a driving car, can be expressed as

E = 1/2 m v2               (1)


E = dynamic energy (J, ft lb)

m = mass of the object (kg, slugs)

v = velocity of the object (m/s, ft/s)

The work made by an impact force slowing down a moving object can be expressed as

W = F s                     (2)


W = work done (J, ft lb)

F = slow down force (N, lbf)

s = slow down distance (m, ft)

In an impact like a car crash the dynamic energy from the object is converted to work. The equations can be combined to

F s = 1/2 m v2                (3)

- expressed as slow down force  

F = 1/2 m v2 / s

- expressed as slow down distance

s = 1/2 m v2 / F

Note! The slow down distance is very important and the key to limit the impact forces acting in car crashes.

Example - Crashing Car

A car with a mass of 2000 kg drives in speed 60 km/h (16.7 m/s) before it crashes in a massive concrete wall. The front of the car impacts 0.5 m (the slow down distance). The impact force can be calculated as

F = 1/2 (2000 kg) (16.7 m/s)2 / (0.5 m) 

   = 558 kN

Note that the gravitation force acting on the car is only

Fw = m g

  = (2000 kg) (9.81 m/s2)

  = 19.6 kN

The impact creates a force 28 times the gravity!!

Impact Force from a Falling Object

The dynamic energy in a falling object at the impact moment when it hits the ground can be calculated as

E = Fw h

   = m ag h              (4)


Fw = force due to gravity - weight (N, lbf)

ag = acceleration of gravity (9.81 m/s2, 32.17405 ft/s2)

h = falling height (m)

The equation can be combined with the equation of work to:

F = m g h / s                 (5)

Example - a Falling Car

The same car as above falls from a height of 14.2 m and crashes with the front down on a massive concrete tarmac. The front impacts 0.5 m (slow down distance) as above. The impact force can be calculated as

F = (2000 kg) (9.81 m/s2) (14.2 m) / (0.5 m)

   = 557 kN

Note! - a car crash in 90 km/h (25 m/s) compares to a fall from 32 m!!

Example - a Person falling from a Table

A person with weight (gravitational force) of 200 lbs (lbf) falls from a 4 feet high table.

The energy of the falling body when it hits the ground can be calculated using (4) as

E = (200 lbf) (4 ft)

   = 800 ft lb

The impact on a human body can be difficult to determine since it depends on how the body hits the ground - which part of the body, the angle of the body and/or if hands are used to protect the body and so on. 

For this example we use an impact distance of 3/4 inch (0.0625 ft) to calculate the impact force:

F = (800 ft lb) / (0.0625 ft)

  = 12800 lbf

In metric units - person with weight 90 kg, falling distance 1.2 m and impact distance 2 cm:

E = (90 kg) (9.81 m/s2) (1.2 m)

   = 1059 J

F = (1059 J) / (0.02 m)

   = 53 kN

Sponsored Links

Related Topics

Related Documents

Tag Search

  • en: impact force energy work acceleration
  • es: fuerza de impacto aceleraci√≥n trabajo de energ√≠a
  • de: Schlagkraft Energiearbeit Beschleunigung
Sponsored Links

Search the Engineering ToolBox

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro .Add the Engineering ToolBox extension to your SketchUp from the SketchUp Pro Sketchup Extension Warehouse!

Translate this page to
About the Engineering ToolBox!


3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Scientific Online Calculator

Scientific Calculator

1 22

Sponsored Links