Custom Search
Impact Force
Impact forces acting on falling objects, crashed cars ..
Sponsored Links
The dynamic energy in a moving object, like a falling ball or a driving car, can be expressed as
E = 1/2 m v2 (1)
where
E = dynamic energy (J)
m = mass of the object (kg)
v = velocity of the object (m/s)
Work made by a impact force slowing down the object can be expressed as
W = F s (2)
where
W = work done (J)
F = slow down force (N)
s = slow down distance (m)
In an impact like a car crash the dynamic energy from the object is converted to work. The equations can be combined as
F s = 1/2 m v2 (3)
or expressed as a function of the slow down distance
F = 1/2 m v2 / s
Note! The slow down distance is very important and a key to limit the impact forces acting in car crashes.
Example - Crashing Car
A car with a mass of 2000 kg drives in 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 impacting 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
F = m g
= (2000 kg) (9.81 m/s2)
= 19.6 kN
Impact Force from a Falling Object
The dynamic energy in a falling object at the impact moment can be expressed as
E = m g h (4)
where
g = acceleration of gravitation (9.81 m/s2)
h = falling height (m)
The equation can be combined with the equationof work as
F = m g h / s (5)
Example - 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 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 that a car driving in 90 km/h (25 m/s) compares to a fall from 32 m(!)
Sponsored Links
Search the Engineering ToolBox
Custom Search
Engineering ToolBox - SketchUp Edition - Online 3D modeling!
Engineering ToolBox - SketchUp Edition - enabled for use with the amazing, fun and free Google SketchUp