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Impact Force

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

E = 1/2 m v2(1)

where

E = kinetic (dynamic) energy (J, ft lb)

m = mass of the object (kg, slugs )

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

In an impact - like a car crash - the work made by the impact force slowing down an moving object over a distance by deforming the crumple zone can be expressed as

W = F avg s                     (2)

where

W = work done (J, ft lb)

F avg = average impact force during deformation (N, lb f )

s = deformation distance, crumple zone (m, ft)

When a crumple zone deforms in a car crash the average impact force is designed to be as constant as possible.

In an impact where the object is not deformed -  the work made by the impact force slowing down the moving object equals to the work done by a spring force - and can be expressed as

W = 1/2 F max s

= 1/2 k s2(2b)

where

W = work done (J, ft lb)

F max = maximum force at the end of the deformation (N, lb f )

k = spring constant

s = deformation distance (m, ft)

In a car crash the dynamic energy is converted to work and equation 1 and 2  can be combined to

F avg s = 1/2 m v2(3)

The average impact force can be calculated as

F avg = 1/2 m v2/ s                  (3b)

The deformation slow-down distance can be calculated as

s = 1/2 m v2/ F avg (3c)

Note! - The deformation slow-down distance is very important and the key to limit the forces acting on passengers in a car crash.

Example - Car Crash

A car with a mass of 2000 kg drives with speed 60 km/h (16.7 m/s) before it crashes into a massive concrete wall. The front of the car impacts 0.5 m (the deformation distance).

The impact force can be calculated as

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

= 558 kN

Note that the gravitation force (weight) acting on the car is only

F w = m g

= (2000 kg) (9.81 m/s2)

= 19.6 kN

The impact creates a force 28 times gravity!!

A person sitting inside the car with seat belts on will de-accelerate with a force 28 times gravity . Note that the National Highway Traffic Safety Administration (NHTSA) states that "the maximum chest acceleration shall not exceed 60 times gravity for time periods longer than 3 milliseconds ". For a car crash with 90 km/h (25 m/s) the de-acceleration will be 64 times gravity (same parameters as above).

  • 60 mph = 96.6 km/h

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 = F weight h

= m a g h              (4)

where

F weight = force due to gravity - or weight (N, lb f )

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

h = falling height (m)

If the dynamic energy from the fall is converted to impact work - equation 2 and 4 can be combined to

F avg s = m a g h                      (5)

The impact force can be expressed as

F avg = m a g h / s                  (5b)

The deformation slow-down distance can be expressed as

s = m a g h / F avg (5c)

Example - a Falling Car

The same car as above falls from height 14.2 m and crashes on the crumple zone 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 avg = (2000 kg) (9.81 m/s2) (14.2 m) / (0.5 m)

= 558 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 (lb f ) 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 lb f ) (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 avg = (800 ft lb) / (0.0625 ft)

= 12800 lb f

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 avg = (1059 J) / (0.02 m)

= 53 kN

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