FORENSIC PHYSICS

 

 

 

AppleMark

 

 

 

Book: Evidence for murder

 

SEVEN CASE STUDIES

 

 

 

Balcony fall in Sydney A young woman died when she fell off a balcony. It appeared

to the police that she was pushed, but the evidence showed that she fell accidentally.

 
 

 

 

Cliff fall in Sydney  

 

It took 13 years to prove that Caroline Byrne was thrown off a cliff

in Sydney. The police initially assumed that she jumped. Gordon

Wood was convicted of murder but was later acquitted on appeal.

 

Fall off parking station in San Jose  

 

A priest rolled off the top floor of a parking lot.

The family assumed he was pushed.

 
    

 

 

In May 2010 Des Campbell was found guilty of pushing

his wife Janet off a cliff near Otford. The evidence included

a shoe print and some broken branches.

 

 


Private Jacob Kovco was the first Australian casualty in Iraq.

An Army enquiry in 2006 and a subsequent inquest in 2008

found that he acted irresponsibly in his room and shot himself.

The possibility of an accident was not fully explored.

 

 

 

 Train fall   A train emerging from a tunnel pushes air ahead of it as a result of the piston effect. A man was sucked into the air stream in the London Underground in 2004.

 

Vehicle Roll  A driver rolled a van when he turned a corner. Five drunk passengers had upset the normal balance.

 


 

FALLING FATALITIES

 

Falls are the leading cause of nonfatal injuries and the second leading cause of brain injuries. In the USA, falls cost $37 billion pa compared with $49 billion for motor vehicle accidents. The cost of stair falls each year in the USA is the same as the cost of building new stairs each year. Huge sums are spent on researching motor vehicle accidents, but almost nothing is spent on researching falling accidents since people (eg lawyers and architects) tend to blame the person who fell rather than focus on potential design or maintenance issues.

 

 

Over the last few years I have been asked by the NSW coroner and by the NSW police to provide reports on a number of falling fatalities, specifically regarding the physics of the fall. There are not many lawyers or police or journalists who understand Newton's laws in sufficient detail to even know where to start. Furthermore, there is very little information available in the forensic literature or on web sites concerning fatal falls. Consequently, I have provided some information below, based on cases I have already investigated.

 

If a person dies as a result of a fall from a height, say off a cliff or a tall building, then it is often very difficult to determine whether the fall resulted from an accident or whether it was a suicide or homicide, especially if there were no witnesses and no suicide note or any other indications of a potential suicide. Psychologists assert that there are many indicators of suicide such as previous suicide attempts, depression, a history of mental disorder, a patient in a psychiatric hospital, the leaving of possessions at a cliff top, etc. None of these factors can be taken as proof of suicide, but together might lead a coroner to conclude that suicide was the most likely explanation.

 

In terms of the physics of the problem, some or all of the following factors may need to be considered, depending on the particular circumstances. The calculation of the trajectory during the fall is the easy part. It is everything else leading up to that calculation that is difficult.

 

 

 

1. The fall height.

 

The height of a cliff or building is not easy to measure. It is probably best left to police experts, but it is wise to check independently whether the measurement seems correct eg with a laser distance finder.

 

2. The horizontal distance travelled during the fall.

 

Again, forensic experts can measure this. It might involve one team abseiling down a cliff and another team taking distance and angle measurements.

 

3. The vertical slope of the cliff or building.

 

The deceased person may have bounced or slid to the point where they were found.

If the fall is into water, then police or navy divers may need to assist with measurements close to the edge of a cliff.

 

4. The first and subsequent points of impact.

 

Forensic experts can identify the initial impact point(s) from damage caused by the impact(s). It should marked with paint and photographed for later examination, even if the initial investigation points to suicide. It might later turn out to be a homicide.

 

5. Identification of potential launch points.

 

The actual launch point is not necessarily the closest point to the landing spot. There may well be several potential launch points, each of which needs to be examined on its merits.

 

6. Available runup distance to each launch point.

 

A horizontal flight distance of 10 m may be easier to explain than a 5 m flight distance if the 10 m distance has an available runup distance of 10 m and the 5 m distance has an available runup of only 0.5 m.

 

7. Slope, slipperyness and nature of runup path.

 

A person jumping off a steep uphill slope cannot jump at the same speed as on a level slope. A person attempting to jump forward off a 40 degree slope might end up jumping backward.

 

8. Flight distance and launch angle.

 

The horizontal flight distance through the air is given by D = VT where V is the horizontal launch speed and T is the flight time in the air. The time in the air depends on the initial launch angle, but the additional time resulting from a launch angle of say 20 degrees, as opposed to say zero degrees, will be small compared to the total time if the vertical fall is 30 m or more. Furthermore the horizontal launch speed will be reduced if the jumper attempts a launch at say 20 degrees rather than say 10 degrees. A jump off a cliff or tall building is not the same as a jump on level ground. In the long jump, the jumper needs to spend as much time in the air as possible. The launch angle is probably not known, but should be incorporated into trajectory calculations to determine whether a launch at say 10 degree degrees above the horizontal makes any significant difference to a 20 degree launch. It won't if the vertical fall distance is 30 m or more.

 

 

9. Takeoff distance.

 

When a person jumps or dives, the position of the centre of mass is in front of the back foot at the instant of takeoff.  The flight distance is the subsequent distance travelled through the air. The takeoff distance is the horizontal distance from the back foot to the centre of mass, and it can add an extra 0.5 m or so to the total horizontal distance travelled. 

 

 

                      

 

10. Orientation of body on impact

 

At high launch speeds, a head first landing implies a dive, and a feet first landing implies a feet first jump. At low launch speeds, a head first landing might result from a feet first jump followed by a somersault. In a high speed jump, a jump speed is less than the runup speed and a head first dive speed is less than a feet first jump speed. The launch speed also depends on the available runup distance, which may even be as small as say 0.2 m, or it might be as large as 20 or 30 m.

 

11. Nature of injuries and cause of death.

 

Forensic pathologists can usually determine the cause of death from the nature of the injuries, and can usually tell whether the deceased landed feet first, head first or feet first then head etc. In a fatal fall from a height, there are usually many injuries. If the fall is into water then the injuries may all be internal.

 

 

12. Physical evidence, if any, left at the launch point.

 

It may be possible to determine what happened purely from evidence left at the top of a cliff or building. In one case I examined, a woman overbalanced on a balcony and left a series of impressions on the outside of a glass panel forming part of the balcony wall, showing exactly what had happened. See Journal of Forensic Sciences, January 2006, for a report on this incident.

 

13. Athletic ability of the deceased.

 

If a person lands say 15 m out from a cliff or building, then the height of the fall and the launch speed are obviously both large. The main question then is whether the deceased had sufficient athletic ability to achieve the required launch speed with the available runup distance.

 

14. Strength and athletic ability of any suspect.

 

If a person is suspected of pushing or throwing another person off a bulding or cliff, then the question might be whether the suspect is physically capable of doing so. Tests can be done by pushing or throwing volunteers into a swimming pool.

 

15.  Prevailing wind and effect of air resistance.

 

Air resistance is relatively small and will make only a small difference to most human body trajectory calculations, unless a strong wind was blowing at the time or the fall is from a very large height.

 

16.  Presence or absence of drugs, alcohol or sedatives.

 

Not really a physics issue, but it might help explain why the deceased fell.

 

17. Statements made by others before or after the event.

 

Was the suspect seen near the scene of the crime? Again, not a physics issue, but one that is obviously important, especially if the suspect was actually somewhere else at the time.

 

The jpeg image here shows the trajectory of each end of an iron bar pushed off the top of a cliff. A person would not necessarily bounce in the same manner, but simulation studies like this are useful in getting a good idea of what might or might not have happened.

 

 

The movie clip here shows a mannequin pushed off

the cliff. People donŐt bounce, but they can slide

and roll at high speed when impacting the side of

a cliff. Movie taken at 300 fps.