Madeline Darnell, of Athens GA, asks :-

Yesterday I fell straight forward on a sidewalk after tripping. I caught myself with both hands near the wrist, then immediately rolled onto the grass. I wasn't hurt. I've been puzzling over the fact that I immediately rolled over after falling on my hands. It seemed an automatic reaction. Does it have something to do with the energy of falling?

John Campbell, a physicist at the University of Canterbury, responded.

I recall an old story of the man who fell off the roof of a 40 storey building. As he passed the 5th floor he was heard to say, "So far, so good."

Falling wont hurt you, it is the impact at the end that does the damage.

To give an extreme, gory, example, a British paratrooper of the 1950s told me that on battalion exercise drops chances were a couple of parachutes wouldn't open. Those poor fellows ended up corpses with their lower legs buried in grass half way up to their knees and their upper leg bones up near their head.

An open parachute was designed to slow the spead of descent down to that of a jumping height that was quite survivable. Paratroopers in training felt honour-bound to jump out of 2nd storey windows rather than use the stairs.

I recall that during my parachuting days in the mid-60s we used the same World War 2 type parachutes which didn't have the slow descent speeds or steerability of today's parachutes. Thus we were trained to ensure on landing our feet were together, legs partially bent and not rigidly straight, and on first impact to turn sideways so the ground contact point progressed up our side and finally by rolling across the back shoulders. This gives the maximum possible contact time, and hence the minimum force, for the impact.

Safety in any impact is through reducing the force (ie deceleration of the impact) usually by increasing the impact time. (Force is mass times the acceleration.) That is done in modern cars through car fronts designed to progressively crumple, seat-belts, and internal air bags. Stunt people falling from large heights land out of sight on stacks of soft cardboard boxes or, more usually today, on large collapsible air bags.

When the only help available is the reaction of the person falling, such as when you tripped forward towards a hard surface, we instinctively extend our arms to help break the fall. If you keep your arms rigid you are likely to break a bone. Using your arms to "lower" yourself down increases the time of moving contact and lessens the force. Rolling takes up some of the energy of the fall but also presents a more padded side of the body to help protect the brittle bits (ribs and head) and also further increases the impact time (muscle and fat can compress a reasonable distance).