I'm going to assume you're a bright Stanford kid, and that your brain will remember this.
Work = F * d
Work = Force * distance
Try punching your arch nemesis for an instant. Just hit him for a second and as soon as you touch him, pull your hand back. Nothing happens.
Now when you punch, punch behind him. Keep pushing, all the way through, pushing pushing pushing. Whoa. Can you believe your own strength? That guy just flew across the room. You can try this with a punching bag, too. Whatever you like.
So when you punched your grandma or whoever, you had a lot of force but you didn't keep applying it over a distance. When you push push push then you do a lot of work to your poor grandma.
So basically the ground/fire-hydrant/tree is going to do a lot of work to you. That amount of work isn't changing, no matter what.
So you've got
WORK = force * distance
Notice how work is bold here. Imagine work is written 10 feet tall on a chalkboard to represent how big work is here.
Now if your distance is small, the force is going to need to be... (guess)
WORK = FORCE * dist
Now if you can find a way to extend your distance.... the force you experience will shrink.
So if you can extend your legs, extend your hands, do a parkour/Ta Kwon Do roll, something, that will dampen your fall.
Another example. Someone hits a baseball super deep into back field. You're playing without a baseball mit. Say you just hold your hand steady and catch it all at once. SMACK. That really hurts your hand. Now say you extend your hand all the way out, and as the ball lands in your hand, your arm moves back until your hand is at your chest, absorbing the blow. Ahhhh. No sweat!
work is going to be same-same
Force needs to be small
so make distance big
The guys comments make a very good point. A lot of people get injured by extending their limbs. What's happening is they don't give their limbs room to bend. They brace for the fall by extending their arms/legs too far out, to protect their body/head. Instead they get locked. OUCH. Extend your arms/legs, but don't lock them forward. That's just going to make the distance very small again, and it's really going to hurt your joints.
So extend, but don't lock your elbows/knees.
Good point guys.
Think of it like falling. Don't lock your legs all the way forward. That's... really going to hurt. But if you extend the toes of your foot, bend your knees a little, and give that ground a good SLAP with your hands when you're landing, that fall is going to feel like butter. You can practice absorbing falls on a set of stairs. Go up one step, jump off, extend your feet/legs, and practice absorbing the whole fall. Then go up another step. Keep working at it until you can run off the top of the stairs and land on the bottom (if you're intense like that). You'll notice that the difference between the 10th step and the 11th step is HUGE. And the difference between the 11th step and the 12th step is INSANE. But that's another physics lesson.
The physics does not apply exclusively to spherical bodies. If you can extend your slide to 35 feet instead of 30 feet, you will SIGNIFICANTLY decrease the trauma you experience.
"Kinetic" energy = moving energy = (mass * velocity^2)/2
velocity = distance/time
That means with any change in velocity will SQUARE the energy.
When you talk about collisions, you're talking about momentum. Specifically the change in momentum ("impulse of momentum") or change in mass * velocity
mass * velocity = Momentum
Mass is staying same-same.
Velocity is changing from 20MPH to 0.
That's a huge CHANGE in momentum.
If you increase the distance of that change in speed 5 ft (16% of 30ft), you decrease the change in momentum.
So remember to find a way to roll if it's safe. Don't add more energy to the impact by rolling into the collision, unless you're going to get substantially more distance. For example, if you're falling and you know how to land on your feet and instead of SLAPPing the ground, you roll forward over your shoulder, that's going to be even better. It takes a little training to learn how to forward roll like that, and I'm assuming that the best way to think of this is simply. I like my models to be simpler than the actual thing I'm describing.
So just think: how can I increase the distance of this collision? I don't want to BOOM hit the pavement all at once. I want to soften the blow by sliding more, absorbing some of the impact, whatever.
I'm not a medical doctor and I don't know very much anatomy. Something tells me we evolved to protect our heads and bodies for a reason. That's where all the really important organ stuff is. The problem with people who crash a lot, like skateboarders, is they tend to fuck up their limbs before they become experts at falling. Just don't lock your elbows/knees by over extending them.
- Second, and probably most important advice: try to brake before the accident. Changing the speed before your collision will reduce the energy SQUARED. Again, try jumping off your stairs. Falling from 13ft is way way harder than falling from 10ft. That's because the little extra speed you have when landing gets squared as kinetic energy. Think of energy as the potential to do work. Like the floor having the potential to work your body into jello.
- Wow, telling people to brake is probably the first advice I should have given.