Crash physics Collisions 1 During Collisions Enormous forces
- Slides: 21
Crash physics Collisions 1
During Collisions Enormous forces are exerted n n Car Occupants have same velocity as vehicle Inertia keeps occupants moving after impact 2
Forces - a little review The push or pull on objects n Contact w Springs w Friction w applied n Distance w Gravity - large spatial separation w magnetism 3
Force You know it as: n F=m*a w m = mass w a = acceleration w a = ∆v/t This is acceleration 4
Therefore. . . F = m * ∆v/t But, Newton saw it this way: § F * t = m * ∆v Momentum HUH? ? !! 5
What does it mean? In order to change the momentum of a car in collision… F* t = m * ∆v . . . you have to apply a force for a period of time 6
Momentum “Mass in motion” Accounts for the “bashing” effect in collisions Mathematically: n P= m * v Why P? ? I don’t know! They just do. 7
Momentum Is a vector quantity It has size (magnitude) Direction 8
What influences momentum? The heavier the object. . . P= m * v the faster it travels. . . or both…. the more momentum it has. 9
Momentum P = m *v 50 m/s P = 1000 * 50 = 50, 000 1000 kg 25 m/s 2000 kg P = 2000 * 25 = 50, 000 10
Back to the crash What happens to the momentum? + momentum - momentum 11
Conservation of momentum Total momenta is conserved. The momentum of the car is +20, 000. The momentum of the truck is -60, 000. The total is -40, 000 before & after. 12
Would you. . try to stop a 15 lb cannon ball? Traveling at 700 mph? P = 15 * 700 10, 500 13
I’ll bet you have!! An unbelted passenger Trying to stop 150 lb person 70 mph P=m*v 150 * 70 = 10, 500 14
Stopping the car The momentum must decrease to 0 n m * ∆v = 0 A force must be applied. Over a period of time. F*t This is called: 15
Impulse F * t =m * ∆v Impulse: 16
Impulse F * t = m * ∆v Slowing the car requires an impulse n A large force over a short time. w. F * t OR n A small force over a large time. w. F * t Both will slow (stop) the car. 17
Collision impulse Begins at initial contact with outside object: n n n Other car Tree Etc. 18
The “give” influence During collision material deforms. n n n Crush Break Bend This “give” of the material takes time. The longer it takes - the better. Why? 19
. . . Impulse Is required to change momentum =F*t If t is larger then F can be smaller. Less force = less injury 20
Conclusion: Rely on impulse. Use the full 3 inches allowed. Use materials that have “memory”. Maximize the collision time. Minimize impact felt by occupant. 21
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