Crash physics Collisions 1 During Collisions Enormous forces

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Crash physics Collisions 1

Crash physics Collisions 1

During Collisions Enormous forces are exerted n n Car Occupants have same velocity as

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

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 =

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:

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

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=

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

Momentum Is a vector quantity It has size (magnitude) Direction 8

What influences momentum? The heavier the object. . . P= m * v the

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,

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

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,

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

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

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 =

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 Impulse: 16

Impulse F * t = m * ∆v Slowing the car requires an impulse

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

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

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

. . . 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

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