In a completely theoretical environment, lets say a 1000 kg car is moving at 40 km/hr with a board on the top of the car weighing 10 kg. If the car crashes, coming to a complete stop, how fast does the board fly off the car? I initially thought obviously 40 km/hr but the conversation of momentum states-
(m+m2)v=mv+m2v2 which means gives 40400=(1000)(0)+10(v2) which gives us a v2 of 4040 km/hr which is way too fast. What went wrong?

Question

In a completely theoretical environment, lets say a 1000 kg car is moving at 40 km/hr with a board on the top of the car weighing 10 kg. If the car crashes, coming to a complete stop, how fast does the board fly off the car? I initially thought obviously 40 km/hr but the conversation of momentum states- 
 (m+m2)v=mv+m2v2 which means gives 40400=(1000)(0)+10(v2) which gives us a v2 of 4040 km/hr which is way too fast. What went wrong?

surry99 · Answer

Hint: When can you apply the conservation of linear momentum?

anonymous · Answer

when no net force acts on the system

anonymous · Answer

right? @surry99

surry99 · Answer

no net external force...correct.

surry99 · Answer

in order for the car to come to rest upon collision it must be hitting something of significance like a wall. How wold you describe the force that the wall exerts on the car?

surry99 · Answer

would

anonymous · Answer

ok I gotcha. THanks!

surry99 · Answer

great!