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                  INERTIA & MOMENTUM

 

           Trains Collision                   Conservation of momentum 

           Head-on Collision             Two Cars Collision                 Back to Physics

 

 

 The law of inertia is most commonly experienced when riding in cars and trucks. In fact, the tendency of moving objects to continue in motion is a common cause of a variety of transportation accidents - of both small and large magnitudes.

Trains Collision

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car hitting wall  

 

                                      

 

 

 

 

Conservation of momentum 

 

 

cart with bricks

The collision between a 3.0-kg loaded cart and a 2-kg dropped brick. It will be assumed that there are no net external forces acting upon the two objects involved in the collision. The only net force acting upon the two objects (loaded cart and dropped brick) are internal forces - the force of friction between the loaded cart and the droped brick. The before- and after-collision velocities and momentum are shown in the data tables.

 

               

  Head-on Collision

 

 

car and truck collision

 

n the collision between the truck and the car, total system momentum is conserved. Before the collision, the momentum of the car is +20 000 kg*m/s and the momentum of the truck is -60 000 kg*m/s; the total system momentum is -40 000 kg*m/s. After the collision, the momentum of the car is -40 000 kg*m/s and the momentum of the truck is 0 kg*m/s; the total system momentum is -40 000 kg*m/s. The total system momentum is conserved. The momentum change of the car (-40 000 kg*m/s) is equal in magnitude and opposite in direction to the momentum change of the truck (40 000 kg*m/s) .

 

Two Cars Collision

 

 

two cars colliding

 

When considering the total momentum of the system before the collision, the individual momentum of the two cars must be added as vectors. That is, 20 000 kg*m/s, East must be added to 10 000 kg*m/s, North. The sum of these two vectors is not 30 000 kg*m/s; this would only be the case if the two momentum vectors had the same direction. Instead, the sum of 20 000 kg*m/s, East and 10 000 kg*m/s, North is 22 361 kg*m/s at an angle of 26.6 North of East. Since the two momentum vectors are at right angles, their sum can be found using the Pythagorean theorem    Go to the top