When 2 vehicles collide, what happens to their momentum?
Look at this example, 2 cars (A & B) both of 1000kg, were traveling in the same direction with velocity 20 ms-1 and 10 ms-1 respectively. They collided. After the collision, car A continued to move forward with 10 ms-1 and car B ramped forward with 20ms-1 . Let's calculate the total momentum before and after collision. (u = initial velocity, v = final velocity)
| Before collision | After collision | ||||||||||||
| mA | uA | mAuA | mB | uB | mBuB | total mv | mA | vA | mAvA | mB | vB | mBvB | total mv |
| 1000 | 20 | 20,000 | 1000 | 10 | 10,000 | 30,000 | 1000 | 10 | 10,000 | 1000 | 20 | 20,000 | 30,000 |
From this example, we conclude that in collision, total momentum before and after collision is the same. That is momentum is conserved.
The Law of Momentum Conservation
In a system with a few objects reacting, momentum is always conserved if there is no external force acting on the system. In collision and explosion, momentum is conserved.
| Total Momentum before collision/explosion | = | Total Momentum after collision/explosion |
Momentum is transferred from one object to another in a system. This is clearly shown in Newton's cradle.
After collision, sometimes the vehicles went separate ways, sometimes they got stuck and move together? Why? To explain this we need to understand 2 types of collision. (elastic & inelastic)
Elastic Collision
In elastic collision, the vehicles went separate way after colliding. Look at the graphic below and pay attention to the change in momentum before and after collision. What is the total momentum before and after collision?
Click here for more examples. (Negative momentum means the object is moving in the opposite direction.)
Inelastic Collision
In inelastic collision, the vehicles got stuck and move together
after colliding.
Look at the graphics below. Pay attention to the total momentum before and after
collision.
Click here for more examples.
In elastic collision, kinetic energy is conserved after the collision. But in inelastic collision, some kinetic energy is lost. Therefore kinetic energy after collision is less than before collision.
However, in both types of collision, momentum is conserved.
| Elastic collision | Inelastic collision | |
| Momentum | conserved | conserved |
| Kinetic Energy | conserved | after collision less than before collision |
| Scenario | separated | stuck together |
You can carry out some experiments here to understand the two types of collision better.
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Last updated: 16 Sep 2002