Starter
Explanation:
Turn the ignition switch to start and release the key immediately or you could destroy the starter.
The car starter is used to cause ignition in the internal combustion engine in order to fire the piston and cause mechanical motion. The starter is used to start the cyclic process of the internal combustion engine.
- Once the engine starts by igniting the starter, it is best to release it.
- The starter ensures that the spark plug is engaged and the motor is brought into work.
- If the ignition is still engaged, the process continues repeatedly and it can damage the starter of the car.
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Let say the two train cars are of masses
and 
now if the speed of two cars are
and 
then we can say that the momentum of two cars before they collide is given by

here two cars are moving in opposite direction so we can say that the net momentum is subtraction of two cars momentum.
Now since in these two car motion there is no external force on them while they collide
So the momentum of two cars are always conserved.
hence we can say that the final momentum of two cars will be same after collision as it is before collision

Answer:
A stripe of magnetic information that is affixed to the back of a plastic credit or debit card.