A car travelling at 100 km/h suddenly brakes to a stop. What happened to the kinetic energy of the car? It was transferred out of the system by work. It was transformed into elastic potential energy. It was transformed into thermal energy. It transformed to gravitational potential energy.

When a car brakes to a stop, the kinetic energy it had while moving is transformed into other forms of energy. The kinetic energy doesn't disappear, it just changes form due to the law of conservation of energy. Here's what happens:

1. Some of the kinetic energy is transferred out of the system as work. The brakes apply a force to the wheels, which slows the car down. This force over a distance is work, which is a transfer of energy.

2. Some of the kinetic energy is transformed into thermal energy. The friction between the brake pads and the wheels generates heat. This is a transformation of kinetic energy into thermal energy.

3. The kinetic energy is not transformed into elastic potential energy. Elastic potential energy is stored in objects as they are stretched or compressed. In this case, the car is not being stretched or compressed, so there is no transformation into elastic potential energy.

4. The kinetic energy is not transformed into gravitational potential energy. Gravitational potential energy is energy an object possesses because of its position in a gravitational field. In this case, the car's position in the gravitational field does not change as it stops, so there is no transformation into gravitational potential energy.

So, the kinetic energy of the car is primarily transferred out of the system as work and transformed into thermal energy.