Answer:
1. Gravity is the force that acts on every object on earth at any time. It is responsible for this car's increasing speed while moving downhill and it's slowing while moving uphill.
Besides gravity, every moving object is affected by the friction force. It is a force resisting the motion and it depends on objects weight and surface features.
2. Kinetic energy of an object is expressed through an equation:
Ek = m • v^2 / 2 (v being the speed)
so we can see that it is directly proportional to the object's speed.
Potential energy, on the other hand, is calculated as:
Ep = m • g • h (h being the height)
so we can see that it's directly proportional to the object's height.
In our case, just before going downhill, the car is at the 25 meters height and it's not moving. That means that it hasn't got any kinetic energy and his potential energy is on the maximum. Starting his downhill movement, the car will lose the height, but it will increase the speed, so his potential energy will start to decrease, while kinetic energy will increase. At the end of the slope, his speed will be at maximum (so will his kinetic energy), but his height will drop (so will his potential energy). When moving uphill, contrary, his height (and potential energy) will rise, but he will lose speed (and kinetic energy), because of the previously mentioned forces. Note that these two energies are opposing; as one increases, other decreases and vice versa.
3. This car started from the hill that was 25 meters high and that was the point of his maximum energy. Third hill is 30 meters high, so that means that it has greater potential energy. Being moved only by gravity, and slowed down by friction which causes the car to lose some energy on overcoming it, the car can never reach higher potential energy on its own, without engines or any other pull force.
