Question

Mountain roads rarely go straight up the slope but wind up gradually. Why?

Answer 1

When a vehicle climbs up an incline, it is moving against two forces - the force of friction acting against the tires, and the force of gravity. As the inclination increases, the component of gravitational force down the incline increases. To counteract this, the car must apply a large force. That is why mountain roads rarely go straight up the slope but wind up gradually.
One other important issue is power. Most vehicles don't have the ability to exert enough force fast enough to climb up a steep slope like that. Or, if they do have enough power, they don't have enough gasoline to do that again and again. Either way, by making vehicles go up a longer, but less steep slope, the vehicle has to exert less power to do so. The idea is that it's easier to exert less power over a longer period of time.