The force of gravity will cause your vehicle to speed up when going downhill, and slow down when going uphill. The energy of motion increases proportionally with the increase in weight, and the energy increases proportionally with the square of the increase in speed. Increase with an increase of your kinetic energy. Gravity decreases your kinetic energy when driving uphill and increases it when driving downhill. Therefore, the force of gravity will make it easier to stop your car if you're going uphill but more difficult to stop your car if you're going downhill.
