text: A nice open question and so much to talk about! Where to begin???\n\nOk I'll talk about what happens when you drive away in a car. The acceleration needed for a car to move from standstill is proportional to the force applied (by the engine). While you have a force from the engine pushing the car forwards, there are always opposing forces - such as friction from the air, the axles and tyres. These reduce the amount by which the car accelerates away. When you pull away you start in 1st gear (usually). This is so that the car has enough power to pull away. A car could not pull away in its highest gear for example. This is because in a low gear, the input speed (from the engine) is converted into such a speed which allows the car to pull away. You can only go slowly in this gear, but at least you can pull away with plenty of power. For higher gears, your car will tend to be already in motion (i.e. it has momentum). It does not therefore require much power to allow an increase in speed. So high gears are good for high speeds, but the car finds it harder to pull away.\n\nWhen you are travelling at a constant speed, the forces are balanced, so the force that the engine applies is equal to the opposing forces such as friction. It is important to quote Newton's 1st law - a body will continue in uniform motion unless acted upon by an external force. This applies in the above situation but friction complicates the matter. Say there was no friction, then for the car to travel at constant speed, no force would need to be applied (ever) once it had reached that constant speed.\n\nIf you need to use the brakes to come to a stop, these use friction to slow you down. Typically disc brakes are connected to the wheel and pads press on the discs to slow the wheels (and therefore the car) down. The brakes use a hydraulic system which only needs the driver to apply a small pressure on the pedal for the pads to exert a larger pressure on the discs. This happens because the freedom of movement in the brake pedal is long, whereas it is small in the brake pads. This difference allows a much larger pressure to be applied to the brake discs. Also note that road surface conditions and tyre conditions have a considerable effect on the braking distance - ice and water increase braking distance considerably. Some cars use an anti-lock braking system which works by releasing the tyres when skidding is detected so that the tyre will gain grip followed by more braking. This technique helps to decrease braking distances. \n\nI could go on but I must stop now!