An object, which is set into motion, has energy, so if you were to push a toy car across a laminate floor, for example, once you release it from your hand it will continue to run over the floor until the friction within moving parts, and air resistance, will slow it to an eventual stop.
To keep matters simple, the degree of kinetic energy present within an object will be conditional upon two factors, its weight (mass) and its speed (velocity). Therefore, a 1,000kg car travelling at 40mph will possess more kinetic energy than the identical vehicle travelling at 20mph. However, a 2,000kg car travelling ay 40mph will possess more kinetic energy than the 1,000kg car travelling at the same speed.
The kinetic energy of an object is directly proportional to the square of its speed, which means that if you double its speed you will quadruple the amount of kinetic energy. This is why the Highway Code tells us that if we double our speed we quadruple our braking distance. See the table below and compare the 20mph data to the 40mph data, as well as the 30mph and 60mph data.
At 20mph a vehicle will be travelling at 8.9 m/s and take 6-metres to stop
At 30mph a vehicle will be travelling at 13.3 m/s and take 14-metres to stop
At 40mph a vehicle will be travelling at 17.8 m/s and take 24-metres to stop
At 50mph a vehicle will be travelling at 22.2 m/s and take 38-metres to stop
At 60mph a vehicle will be travelling at 26.7 m/s and take 56-metres to stop
At 70mph a vehicle will be travelling at 31.1 m/s and take 75-metres to stop
Distances will vary according to condition of road surface upon which the vehicle concerned is being driven, but will always remain relative to each other.
The most well known method of stopping a car is to use the brakes, but brakes cannot do the job on their own. Whilst the brakes will apply stopping power to the wheels, the wheels have to rely on their tyres gripping the ground over which the car is travelling if they are to succeed.
Tyres is only able to grip the road trough the existence of friction between the contact surface (footprint) and where that is contact with the ground, and the harder you push that tyre onto the ground the more friction is created. In layman’s terms, friction, in tyre–talk, is grip, and it is easy to think that you can gain greater grip by making the vehicle heavier so as to push the tyres onto the ground with more force.
The amount of Kinetic energy is calculated by mass x velocity², which means the greater the weight (mass) of the vehicle, the greater the amount of energy will be required to accelerate it. It also follows, therefore, if more energy is required to accelerate a 2–tonne vehicle than a 1–tonne vehicle, to the same rate of speed (velocity), more energy is required to bring the heavier vehicle to a halt.
This page was last updated
Tuesday, 15-Feb-2011