I am a motorcyclist, who knows next to nothing about physics 🙁 , and was wondering if you could help me understand how to calculate the force of an impact, at a given speed:
if the rider hits a hard object, e.g. a wall, and comes to a sudden stop, and
if the rider hits the tarmac, then slides across the tarmac; in this case I’d like to understand the force of the initial impact with the tarmac.
The position of the rider, distance from the floor, lean angle, etc. will all be factors, but I guess some simplifying assumptions, just to get an idea of the order of magnitude we are talking about, would help.
I ask because European standard are very specific about the protective properties of motorcycle armour: a level 2 protector must transmit no more than 20 kN of energy when struck with a force of 50 Joules, (compared to 35 kN for Level 1). However, I have no clue whatsoever what these figures correspond to! I imagine the variables involved will be plenty, but I’d like to get a rough sense: a force of 50 Joules corresponds to… what? Kisisng the floor at 30 kmh? At 60?
All I have managed to calculate with my distant memories of high-school physics is that an impact at 50 kmh corresponds roughly to falling from a height of 10 metres.
I suppose hitting a wall at 50 kmh may easily kill you, but GP pilots kiss the floor at crazy speeds, slide and walk away unhurt, so I'm assuming hitting the floor and sliding involves much lower forces.
Best Answer
Ok first let's get the numbers clear.
$20kN$ is a force, not energy. $50J$ is energy, not force.
I think in your post you're talking about two scenarios. If you "fall off" your motorcycle and slide on the ground the danger is not so much the force slowing you down (which is basically only friction) but the friction damaging your skin, flesh, bones etc. If you hit a brick wall at a certain speed, you have kinetic energy $E_{kin}=\frac{1}{2}mv^2$, where $m$ is your mass, and $v$ your velocity. The wall will slow you down with very little space (and time) to do so, thus the force is extremly high. Protective equipment is on the one hand designed to endure the abrasive friction to protect your skin and on the other to reduce the force created by the impact, to protect your organs, mainly your spine and head.
On wikipedia it says that the damping properties are measured by dropping a 5kg cylinder on the protective equipment and measuring the transmitted force. You can calculate the energy of the cylinder as followed: $E_{pot}=m\cdot g \cdot h$ where $m$ is the mass of the cylinder, $g=9.81\frac{m}{s^2}$ the gravitational acceleration and $h$ the height, from which the cylinder is dropped.
Theoretically you can calculate the reactive force, if you know how much space you have to dissipate the kinetic energy of the cylinder: $E_{dis}=F\cdot d$, where $F$ is the force and $d$ the distance, but it is hard to predict $d$ as it depends on a variety of factors and material properties.