3d helmet
Helmets are useful as safety gear to prevent injuries in an uncontrolled environment. If you can't prevent a crash or impact, but you know it will occur, a helmet can prevent or minimize injury to the head and brain. Human brains can be injured by impact, of course, or by exceptionally violent rotation of the head, when the brain remains stationary, giving blood vessels and nerves a yank. Internal blood vessels and nerves yank parts of the brain around too in different ways, straining the vessels and nerves in the process.
Helmets designed to handle major crash energy generally contain a layer of crushable foam. When you crash and hit a hard surface, the foam part of a helmet crushes, controlling the crash energy and extending your head's stopping time by about six thousandths of a second (6 ms) to reduce the peak impact to the brain. Rotational forces and internal strains are likely to be reduced by the crushing. Thicker foam is better, giving your head more room and milliseconds to stop. If the foam is 15mm thick it obviously has to stop you in half the distance of a 30mm thick foam. Basic laws of physics result in more force to the brain if the stopping distance is shorter, whatever the "miracle" foam may be. Less dense foam can be better as well, since it can crush in a lesser impact, but it has to be thicker in order to avoid crushing down and "bottoming out" in a harder impact. The ideal "rate sensitive" foam would tune itself for the impact, stiffening up for a hard one and yielding more in a more moderate hit.
If the helmet is very thick, the outer circumference of the head is in effect extended. If the helmet then does not skid on the crash surface, that will wrench the head more, contributing to strain on the neck and possibly to rotational forces on the brain. In short, there are always tradeoffs, and a super-thick helmet will probably not be optimal. It will also fail on consumer acceptance. If there are squishy fitting pads inside the helmet they are there for comfort, not impact. The impact is so hard and sharp that squishy foam just bottoms out immediately. In most helmets a smooth plastic skin holds the helmet's foam together as it crushes and helps it skid easily on the crash surface, rather than jerking your head to a stop. In activities that involve forward speed on rough pavement, rounder helmets are safer, since they skid more easily. Different types of helmets seem indistinguishable to most consumers, and you can't test the impact protection unless you have a lab and are willing to destroy the helmet. So the industry uses standards to designate performance levels.
Standards define laboratory tests for helmets that are matched to the use intended. If a helmet can pass the tests for a sport or activity, it provides adequate impact protection. A construction helmet will not pass the more severe bicycle helmet tests. A bicycle helmet will not pass the more severe motorcycle helmet tests. None of them provides the protection against shrapnel that is required of a military helmet. Standards also define other tests for such parameters as strap strength, shell configuration, visor attachments, and the head coverage that must be provided, depending on the activity. A typical standard specifies impact tests, strap tests, characteristics of materials to be used, required coverage, labeling and other requirements. Some have tests to simulate low temperature performance, hot performance, wet performance and sunlight ageing. Test equipment is described as well as the severity of the testing.
An ideal helmet should manage as much energy as possible in a very hard crash, keeping g levels in lab testing as low as possible, but certainly below 200 g for a two meter drop. In a lesser crash it should keep g's below 75. It should be able to handle multiple impacts. A helmet should have a strong strap that keeps it on your head after the first impact (car) for the second impact (street). Child and toddler helmets should also have a buckle that holds firm in a crash but releases after 5 seconds of steady pull to avoid strangling a child who climbs trees or uses playground equipment with their helmet still on and gets caught. A helmet should also be easy to adjust properly or be self-adjusting, and designed to encourage a good fit without excessive fiddling. Once adjusted, the adjustments should stay put. It should be comfortable to wear: cool, light, unobtrusive to the user and fashionable in appearance. The quality of the materials should be apparent. The ideal helmet should be as round and smooth as possible to prevent catching in a crash. A helmet should be durable, easily cleaned, and should not scuff or dent in normal use. Lastly, a helmet should be cheap and readily available in retail stores, including local bicycle shops.
Helmets designed to handle major crash energy generally contain a layer of crushable foam. When you crash and hit a hard surface, the foam part of a helmet crushes, controlling the crash energy and extending your head's stopping time by about six thousandths of a second (6 ms) to reduce the peak impact to the brain. Rotational forces and internal strains are likely to be reduced by the crushing. Thicker foam is better, giving your head more room and milliseconds to stop. If the foam is 15mm thick it obviously has to stop you in half the distance of a 30mm thick foam. Basic laws of physics result in more force to the brain if the stopping distance is shorter, whatever the "miracle" foam may be. Less dense foam can be better as well, since it can crush in a lesser impact, but it has to be thicker in order to avoid crushing down and "bottoming out" in a harder impact. The ideal "rate sensitive" foam would tune itself for the impact, stiffening up for a hard one and yielding more in a more moderate hit.
If the helmet is very thick, the outer circumference of the head is in effect extended. If the helmet then does not skid on the crash surface, that will wrench the head more, contributing to strain on the neck and possibly to rotational forces on the brain. In short, there are always tradeoffs, and a super-thick helmet will probably not be optimal. It will also fail on consumer acceptance. If there are squishy fitting pads inside the helmet they are there for comfort, not impact. The impact is so hard and sharp that squishy foam just bottoms out immediately. In most helmets a smooth plastic skin holds the helmet's foam together as it crushes and helps it skid easily on the crash surface, rather than jerking your head to a stop. In activities that involve forward speed on rough pavement, rounder helmets are safer, since they skid more easily. Different types of helmets seem indistinguishable to most consumers, and you can't test the impact protection unless you have a lab and are willing to destroy the helmet. So the industry uses standards to designate performance levels.
Standards define laboratory tests for helmets that are matched to the use intended. If a helmet can pass the tests for a sport or activity, it provides adequate impact protection. A construction helmet will not pass the more severe bicycle helmet tests. A bicycle helmet will not pass the more severe motorcycle helmet tests. None of them provides the protection against shrapnel that is required of a military helmet. Standards also define other tests for such parameters as strap strength, shell configuration, visor attachments, and the head coverage that must be provided, depending on the activity. A typical standard specifies impact tests, strap tests, characteristics of materials to be used, required coverage, labeling and other requirements. Some have tests to simulate low temperature performance, hot performance, wet performance and sunlight ageing. Test equipment is described as well as the severity of the testing.
An ideal helmet should manage as much energy as possible in a very hard crash, keeping g levels in lab testing as low as possible, but certainly below 200 g for a two meter drop. In a lesser crash it should keep g's below 75. It should be able to handle multiple impacts. A helmet should have a strong strap that keeps it on your head after the first impact (car) for the second impact (street). Child and toddler helmets should also have a buckle that holds firm in a crash but releases after 5 seconds of steady pull to avoid strangling a child who climbs trees or uses playground equipment with their helmet still on and gets caught. A helmet should also be easy to adjust properly or be self-adjusting, and designed to encourage a good fit without excessive fiddling. Once adjusted, the adjustments should stay put. It should be comfortable to wear: cool, light, unobtrusive to the user and fashionable in appearance. The quality of the materials should be apparent. The ideal helmet should be as round and smooth as possible to prevent catching in a crash. A helmet should be durable, easily cleaned, and should not scuff or dent in normal use. Lastly, a helmet should be cheap and readily available in retail stores, including local bicycle shops.
key concepts
ACCELERATION is a change in speed over a period of time; the higher the acceleration, the faster the change in speed.
COEFFICIENT OF FRICTION is the measurement of the level of friction embodied in a particular material. The formula is μ = f/N, where μ is the coefficient of friction, f, is the amount of force that resists motion, and N is the normal force. Normal force is the force at which one surface is being pushed into another.
CRUMPLE ZONES are areas of an object designed to deform and crumple in an impact, as a means to absorb the energy of a collision. The fronts of most automobiles are designed as crumple zones to protect the passengers from frontal collisions.
DRAG is a term used in fluid dynamics that is sometimes referred to as air resistance or fluid resistance. Friction is one of multiple factors that influence the amount of drag encountered by a body moving through a fluid such as air or water.
INERTIA: when an object remains still or moves in a constant direction at a constant speed.
G FORCE: a force acting on a body as a result of acceleration or gravity, informally described in units of acceleration equal to one g.
FRICTION is a force that resists motion when two objects or surfaces come in contact.
FORCE causes masses to accelerate; they are influences that cause a change of movement, direction, or shape. When you press on an object, you are exerting a force on it. When a robot is accelerating, it does so because of the force its wheels exert on the floor. Force is measured in units such as pounds or newtons. For instance, the weight of an object is the force on the object due to gravity (accelerating the object towards the center of the earth).
KINETIC FRICTION (or dynamic friction) occurs when two objects are moving relative to each other and rub together (like a sled on the ground).
NEWTON'S SECOND LAW OF MOTION
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
COEFFICIENT OF FRICTION is the measurement of the level of friction embodied in a particular material. The formula is μ = f/N, where μ is the coefficient of friction, f, is the amount of force that resists motion, and N is the normal force. Normal force is the force at which one surface is being pushed into another.
CRUMPLE ZONES are areas of an object designed to deform and crumple in an impact, as a means to absorb the energy of a collision. The fronts of most automobiles are designed as crumple zones to protect the passengers from frontal collisions.
DRAG is a term used in fluid dynamics that is sometimes referred to as air resistance or fluid resistance. Friction is one of multiple factors that influence the amount of drag encountered by a body moving through a fluid such as air or water.
INERTIA: when an object remains still or moves in a constant direction at a constant speed.
G FORCE: a force acting on a body as a result of acceleration or gravity, informally described in units of acceleration equal to one g.
FRICTION is a force that resists motion when two objects or surfaces come in contact.
FORCE causes masses to accelerate; they are influences that cause a change of movement, direction, or shape. When you press on an object, you are exerting a force on it. When a robot is accelerating, it does so because of the force its wheels exert on the floor. Force is measured in units such as pounds or newtons. For instance, the weight of an object is the force on the object due to gravity (accelerating the object towards the center of the earth).
KINETIC FRICTION (or dynamic friction) occurs when two objects are moving relative to each other and rub together (like a sled on the ground).
NEWTON'S SECOND LAW OF MOTION
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.