Momentum & Safety Features
- Since force is equal to the rate of change in momentum, the force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs
- The contact time is the time in which the person is in contact with what they have collided with
- Therefore, safety features are created to reduce the impact of a force, such as in:
- Vehicles
- Playgrounds
- Bicycle helmets
- Gymnasium crashmats
Vehicles
- Vehicle safety features are designed to absorb energy upon an impact by changing shape
- The main vehicle safety features are crumple zones, seat belts and airbags
- For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place
- This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest
- The increased time reduces the force and risk of injury on a passenger
- The usefulness of safety equipment depends on two main factors: mass and velocity
- If the impact is from a large mass, for example, a truck travelling very fast and colliding with a wall, the momentum will be very large
- The change in momentum (ie. from a high speed to rest) will also be very large
- This means that a very long contact time is needed to reduce the force of impact
The seat belt, airbag and crumple zones help reduce the risk of injury on a passenger
- Seat belts
- These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop
- They are designed to stretch slightly to increase the time for the passenger’s momentum to reach zero and reduce the force on them in a collision
- Airbags
- These are deployed at the front on the dashboard and steering wheel when a collision occurs
- They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact
- Crumple zones
- These are designed into the exterior of vehicles
- They are at the front and back and are designed to crush or crumple in a controlled way in a collision
- This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions
- The crumple zones increase the time over which the vehicle comes to rest, lowering the impact force on the passengers
Crash Mats
- Crash mats used in gymnasiums help reduce the risk of injury for falls in gymnastics and climbing
- They are thick and soft to offer shock absorption of the force created by the person landing on the mat
- When a person lands on a crash mat with a large force, for example after jumping, the soft landing means their body is in contact with the mat for a longer period of time than if it was otherwise not there
- This increases the contact time over which their momentum is reduced creating a smaller impact force and a lower chance of injury
A bouldering mat is a type of crash mat used to reduce the chance of injury in falls whilst climbing
- In a similar way, playgrounds utilise cushioned surfaces as children will often fall onto these with a large force
- The cushioned surface reduces the risk of a severe injury by increasing their contact time with the ground
- Meanwhile, a child in a gymnasium can use a thinner crash mat than an adult due to having a lower mass
- This is the same for activities where a person/adult will fall with a low velocity such as falling from lower heights
- Therefore, thin crash mats are suitable for low-impact activities
- Safety features are intended to reduce the chance of serious injury but do not completely prevent it in all cases
转载自savemyexam
