Impulse_and_Momentum_in_a_Car_Collision

PART 1: Momentum and Impulse As modern automobiles have become more powerful and able to obtain higher velocities the need for effective safety features has become more crucial. A deeper comprehensive knowledge of the forces and processes present during collisions has led to the development of many life saving devices and smarter, more effective road design. Mo-men-tum(noun) ‘The quantity of motion of a moving body’. Oxford Dictionary, 1991 Momentum is defined as the product of an objects mass and its velocity. It is expressed by: Im-pulse(noun) ‘Change in momentum produced by a force’. Oxford Dictionary, 1991 An impulse is defined as the product of force and the time in which the force was applied. Impulse is expressed by: I = impulse f = force t = time Momentum and kinetic energy are closely related. Kinetic energy is the energy an object has due to its momentum. Kinetic energy is expressed by the formula: Low Speed Zones in Built-up Areas Speed is one of the greatest causes of death and serious injury on Australian roads. There are several ways in which speed can increase the risk of an accident but in its relation to momentum and impulse, it is crucial in the relationship between force and energy. From the formula: We can see that the kinetic energy is proportion to the velocity squared which means that if the speed is doubled, the energy is quadrupled. The rapid transfer of kinetic energy is the cause of injuries in car accidents. So managing the amount of kinetic energy is crucial. e.g. if two cars, one travelling at 50km/h and the other travelling at 60km/h crash into a brick wall. What will their energy be if both cars have an equal mass of 5000kg' Driver 1: KE = mv2 / 2 Driver 2: KE = mv2 / 2 = 5000x13.892 / 2 = 5000x18.752 / 2 = 482.3 KJ =878.9 KJ This example illustrates how small increases in velocity can have a huge impact on the amount of energy present. As kinetic energy is the energy an object has due to its momentum, the more kinetic energy an object has, the more momentum it has. The example also shows how lowering the speed limit in built up areas to 50km/h drastically lowers the kinetic energy the car has. This means that the momentum is also lowered; therefore a smaller impulse is required to bring the car’s momentum to zero. If a car has less kinetic energy a smaller stopping force and time period are needed to dissipate the energy. This is crucial in built up areas, especially residential areas as there are a higher chance of obstacles which required sudden braking ie people, pets, other vehicles. |Speed km/h |Risk Relative to 60km/h | |65 |Double | |70 |4 times | |75 |11 times | |80 |32 times | http://www.rta.nsw.gov.au/roadsafety Figure 1 Figure 1 demonstrates how by increasing the speed of a car also increases the risk of an accident as well as the severity of the crash which was mentioned above. As velocity increases, the required stopping distance also increases. Figure 2 illustrates this connection. Braking distance is proportional to the speed squared. 50km/h speed limits lower the risks of accidents but also the severity of accidents. By reducing the speed limit to 50km/h there is less kinetic energy and momentum involved which allows drivers shorter time periods to bring their momentum to zero. This ability is important in built up areas as driving can be unpredictable. Figure 2 Air Bags The main purpose of an airbag is to increase the time for an occupant. By increasing the time of the impulse, a smaller force is applied to the car passengers. To dissipate the momentum of a moving object to zero, as in the case of a car collision, an impulse must be exerted. e.g. if an impulse of 1750 Ns is applied on two cars with equal momentum. Calculating the force exerted on the two drivers if one driver hits the windshield in 0.002s while the other hits and deflates an airbag in 0.5s Driver 1: F = I / T Driver 2: F = I / T = 1750 / 0.002 = 1750 / 0.5 = 875 000 N = 3500 N As you can see from this hypothetical scenario, small increases in the time in which the force is applied greatly reduce the amount of force applied to the occupants during a collision. It is for this reason that air bags are essential in all cars. Modern cars now travel with higher velocity and therefore also travel with more momentum. Therefore a greater impulse is needed to overcome this momentum. This means that greater forces will be exerted on the passengers, thereby increasing the risk of injury. This also makes the use of airbags fundamental to ensure that the risk of serious injuries or fatalities is minimised. Another function of airbags is to spread the force across a larger area of the body. This helps distribute the stopping force and helps minimise the chance a serious head trauma. When an occupant is in a serious front on collision, the head is usually first to collide with other objects (steering wheel, dashboard). This means that the head absorbs most of the force, which can cause serious injury. An air bag spreads the force across a greater proportion of the body which minimises the impact of the force. (refer to figure 3). Figure 3 Crumple Zones It a front on collision, a car can either crumple or rebound. (refer to figure 2) Figure 4 As figure 4 shows, a car that rebounds experiences a greater change in momentum than a car that crumples upon impact. A greater change in momentum also means a greater impulse and therefore a greater force would be experienced by the occupants, thus increasing injuries. To minimise the force exerted upon the vehicle, crumple zones have been added to all modern cars. Crumple zones, collapse upon impact which reduces the risk of rebounding and thus minimising the change of momentum. Like Airbags, crumple zones also increase the stopping distance and absorbs the force of the collision. They create a longer stopping distance which lowers the force needed to stop the car’s momentum. All of these safety features used in conjunction help to protect occupants from the forces and pressures inflicted upon a person during a collision. Each is preforms a unique task in ensuring the safety of all the occupants of the car. ----------------------- I = fΔt