The dynamics of collisions, impulse and momentum
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“Mutationem motus proportionalem esse vi motrici impressæ, et fieri secundum lineam rectam qua vis illa imprimitur. “
Isaac Newton , Lex secunda
This photo is related to the spectacular Parisian accident of 1895, the locomotive and some wagons join the roadway outside the Montparnasse station and the windows of the high floor of the station itself. The engine seems almost complete after breaking through the barriers, the wall and a flight of several meters in height. The locomotive is built to resist, the material is designed to undergo a small deformation even for intense shocks.
An even more evident example is observable in the Formula 1 car races, where the body of a single-seater car, in frontal collision, seems to suffer a sort of explosion (see the photo below the Robert Kubica incident in the Montreal circuit in 2007) , where thousands of pieces of the car, along with the front tires, move away at great speed from the cockpit that protects the pilot.
If we want to reduce the forces (therefore the effects) suffered by people inside vehicles, we need to maximize the duration of the impact. From another point of view, a substantial part of the car’s kinetic energy is used for breaking the bonds of materials or in the work of permanent deformation efforts. In the case of non-sports cars, the joint action of safety belts and airbags brakes the impact of the driver and passengers with the interior walls of the passenger compartment. The airbags are driven by sensors that measure the deceleration of the car in the collision and have activation times of a few tenths of a millisecond (usually between thirty milliseconds and fifty milliseconds). At this point a soft and inclined head restraint is essential to soften the blow that the head receives at the end of the collision when, after the impact with the airbag, it returns backwards.
The human body resists intense accelerations (50 g) only for very short time intervals (a few hundredths of a second). In the collision, the passenger without a seat belt certainly reports very serious injuries.
On the contrary, if the passenger has a seat belt fastened, the passenger moves in solidarity with the car and then stops with the same deceleration, about 30 g, which his body can tolerate without permanent damage.
The only problem is the head, which is not held by the belt and continues to move forward and then downward, until it hits the steering wheel or the dashboard, against which it stops with a very small stopping distance.
To prevent this traumatic contact, cars are equipped with airbags, that is, balloons that swell within a few hundredths of a second from the impact. The airbag provides the head with a sufficiently large stopping space and therefore reduces its deceleration to values that do not cause permanent damage.
Ex 3: Working in small groups, solve the problem and bring the solution back into the Padlet
In the crash test under examination a car is launched frontally at the standard speed of 56 km / h against a 1 m wide and 54 cm deep aluminum barrier. The angle between the direction of motion and the impact plane of the barrier is 90 °.
From the point of view of the manikin the initial acceleration is zero, at the instant of the impact its head undergoes an acceleration that has three components with respect to the barrier. The acceleration module, in a few tenths of a second, reaches a maximum and then decreases to values close to zero, both for the action of the belts and the airbag, and for the car’s stop.
Limiting the time interval between 40 ms and 110 ms and examining only the progress of the driver’s head towards the steering wheel with the measurements of the graph below, calculate the average acceleration and intensity of the force, setting the total mass is an indicative value of 1300 kg.
The variation of kinetic energy is also calculated.
The activity is based on the verification of the Second Principle of Dynamics. Initially, with a series of activities (glossaries, exercises, mind maps,etc.) both the linguistic and the material prerequisites occur, activities are carried out in pairs and/or in small groups. Then the pratical test is carried out, the students, always in group must take note of the data obtained, after which, they must complete the Padlet prepared with the solution to the problem found.
The teacher will evaluate, with appropriate rubric, the material produced and will carry out futher activities to obtain a feedback on the understanding of the topic and on acquisition of the specific language.
All the activities involve the use of devices, all the material provided by the teacher is digital, published on Moodle, all students are in fact equipped with a device (PC or tablet).
Description classroom
The 27 students attend the 3TH year of Mechtronics course and have, on average, a B1-B2 English level
Setting
The lesson was held in a TEAL laboratory equipped with video projectors and LIM
Assessment
The lesson includes a series of activities immediately verified in the results, in this way it was easier to monitor the learning, both linguistic and content.
Bibliography:
– http://www.treccani.it/scuola
– https://www.youtube.com/watch?v=S3xkya4HxvI
– http://www.treccani.it/scuola/lezioni/fisica/dinamica2
– https://www.youtube.com/watch?v=eP1_POQPJVw&t=104s
– Physics Mechanica – E.Anzola, S.Borracci – Scienze Zanichelli
Published: Jul 24, 2018
Latest Revision: Jul 24, 2018
Ourboox Unique Identifier: OB-511990
Copyright © 2018
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