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Joo Koon MRT collision converted into mock O-level physics exam questions for 2018


Two trains booped each other at Joo Koon MRT station on Nov. 15, 2017.

This marked the second time something like this has happened after the first incident in 1993, sending the usual shock waves across Singapore.

Within several hours, the Internet was again abuzz as the collision was converted into a set of mock O-level exam questions, effectively turning knowledge into application.

The meme is presumably created by teenagers, who are or were mugging for prelims.

This is the mock physics exam with two parts inspired by the Joo Koon MRT collision:

1) A train weighing 140000 kilograms was moving at 20ms-1 before colliding into another train at the station.

a) Assuming that all of its kinetic energy was imparted to the other train, calculate the average retarding force experienced by the train given that it moved 5 metres forward before stopping. [3]

b) An official then makes a televised speech about how the situation was inevitable that lasts for about 15 minutes. Assuming that 2.5 million people watched it on their television from start to end and that the average wattage of a television is 20W, calculate the total cost incurred given that one kWh of electricity is 15 cents. [3]

Attempts to solve it

As always, once a challenge has been proposed on the Internet, it is promptly accepted.

Solutions attempting to solve the questions can be found on Reddit Singapore:

1a) Collision is elastic. Hence, K.E. of moving train = K.E. of second train = 1/2•m•v2 = 1/2(140000)(20)2 = 2.8 × 107 J = 28 MJ

Since W = F•d, average stopping force experienced by 2nd train,

F = 2.8 × 107 ÷ 5 = 5.6 × 106 N = 5.6 MN

b) Total energy used by all televisions,

E = n•P•t = (2.5 × 106)(20)(15 × 60) = 4.5 × 1010 J = 45 GJ

1 kWh = 3.6 × 106 J, hence

45 GJ = (4.5 × 1010) ÷ (3.6 × 106) = 12 500 kWh

Cost incurred = 12 500 × 0.15 = $1875

Poorly phrased

Even though the meme looks well put together, it has been flamed for being poorly phrased or illogical, as another Reddit user pointed out:

Question is flawed. Meme questions are shit questions.

The question mentions that KE is fully imparted, so under that assumption, it implies that KE of system has been conserved, and thus is a elastic collision. And since KE is imparted fully, the first train must stop.

Except it cannot be an elastic collision if the first train collides and doesn’t stop immediately. An elastic collision lasts for a instant (approximately the speed of sound in solid to be precise), and so the train cannot ‘move forward by 5 meters’ after collision. It must stop immediately after colliding.

I’ll end the explanation here for the layman. However, if you want to understand my assumption that elastic collisions last for a split second, read on.

Why do I know it must stop immediately? Because for an elastic collision, the relative speed of approach must equal the relative speed of separation, and so the trains separate as fast as the approach when they collide. Thus, the collision lasts for a single instant.

However, if all exams were like that, learning would definitely be more fun.

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