Physics Past Paper Question

[elarohh]

I'm really confused by this question. I thought the answer would be B, because the lift's motor allows the lift to move, hence electric to kinetic energy, but the markscheme says the answer is D. Can anyone explain why? 

Thanks in advance!

[Vioh]

I'm really confused by this question. I thought the answer would be B, because the lift's motor allows the lift to move, hence electric to kinetic energy, but the markscheme says the answer is D. Can anyone explain why? 

Thanks in advance!

 

This is a pretty common mistake among physics students. The answer (B) is wrong. The electrical energy (being an external force) is not used to change the kinetic energy of the elevator because the elevator travels at a constant speed. Yes, the kinetic energy does increase as the elevator accelerates from rest. However, throughout most of the elevator's journey, the speed is constant, and that is exactly why the question asks for the main energy transformation. Similarly, you can basically eliminate option (A).

 

Now, let's analyze why (D) is the right answer. The following is the general rule:

 

In other words, without any electrical power, the elevator will drop itself in a free fall, increasing its speed all the time until it reaches the terminal velocity. However, the electrical motor acts as a 'resistive' force against the force of gravity, thus making sure that the elevator travels at a constant speed. This is very similar to how air resistance can force an object in free fall to reach its terminal constant speed, therefore transforming gravitational potential energy into thermal energy (heat). However, in the case of our elevator, it is the electrical energy (i.e. an external energy) from the motor that does the work to decrease this gravitational potential energy of the elevator (at floor 10) and then transform it to heat. So the main energy transformation must be electrical --> thermal.

 

If there are any further questions, feel free to ask

Edited December 24, 2014 by Vioh