Conservation of Angular Momentum & Energy question

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1. Mar 20, 2015

15ongm

Question:
A streetcar is freely coasting (no friction) around a large circular track. It is then switched to a small circular track. When coasting on the smaller circle its speed is:

a) greater
b) less
c) unchanged

Relevant
Formulas:

w = v/r
KE = 1/2mv2

My teacher said the normal force from the track on the train doing work as the train switches from the larger to the smaller circle and something about how the conservation of momentum is not valid in this situation. And when I looked this question up online, they saw that energy is conserved. However, I'm still confused on how to solve this problem and what the answer is (I think it's unchanged?).

2. Mar 20, 2015

Staff: Mentor

Does the force from the track do work on the train? That's key.

3. Mar 24, 2015

15ongm

I think so? I remember my teacher saying something how the normal force does work on the train but I'm confused as to how it does.

4. Mar 25, 2015

haruspex

You are right to question it. What direction will the force from the rails be in, in relation to the direction of the train's velocity?

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