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Simple question

I have a question where i need to find the equilibrium temp between ice and silver ... i know what to do i'm just wondering this:

Ice starts at 260k and silver at 250k so then that means the ice will lose energy and cool down yes? silver will gain and heat up.

So that made me think: Qs = -Qi since ice will lose energy, but the mark scheme has it as Qi = -Qs why is this?

And also while im on the subject of signs .... why is the force of friction acting on a car negative? - is it because the car transfers energy to overcome the friction and so loses energy?

Reply 1

Aurel-Aqua

Carlo08

Qi = -Qs, multiply both sides by -1: -Qi = Qs, Qs = -Qi

And for friction, friction opposes motion in the positive direction. If we define the resultant force in one direction, and frictional force acts in another direction, it'll just give a negative sign to the friction.