# Why is energy conserved here?

Two identical discs of mass M and radius R are connected by a light rod. The assembly rests at a corner, the vertical wall is smooth and there is sufficient friction on the floor to ensure pure rolling of disc B. The system starts from position θ=0.

This is our scenario and I've been told energy is conserved in this scenario and I don't understand why that is.

Aren't the normal forces from the wall and the floor external forces ?

• Looking at the answer in the link: Have $V_A \cos \theta = V_B \sin \theta$. Surely the next line $\implies V_A = V_B \sin \theta$ is incorrect?
– jim
Jan 4, 2021 at 10:45
• Energy is always conserved so you mean "how" is energy conserved. Jan 4, 2021 at 11:30
• @my2cts Seems fine to me sir/ma'am. Why is energy conserved here in the presence of external forces ? Is it grammatically incorrect? Jan 4, 2021 at 15:44
• @Glowingbluejuicebox my2cts is refering to first law of thermodynamics-"energy can neither be created nor be destroyed" and indeed has a point.You must specify a system for which you are try ing to conserve energy .Energy is always conserved irrespective of your ability to represent it mathematically. Jan 5, 2021 at 10:00