Question

Object A is stationary while objects B and C are in motion. Forces from object A do 20 J of work on object B and -5 J of work on object C. Forces from the environment do 4 J of work on object B and 8 J of work on object C. Objects B and C do not interact. What are ΔKtot and ΔU if (a) objects A, B, and C are defined as separate systems and (b) one system is defined to include objects A, B, and C and their interactions?

Answer 1

Answer:

Explanation:

Since these are forces exchanging work I assume they are affecting the kinetic energy of the objects, and not their internal energy. (Internal energy might be affected by heat or work done by shafts).

For object A:

$\Delta K = 5 - 20 = 15 J\\\Delta U = 0$

For object B:

$\Delta K = 4 + 20 = 24 J\\\Delta U = 0$

For object C:

$\Delta K = 8 + 5 = 13 J\\\Delta U = 0$

For all objects consiered as a system:

$\Delta K = 8 + 4 = 12 J\\\Delta U = 0$