Paper 3 Conservation Laws in Physics Answers

Q1 System Accounting

1. Thermal energy $=180-120=60\mathrm{J}$. [2]
2. Some mechanical energy is transferred to internal/thermal energy due to friction, so kinetic plus gravitational potential energy is not constant. [2]
3. If the block, Earth and surroundings are included, the thermal energy increase is counted, so total energy remains conserved. [2]

Q2 Inelastic Collision

1. $0.50(3.0)=(0.50+0.30)v$, so $v=1.875\mathrm{m}{\mathrm{s}}^{-1}\approx 1.9\mathrm{m}{\mathrm{s}}^{-1}$. [3]
2. $K_i=\frac{1}{2}(0.50)(3.0{)}^2=2.25\mathrm{J}$ and $K_f=\frac{1}{2}(0.80)(1.875{)}^2=1.41\mathrm{J}$. [3]
3. $\Delta K=2.25-1.41=0.84\mathrm{J}$. [1]
4. For a system containing both trolleys and the relevant surroundings, kinetic energy is transferred to internal, thermal, sound or deformation energy [2]; the total energy of that closed system remains constant [1]. [3]
5. Any one of internal energy, thermal energy, sound or permanent deformation. [1]