1. Q0 (review question) A block of 2kg slides on a surface with an initial velocity 5m/s. The frictional force between the block and the surface is 5N. How long can the block travel before coming to a stop? A) 1m; B) 10m; C) 5m; D) 4m..

2. Phy100: More on Energy conservation Goals: 1) Conditions for the mechanical energy to be conserved: Effects of frictions (or air drag, inelastic distortion) 2) Conversion between mechanical energies and heat.

3. Mechanical energy (E=K+U) is conserved when there are No dissipations say due to frictions or the system is frictionless, or due to inelastic collisions (as seen in the demo). Energy in a particular form (mechanical) is conserved only conditionally.

4. Effects of Frictions Work-Energy view: When a frictional force is present, the decrease of mechanical energy is equal to the amount of work done by the friction. Energy conversion view: Alternatively, we say the total energy of our system is still conserved if a) we define E=K+ U+ H, H is the heat or thermal energy; b) system is refined to include the object and surface.

5. Energy conservation including thermal energy Esys is conserved for an isolated system

6. Q 2 Simulations of energy conservation--- Energy skate park Assume that the weight of skater is 60kg and initially he stands 5m above the lowest point of the track. Approximately how much heat is generated when he comes to a stop: 1) 30J; 2) 300J; 3) 3000J; 4) 30,000 J.

7. Q3 Coming down from 5m above the lowest point, John generates more heat before coming to a stop at the bottom when A) The track is almost frictionless and he makes many up-down trips on it; B) The track is very frictional so that he stops very quickly; C) The track is made steeper so that he has more kinetic energy at the bottom; D) None of above.

8. Q4 A block slides on a surface with an initial velocity 5m/s. If the surface becomes rougher, 1)There will be more heat generated; 2)There will be less heat generated; 3)The same heat generated; 4) The heat generated depends on the distance it travels.