1. take 20 * 0.5 to get 10 m/s
2. 0.8/0.4 = 2 Hz
Friction occurs when two surfaces come into contact. Excess friction hinders movement, and it wastes energy. Friction refers to a force that opposes motion. It is important because it is applied in almost all life processes.
Potassium Does! Hope that helps
It is endothermic because heat energy is supplied to make the water of crystallization evaporate and make the compound anhydrous.