Energy

Energy can neither be created nor destroyed; it only changes from one form to another

Step 1: The Law of Conservation of Energy states that the total energy of a system always remains constant. Step 2: Energy cannot be created from nothing – all energy comes from some other form. Step 3: Energy cannot be destroyed – when a body stops due to friction, its kinetic energy converts into heat energy, not destroyed. Step 4: Only kinetic energy remaining constant is only true in the absence of friction for mechanical energy – it is not the general law. Step 5: The correct statement covers ALL forms of energy, not just kinetic energy.

40,000 J

Step 1: At position A (top), the roller coaster is stationary, so KE = 0 J and PE = 40,000 J. Total energy = 40,000 J. Step 2: By the Law of Conservation of Energy (ignoring friction), total energy at any point = 40,000 J. Step 3: At position B (bottom/ground level), height = 0, so PE = 0 J. Step 4: Since Total Energy = PE + KE → 40,000 = 0 + KE → KE = 40,000 J. Step 5: All the potential energy has completely converted into kinetic energy at the bottom.

Electric bell

Step 1: Let us check each option for its energy conversion. Step 2: Electric motor converts electrical energy into mechanical (kinetic) energy – used to run fans, machines. Step 3: Microphone converts SOUND energy into electrical energy – it is the reverse process. Step 4: Electric bell converts electrical energy into sound energy when it rings – this is the correct answer. Step 5: Solar cell converts light energy into electrical energy. Therefore, the electric bell is the correct choice for electrical → sound energy conversion.

PE = E/2, KE = E/2

Step 1: At the starting point (top), PE = E and KE = 0 (body is at rest). Step 2: At the bottom (ground), PE = 0 and KE = E (all PE converted to KE). Step 3: At the midpoint (half the height), the body has fallen through half the total height. Step 4: Since height is halved, PE = E/2. Since total energy is conserved: KE = E – PE = E – E/2 = E/2. Step 5: At the midpoint, PE = KE = E/2. This is a direct application of the conservation of mechanical energy.