E f −−−−− E i = W nc where W nc is the total work done by the non-conservative forces over the path. Note that Our electricity bills carry the energy consumption in units of kWh. Note that kWh is a unit of energy and not of power. Example .
An elevator can carry a maximum load of kg (elevator + passengers) is moving up with a constant speed of m s – . The frictional force opposing the motion is N. Determine the minimum power delivered by the motor to the elevator in watts as well as in horse power. Answer The downward force on the elevator is F = m g + F f = ( × ) + = 22000 N The motor must supply enough power to balance this force.
Hence, P = F. v = 22000 × = 44000 W = hp ⊳ . COLLISIONS In physics we study motion (change in position). At the same time, we try to discover physical quantities, which do not change in a physical process.
The laws of momentum and energy conservation are typical examples. In this section we shall apply these laws to a commonly encountered phenomena, namely collisions. Several games such as billiards, marbles or carrom involve collisions.We shall study the collision of two masses in an idealised form. Consider two masses m and m .
The particle m is moving with speed v 1i , the subscript ‘ i ’ implying initial. We can cosider m to be at rest. No loss of generality is involved in making such a selection. In this situation the mass m collides with the stationary mass m and this is depicted in Fig.
Collision of mass m , with a stationary mass m . The masses m and m fly-off in different directions. We shall see that there are relationships, which connect the masses, the velocities and the angles. u unlike the conservative force, W nc depends on the particular