21A pulley mainly works by changing the direction of what?
22Mechanical advantage of a machine is calculated as load divided by what?
23Momentum of a body is the product of its mass and its what?
24According to the law of conservation of energy, energy can neither be created nor what?
25The law of conservation of momentum applies when there is no what acting on a system?
26In the absence of air resistance, all objects near Earth’s surface fall with the same what?
27Frictional force always opposes what between two surfaces?
28The coefficient of friction mainly depends on the nature of what?
29What is the SI unit of frequency?
30Electricity bills commonly measure energy consumption in which unit?
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