You are using a geiger counter to measure the activity of a radioactive substance over the course of several minutes. if the reading of 400. counts has diminished to 100. counts after 66.7 minutes , what is the half-life of this substance? express your answer with the appropriate units.

Asteel wire of 2 mm diameter is fixed between two points located 2 m apart. the tensile force in the wire is 250n, if its density of steel is given by 7830 kg/m3 the fundamental frequency of vibration hz? ?

Determine the time of the day and month of the year at which the peak sensible cooling load occurs for a top floor, north-eastern corner room of an office building in durban for the following conditions: floor area: 8 x 8 x 3 m east and north walls: 115 mm face brick (outside), 20 mm air space, 115 mm ordinary brick (inside) with 15 mm plaster roof construction: suspended plasterboard ceiling, 450 mm air space, 150 mm concrete, 75mm screed, waterproofing no heat transfer across other surfaces window area 3x 1,5 m high in north wall only. ordinary glass with venetian blinds. lights and occupants : from 07: 00 to 18: 00 average light density: 25 w/m2 number of occupants : 5 seated, light office work room temperature 24°c

Which one of the following faults cause the coffee in a brewer to keep boiling after the brewing cycle is finished? a. an open circuit in the heating element b. a short circuit at the terminals c. welded contacts on the thermostat d. a leak around the gasket