120 Domestic Science 



force is the end of the rope, and the work done by the 

 body i.e. the moving car against the force exerted 

 by the man is measured by the product of the force 

 into the distance through which the end of the rope 

 travels. It will be seen that the force brought into 

 play by the man tends to produce motion in the car, 

 but this tendency is overcome by the resistance afforded 

 by the force which propels the car in the opposite 

 direction. 



75. Another point deserves attention. If a 14 Ib. 

 weight be allowed to fall from the top of a table 2 ft. 6 in. 

 high to the floor, the work done by the force of gravity 

 on the weight is (14 x 2J) = 35 foot-pounds. The 

 weight may, however, be removed from the top of 

 the table to the floor by another route. For instance, 

 it may be caused to slide down a smooth board sloping 

 from the table-top to the floor. The actual distance 

 travelled over by the weight is then greater, but the 

 distance moved over by the point of application of 

 the force of gravity i.e. the centre of gravity of the 

 14 Ib. weight in the direction of the line of action of 

 the acting force is the same as before, viz. 2J ft. Thus 

 the work done by the force of gravity is the same, 

 and is independent of the actual path taken by the 

 weight in moving from the table-top to the floor. 



76. A new idea is introduced when we consider 

 the time taken to perform a given quantity of work. 

 Thus the 14 Ib. weight we have just been dealing with 

 may be raised 3 feet in the air in 1 second or in 3 minutes. 

 The work done in either case is 42 foot-pounds, but 

 the rates of doing it are very different. Taking the 

 second as the unit of time, the rate of working in 

 the former case is 42 foot-pounds per second, while 

 in the latter it is 42 foot-pounds per 3 minutes, or 



