60 MOTION AND FORCE 



11. Which has the faster vibration rate, a short or a long 

 pendulum? If a clock loses time, would you make its pendulum 

 longer or shorter in regulating it ? 



12. Since a pendulum is made to vibrate by the force of grav- 

 ity, would it swing faster or slower on a mountain top than in a 

 valley? (See 29.) 



SECTION III 

 WORK AND MACHINES 



68. Work. Work is said to be done whenever a force 

 causes motion. From this it is clear that work may be 

 measured in terms of the amount of motion caused by a 

 certain force. The amount of work done* by a force is 

 commonly expressed in foot pounds. One foot pound 

 is the amount of work done in raising one pound of 

 matter a distance of one foot against gravity. 



The rate at which work may be done is sometimes 

 called power. The ability of an engine, for example, to 

 do work is expressed as so many "horse power. One horse 

 power is the ability to do thirty-three thousand foot pounds 

 of work a minute. 



69. Machines. A machine is a device which helps 

 man to do work. Note that a machine cannot of itself 

 do work ; it cannot make energy. It can only help in 

 applying force to good advantage ; and as every machine 

 uses up some of the energy in its own motion, none gives 

 us quite as much work as is done upon it. 



70. Uses of Machines. In spite of this fact, however, 

 there are several things gained by the use of machines, 

 which more than make up for this loss in work. 



