LAWS OF MOTION 67 



16. Give the kind of energy present in the following examples: 

 pile-driver haihmer 40 in. in the air; gunpowder; moving ship; 

 water running over a dam; water in a lake on a mountain; water 

 in a reservoir; charged storage battery; coal; wood; recoil of a 

 gun; escaping steam. 



16. Why is oil thrown off from gears and pulleys? Why is 

 mud thrown off automobile wheels? 



17. Why does a lathe continue to move after the switch is 

 turned off? 



Problems 



1. How much energy does a mass weighing 3 tons acquire in 

 falling through 100 ft.? 



2. A machinist exerts upon a file a force of 11 Ibs. downward 

 and 15 Ibs. forward. How much work does he do in 41 horizontal 

 strokes each 6 in. long? In what units is the result expressed? 



3. A pile-driver weighing 510 Ibs. drops from a height of 15 ft. 

 pushing the pile down 6 in. What was the potential energy of the 

 weight before it started to fall? What is the resisting force of the 

 pile? 



4. If a drop hammer weighs 600 Ibs. and falls from a height of 

 24 in., what kind of energy will it possess before falling, and how 

 much energy will be expended? 



5. A large weight of 700 Ibs. is allowed to fall a distance of 18 

 ft. in order to break old car-wheels. What is the kind and the 

 amount of energy? 



6. An elevator in a public building weighs 3^ tons. How 

 much energy will be necessary to lift it from the first floor to the 

 second, a distance of 20 ft.? 



7. A pile-driver weighs 265 Ibs. and falls from a height of 16 ft. 

 What is the energy at the. time it strikes the pile? 



