1. Explain why the wind is able to do the work of turning a windmill.

2. When the wood-chopper chops wood he usually swings the axe high when he comes to a knotty piece. Why?

3. Why is it more comfortable to ride in a carriage with pneumatic tires and springs than in a farm wagon with neither?

4. Explain why fortifications are usually made of earthwork and not masonry.

5. When an automobile runs too fast around a corner it "skids." Why?

6. Explain why a person riding in a rapidly moving railway car is thrown forward when the car stops suddenly.

7. Explain why a person standing in a street car is thrown back when the car starts suddenly.

8. (a) In attempting to kick the panel out of a door why does one experience a pain from the kick? (b) This pain is not severe when you kick a canvas curtain. Why?

9. Why does a man lean forward when he climbs a hill?

10. (a) What is the ballast of a ship? (b) What is the object of the ballast?

11. Why is it unsafe to stand in a canoe?

12. Which is more steady (stable) a load of wood or a load of metal equal in volume?

13. Explain the principle of a revolving clothes-dryer used in laundries.

14. The outer rail of a railroad curve is higher than the inner one. Why?

15. Give the kind of energy present in the following examples: pile-driver hammer 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 lbs. downward and 15 lbs. 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 lbs. 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 lbs. 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 lbs. 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 1/2 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 lbs. and falls from a height of 16 ft. What is the energy at the time it strikes the pile?