FORCE AND MOVING BODIES 179 



30. How does applying brakes to the wheels of a train reduce its 

 speed? 



31. What force do we overcome when we drive a nail? 



32. The earth travels around the sun in a nearly circular path. Can 

 you explain why the attraction of gravity does not cause it to fall into 

 the sun? 



33. Recalling the shape of the earth, decide whether a piece of gold 

 weighing exactly an ounce at the north pole would weigh more or less 

 if carried to the equator? 



34. Where would a body weigh more, at sea-level or on a mountain 

 top? Why? 



35. The planet Jupiter is much larger than the earth. If it has the 

 same density as the earth, would a man weigh more or less if transported 

 to Jupiter? Why? 



36. Suppose the earth to rotate twice as fast as it does at present. 

 What effect would this have on the weight of objects? Why? 



37. Name the kinds of equilibrium represented by the following: a 

 ball on a sloping surface, a moving pendulum, a cone resting on its side, 

 a cone resting on its base, a spinning top, a cube of wood, an egg, walking 

 a rope, a man standing. 



38. When one stands erect, is the center of gravity above or below the 

 point of support? 



39. Why is it so difficult to stand up in a small boat? 



40. Why do we lean forward when getting up from a chair? 



