FORCE AND MOTION. 



45 



sailing at the rate of 12 ft. a second. Over what space does he 

 actually move during 20 seconds ? 



6. A foot-ball simultaneously receives three horizontal blows ; one 

 from the north having a force of 10 pounds; one from the east having 

 a force of 15 pounds, and one from the south-east having a force 

 of 804 kinetic units. Determine the direction of its motion. 



7. Why does a cannon recoil or a shot-gun " kick " when fired ? 

 Why does not the velocity of the gun equal the velocity of the shot ? 



8. If the river mentioned in the third problem be one mile wide, 

 how far did the boat move, and how much longer did it take to cross 

 than if the water had been still ? 



9. A plank 12 feet long has one end on the floor and the other end 

 raised feet. A 50- pound cask is being rolled up the plank. Resolve 

 the gravity of the cask into two components, one perpendicular to 

 the plank to indicate the plank's upward pressure,* and one parallel 

 to the plank to indicate the muscular force needed to hold the cask 

 in place. Find the magnitude of this needed muscular force. 



Recapitulation. To be amplified by the pupil for 



review. 



f ELEMENTS. 



