Motion  by  Impact  and  Gravity.  357 
city  of  c after  the  ftroke  be  reprefented  by  x,  and  that  of 
A or  b in  the  direction  ac  or  cb  by  y,  and  fuppofe 
p ’.q ::  rad. : cofin.  lcd.  Then,  becaufe  ma  the  quantity 
of  motion  before  the  ftroke  is  equal  to  the 
quantity  of  motion  after  the  ftroke,  and  ma1  is  equal  to 
mxz •¥  myy  becaufe  the  quantity  of  force  is  not  altered 
by  the  collifion ; he  eafily  finds  x ~ and  y - 
tlpqma 
pzm-{-  2 qzn* 
There  is  no  problem  which  deferves  to  be  more  con- 
fidered  than  this  by  a perfon  defirous  of  having  a clear 
Zn  idea 
