24.  Resultant  of  two  forces  acting  in  the  same  line.  The
resultant  of  two  forces  is  defined  as  that  single  force  which  will
produce  the  same  effect  upon  a  body  as  is  produced  iy  the  joint
action  of  tlie  two  forces.
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COMPOSITION  AND  EESOLUTION  OF  FOECE      15

In  general,  when  a  single  force  acts  upon  a  body  which  is
free  to  move,  the  body  moves  in  the  direction  in  which  the
force  acts;  but  if  two  oppositely  directed  forces  act  simulta-
neously upon  the  same  body,  as  when  two  boys  pull  in  opposite
directions  on  a  cart,  the  eflFect  upon  the  motion  of  the  cart  is
just  the  same  as  though  it  were  acted  upon  by  a  single  force
equal  to  the  difference  between  the  two  forces  and  acting  in  the
direction  of  the  greater  force.  For  example,  if  one  boy  pulls
back  -on  the  cart  with  a  force  of  50  lb.,  while  another  pulls
forward  with  a  force  of  75  lb.,  the  effect  upon  its  motion  is
obviously  the  same  as  though  it  were  pulled  forward  with
a  single  force  of  magnitude  25  lb. ;  i.e.  the  resultant  of  two
oppositely  directed  forces  applied  at  the  same  point  is  equal
to  the  difference  between  them,  and  its  direction  is  that  of  the
greater  force.
