one-forty-eighth   of   a   foot.   You   gain   force   at   the expense of distance. FRICTION Suppose you are going to push a 400-pound crate up  a  12-foot  plank;  the  upper  end  is  3  feet  higher than  the  lower  end.  You  decide  that  a  100-pound push  will  do  the  job.  The  height  you  will  raise  the crate is one-fourth of the distance through which you will   exert   your   push.   The   theoretical   mechanical advantage is 4. Then you push on the crate, applying 100  pounds  of  force;  but  nothing  happens!  You’ve forgotten about the friction between the surface of the crate and the surface of the plank.  This friction acts as  a  resistance  to  the  movement  of  the  crate;  you must  overcome  this  resistance  to  move  the  crate.  In fact, you might have to push as much as 150  pounds to move it. You would use 50 pounds to overcome the frictional  resistance,  and  the  remaining  100  pounds would be the useful push that would move the crate up the plank. Friction is the resistance that one surface offers to its  movement  over  another  surface.  The  amount  of friction depends upon the nature of the two surfaces and the forces that hold them together. In many instances fiction is useful to you. Friction helps you hold back the crate from sliding down the inclined  ramp.  The  cinders   you   throw   under   the wheels   of   your   car   when   it’s   slipping   on   an   icy pavement  increase  the  friction.  You  wear  rubber- soled   shoes   in   the   gym   to   keep   from   slipping. Locomotives  carry  a  supply  of  sand  to  drop  on  the tracks in  front  of  the  driving  wheels  to  increase  the friction between the wheels and the track. Nails hold structures  together  because  of  the  friction  between the nails and the lumber. You make friction work for you when you slow up an  object  in  motion,  when  you  want  traction,  and when  you  prevent  motion  from  taking  place.  When you  want  a  machine  to  run  smoothly  and  at  high efficiency, you eliminate as much friction as possible by   oiling   and   greasing   bearings   and   honing   and smoothing rubbing surfaces. Where  you  apply  force  to  cause  motion,  friction makes the actual mechanical advantage fall  short  of the   theoretical   mechanical   advantage.   Because   of friction,   you   have   to   make   a   greater   effort   to overcome the resistance that you want to move. If you place  a  marble  and  a  lump  of  sugar  on  a  table  and give   each   an   equal   push,   the   marble   will   move farther. That is because rolling friction is always less than sliding friction. You take advantage of this fact whenever  you  use  ball  bearings  or  roller  bearings. See figure 7-7. Figure 7-7.—These reduce friction. Figure 7-8.—It saves wear and tear. The Navy takes advantage of that fact that rolling friction  is  always  less  than  sliding  friction.  Look  at figure 7-8. This roller chock cuts down the wear and tear  on  lines  and  cables  that  are  run  through  it.  It also reduces friction and reduces the load  the   winch has  to  work  against. 7-4