                                                
        3 1 a                                                           F         3 1 b                                                                                     F            
                                                                                       3 1c                       P  P              
                                                                                                                              
   a      b              b             c                              3 1                       
           m    u     mu          M        V      MV          
   mu＋M u =0                                                                                                                                      3 3 
                      mu M u                         u V            
  mu =M u                                                                                                                                             3 4 
                  
         5kg      7m\s          100 kg     3 4        V         
         5×7 =100V                                                            3 5 
                     100 
    V =                                                        3 6 
                   5×7         
V =0.35m\s                                                                  3 7 
                                            