What is meant by perfect machine

When seismic waves hit a fault, the waves are reflected off the fault. seismograph can detect these reflected seismic waves. geologists then use these data to map the fault's length and depth

I'm having some difficulty seeing the diagram clearly.but the question says there's a net inward force on the object.so we know right away,  from newton's 2nd law of motion, thatthat baby is accelerating.  it doesn't even matter which way thenet force is pointing.  if there's any net force, then either theobject's speed or else its direction must be changing.