Active or passive voice?

bmojtaba'The modern army cannot afford to be slowed up with heavy baggage'. 'sSlow up' means 'to become slower' or 'to make something slower', so whyis the passive voice is used???

They do not want the baggage to slow up the army.

I know what that means....i think ' to slow up 'is correct for the above sentence...dont u think so???

bmojtabaiI think ' to slow up 'is correct for the above sentence...don't you think so? ??

The passive is correct in your original sentence. I have a version in which the active is correct.

I didn't usderstand what you mean,could you explain more?

bmojtaba'The modern army cannot afford to be slowed up with heavy baggage'

It is costly/risky/dangerous for a modern army to be slowed up with heavy baggage.
It is costly/risky dangerous for a modern army if the carrying of heavy baggage slows them up.
Heavy baggage would slow them up.
They would be slowed up by heavy bagga

The meaning of 'slow down' is 'to become slower' it means we don not need to use passive voice 'to be slowed up' and we can easily say 'It is dangerous for modern army to slow up... ' ,could explain for me why passive voice is used in the above sentence????

bmojtabaThe meaning of 'slow down' is 'to become slower'

It can aso mean 'make slower'.

Ok,but it can also mean 'become slower' ,so when we need to say 'become slower ' we should say 'to slow down' or 'to be slowed down' both active and passive voice have the same meaning  'become slower',so what is difference???  

There's a difference of emphasis: If I slow down, I become slower. If I am slowed down, something causes me to slow down.