In TCP/IP protocol the IP layer gives the minimum functionlity .It does not provide any flow control or doesn't have provision for acknowledgement. Why is this done ?why is the functionality of IP kept at a bare minimum?
IP only does a minimum amount of work because sometimes you only want a minimum amount of work to be done. For example, you talked about flow control and acknowledgement in your question, but neither of these make sense for a number of networking protocols. Nor do they make sense for most Multicast traffic. If IP had this functionality, you would have to explicitly disable the functionality for this traffic.
Instead of loading down the packets with overhead for functionality they may not be using, IP takes the approach of allowing you to layer on functionality as you need it. If you want flow control and retransmissions and guarantees about the order of packet delivery, layer TCP on top of your IP.
Additionally, internet routers only need to care about the relatively simple IP protocol and are free to ignore everything above it. This allows vendors to build specialized hardware that is very fast and relatively affordable and not waste silicon on handling special cases in IP. (note: IP options make a mess of this last point, but that's why major routers have a software process that can handle those rare packets where the options are set).
The IP protocol defines the network addressing. The TCP protocol is one of many protocols that can ride on top of the IP protocol. TCP and UDP are the two most talked about layer-4 protocols that ride on the layer-3 IP protocol, but there are others. This provides flexibility in networking that would otherwise be unavailable.
TCP/IP is synonymous with Internet protocol suite. Notice the 4 conceptual layers of the protocol suite and that IP is at the Internet layer while TCP, which does have flow control and acknowledgements, is at the transport layer.
Clear separation of functionality between layers is key to the architecture. The network layer is said to be encapsulated by the transport layer.