Is the transactions in one block effect the hash of the next block? How? What I understand about blockchain is that: 
Blocks are secured by the hash. 
Transactions are secured by the markle-tree.

What i'm asking is, does the contents -transactions- of one block effect the hash of the next block? if so, how it happens?  

If not, doesn't this mean that we can manipulate the transactions inside a block? because if we do so, the next block will have no idea what happens the previous block.

For example, lets say that we mined 10 blocks.

Then, we hacked block number 5 and changed the transactions.

How is this going to effect blocks number 6, 7,....?

I understand that if the majority of the nodes have the same values then the blockchain will be safe.

But what about a small blockchain owned by one node. In theory, does this mean that the block contents can be manipulated very easily?

Thank you
What i'm asking is, does the contents -transactions- of one block effect the hash of the next block? if so, how it happens?


Yes. The block hash of block X is explicitly included in the header of block X+1. So if you change the transactions in block X, it will affect the hash of block X, and that would change block X +1 as well. 


  For example, lets say that we mined 10 blocks. Then, we hacked block number 5 and changed the transactions.


The nodes on the network that already know about block 10 would not accept your new block 5 because it is a shorter chain (less work than the chain that is at block 10). They can recognize the new block 5 as being altered because the longer chain does not include it.