1. A block of metal of mass 2kg is resting on
a frictionless floor. It is struck by a jet
releasing water at the rate of 1kg/sec at a
speed of 5ms-1. What will be the initial
acceleration of the block?

The acceleration is  

Explanation:

From the question we are told that

   The mass of the metal block is

    The mass flow rate of the water is  

    The speed of the water of the water release is

Generally according to the law of conservation of linear momentum

Now   is the initial momentum of the system which mathematically represented as

Now is the mass of water at the point of contact with the block which can be deduced as

Now since at initial the block is at rest

So

And   is the final  momentum of the system which mathematically represented as

So   is the final velocity of water which is zero due to the fact that when the water hits the block it losses its momentum and eventually the velocity becomes zero

    So

Thus  

Thus  

Now the average momentum change is  

Now the force acting on the block is  

and from the question the initial movement of the block took 1s as it is a mass of water moving at a rate of 1kg/s that caused the first movement of the block

  So

Now the acceleration is  

=>    

higher potential

explanation:

because it is in a high speed so it must move into a of high potential

3. dark matter

empty space could also be a simple vacuum