the velosity of a particle moving along the x axis varies according to the expression Vx=40-5t2 whereVx is metre per second and t is in seconds the average accelaration in the time interval t=0 to t=2s is​

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  -10 m/s²

Explanation:

The acceleration is the rate of change of velocity. The average rate of change of velocity will be the difference over the interval, divided by the width of the interval.

 (V(2) -V(0))/(2 -0) = (40 -5(2²)) -(40 -5(0²)) = (20 -40)/2 = -10 . . . m/s²

The average acceleration on [0, 2] is -10 m/s².

Additional comment

The average rate of change of a quadratic function on an interval is exactly equal to the derivative of the function at the midpoint of the interval:

  Vx'(t) = -10t   ⇒  Vx'(1) = -10 . . . m/s² (midpoint of [0, 2] is t=1)

Example of positive - sun gives heat such as thermal energy and thermal energy gives nutrients in plants and plants give foods to able for us to survive. 
Example of negative is - too much heat can lead into radiant energy and too much radiant energy can cause an explosion. also too much heat can cause you to a severe burn such a second degree burn. high temperature can lead to dehydration. 
Hope this helps