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Q.
The ball is projected up from ground with speed $30\, m / \sec$. What is the average velocity for time $0$ to $4 \sec$?
Motion in a Straight Line
Solution:
We should know the displacement in this time.
$\vec{ S }=\vec{ ut }+\frac{\vec{ at }^2}{2} $ (we take upward as positive)
$S=30 \times 4-10 \times 4 \times \frac{4}{2}=40 \,m $
The average velocity will be $10 \,m / \sec$.