What was the initial velocity of a rock throw upwards if the velocity after 8 s is 90m/s?

Initial velocity of rock = 11.6 m/s

Explanation:

Given:

Final velocity of rock = 90 m/s

Time taken by rock = 8 seconds

Find:

Initial velocity of rock

Computation:

Using first equation of motion;

v = u + at

For rock

v = u + gt

Acceleration due to gravity = 9.8 m/s²

90 = u + (9.8)(8)

90 = u + 78.4

u = 90 - 78.4

u = 11.6

Initial velocity of rock = 11.6 m/s

we need to know the time interval during which the force acts. impulse of force = momentum change