Question

A rock is thrown straight up into the air with an initial speed of 55 m/s at time t = 0. Ignore air resistance in this problem. At what times does it move with a speed of 36 m/s? Note: There are two answers to this problem.

Answer 1

Answer:

After 1.938 sec velocity of rock will be 36 m/sec

Explanation:

We have given initial velocity at which rock is thrown u = 55 m/sec

Final velocity v = 36 m/sec

Acceleration due to gravity $g=9.8m/sec^2$

According to first equation of motion we know that $v=u+gt$, here v is final velocity, u is initial velocity, g is acceleration due to gravity and t is time

So $36=55-9.8t$ ( Negative sign is due to rock is thrown upward )

So $9.8t=19$

t = 1.938 sec

So after 1.938 sec velocity of rock will be 36 m/sec