Question

A ball is thrown up with a velocity of 20 m/s. What is the total time of flight, neglecting air resistance? a. 8 sec b. 1 sec c. 2 sec d. 4 sec​

Answer 1

Answer:

4 sec

Explanation:

Using kinematic equation, v = u + a× t, when ball reaches to top of its trajectory, it's velocity becomes zero. Ball is thrown against gravity of earth, therefore a = - g, u = 20 m/s, we can find time of ascend as

0 =20 - 10 × t

20 = 10 × t

t = 20/10 = 2 sec

Time of ascend is same as time of descend = 2 sec.

Time of flight = time of ascend + time of descend

Therefore time of flight = 4 sec.

Answer 2

The correct answer to the following question is (d), and the total time of flight, neglecting air resistance is 4 seconds.

Given: initial velocity = 20 ms⁻¹

To find: total time of flight

Solution: As the ball is thrown upward, the final velocity will be zero.

According to the equation of Law of Motion, neglecting the air resistance,

V - U = at

Final velocity = 0,

Initial velocity = 20ms⁻¹

Acceleration = - 10ms⁻²   ( negative because the motion is against the acceleration due to gravity )

Putting the values in the equation, 0 - 20 = -10 × t

-20 = -10t

t = 2 second

Now, the time of ascending is equal to the time of descending, so the time of flight = 2×2 second = 4 second

Therefore, the total time of flight is 4 seconds.