Question

1. The boy drops the ball from a roof of a house, which takes 3 seconds to hit the ground. Calculate the velocity before the ball crashes to the ground.

2. John throws the ball straight upward and after 1 second it reaches its maximum height then it does free fall motion which takes two seconds. Calculate the maximum height and velocity of the ball before it crashes to the ground.

3. An object does free fall motion. It hits the ground after 4 seconds. Calculate the velocity of the object after 3 seconds and before it hits the ground. What can be the height it is thrown?

4. A rock is fire vertically upward with an initial velocity of 29 m/s. find the rocks’ maximum altitude.

5. A basketball player jumps straight up to grab a rebound. If he was in the air for o.80 second, how high did he jump?

6. What is the instantaneous speed of a book dropping from the 25th floor after 2.5 seconds?

7. How high is a building if it takes 5 seconds for a ball to hit the floor?

8. The world’s tallest building in Burj Dubai. Rounded to the nearest 10, the building is 830 meters high. How long will it take for an object to hit the floor?

9. A.) how long does it take a ball to fall from a roof to the ground 7.0m below?
B.) with what speed does it strike the ground?

10. A rock is dropped of a cliff 115 meters high. How long does it take for a rock to reach the ground?

Answer 1

1.

we can use kinematics to find the final velocity

$v_f = v_i + at$

$v_f = 0 + 10*3 = 30 m/s$

2.

Total time of the motion of ball T = 2 s

so we can use

$T = \frac{2*v_i}{g}$

$2 = \frac{2*v_i}{9.8}$

$v_i = 9.8 m/s$

now maximum height will be

$H = \frac{v_i^2}{2g}$

$H = 4.9 m$

also the final speed at which ball collide with the ground will be same as initial speed so its 9.8 m/s.

3.

velocity of object at any instant during free fall is given by

$v_f = v_i + at$

After t = 3 s

$v_f = 0 + 9.8*3 = 29.4 m/s$

after t = 4 s

$v_f = 0 + 9.8*4 = 39.2 m/s$

height from which it is dropped is given by

$d = \frac{v_f + v_i}{2}*t$

$d = \frac{0 + 39.2}{2}*4 = 78.4 m$