To solve this problem we will apply the linear motion kinematic equations, specifically the concept of acceleration as a function of speed and time, as well as Newton's second law.
PART A) Acceleration can be described as changing the speed in a period of time therefore,

Force is the proportional change between mass and acceleration therefore


PART B) We will apply the same concept given but now we will change the time to 21s therefore:

Now the force


Answer:
With sonar, what happens to sound pulses from a ship after they hit the ocean floor? ... They bounce back to the ship.
Explanation:
I believe we live in the Cenozoic era
We live in the Holocene Epoch, of the Quaternary Period, in the Cenozoic Era (of the Phanerozoic Eon).
This is very good conceptual question and can clear your doubts regarding work-energy theorem.
Whenever force is perpendicular to the direction of the motion, work done by that force is zero.
According to work-energy theorem,
Work done by all the force = change in kinetic energy.
here, work done = 0.
Therefore,
0=change in kinetic energy
This means kinetic energy remains constant.
Hope this helps