Kinematics and Motion Homework

Question 45: "If a sprinter accelerates at 2.2 m/s^2 for 2.5 s, what is her velocity after this time, assuming initial velocity is 0?"

Answer: I chose to substitute the values into Mommy because that equation had all the values I had and needed. I just isolated final velocity and solved for it. Easy.

Question 47: "A fastball pitcher can throw a baseball at 100 km/h. If the windup and throw take 1.5 s, what is the acceleration of the ball?"

Answer: It took me a while to come up with this answer, because at first I didn't get that by throw, they meant the ball only moves for 1.5 seconds. After that, using Mommy (her smile gets bigger and bigger, I swear), substituting values, and remembering to convert to m/s wasn't so bad.

Question 56: "A car travelling at 40 km/h accelerates at 2.3 m/s^2 for 2.7 s. How far has it travelled in that time? What is its final velocity?"

Answer: At first I got this wrong because I accidentally did the next question, which was the exact same thing but with -2.3m/s^2. Also, it doesn't matter afterward whether you use girl or boy because you have the values for and can find your values through both, but I chose the boy because the girl is used more often in these questions. Give him a chance. 

Question 58: "If 100m sprinters accelerate from rest for 3.5s at 2.8m/s^2, how far have they run to this point? How long will it take them to complete the 100m sprint, assuming they maintain their speed the rest of the way?"

Answer: I did the beginning right initially, but made a mistake later because I didn't realize the sprinters had stopped accelerating. Oops. But the answer below is right.

Question 60: "An object is thrown up from a cliff at 10 m/s and reaches a velocity of 20 m/s down as it lands. If the acceleration due to gravity is 9.8 m/s^2, what is the object's displacement? How long did it take the object to land from the time it was thrown up?"

Answer: This involves vector adding. Basically we can assume that up is the positive direction and down is the negative direction. Then the velocities are 10 m/s and -20 m/s. Also, Daddy finally comes back into the picture :) !