Ch 3 Assignment Answers

Ch 3 Review Answers:

  1. The downward (vertical) motion of a projectile is identical to free fall. 
  2. Both balls hit at the same time. Since the vertical motion of the projectile is free fall, the vertical motion of both balls is the same. 
  3.  
  4.   
  5. The ball will be moving at 20 m/s when you catch it. It goes up for about 2 seconds, then falls from rest for about 2 seconds.

Ch 3 Think & Explain Answers:

  1. The fact that the falling rain's speed is about the same 10 meters above the ground as it is just before it hits the ground tells me that the rain is not in free fall - air resistance must be a factor in its motion. If the rain were in free fall, it would accelerate at 10 m/s/s all the way to the ground. Since the rain's velocity is not changing, it is not accelerating, so it is not in free fall. That's a good thing - who wants to be hit by a 300 mi/h raindrop!

Ch 3 Think & Solve Answers:

  1. Harry's horizontal motion is at constant velocity:
    d = vt = (100 m/s)(2 s) = 200 m
  2. First determine how long it would take them to free fall 45 meters from rest:
    T&S#3, part 1
    So, how fast will they have to be moving horizontally to move 15 meters in 3 seconds?
    d = vt
    15 m = v (3 s)
    v = 5 m/s
    They must leave the balcony with a horizontal speed of 5 m/s.
  3. A kid fires a slingshot pellet directly at a target that is far enough away to take 1/2 second to reach.
    1. How far below the target will the pellet hit?
      How far will an object free fall from rest in 0.5 sec?
      d = 0.5at2 = 0.5(10 m/s2)(0.5 s)2 = 1.25 m
      The pellet will hit 1.25 meters below the target.
    2. How high above the target should she aim?
      The easy answer is that she should aim 1.25 m above the target, and this answer would be acceptable for this question, but actually she will have to aim higher than that - do you see why?
  4. A boy throws a ball horizontally from a tower 20 m tall so that the ball lands 60 m from the base of the target. What was the boy's throwing speed?
    The ball will be in the air for 2 seconds, since this is the time it takes for an object to fall 20 meters from rest (d = 0.5at2). If the ball moves horizontally 60 meters in 2 seconds, it has a horizontal speed of 30 m/s (d = vt).
  5. First, how much time does it take the crate to fall 125 m? This is free fall (constant acceleration), so:
    1. d = (1/2) at2
      125 m = (1/2)(10 m/s2)t2
      t2 = 25 s2
      t = 5 s

    Now, how far will the crate move horizontally (at constant velocity) during this 5 second interval?

    d = vt = (50 m/s)(5 s) = 250 m

last update September 7, 2007 by JL Stanbrough