1. ConcepTest 6 Followup Which cannon ball hits the ground first?
0 = -0.5*9.8t2 + Voyt,
4.9t2 - V0sin(q)t = 0.
t{4.9t - V0sin(q)}= 0.
Thus, t = 0 or t = V0sin(q)/4.9
Cannon Ball A
Cannon Ball B
Both cannon balls hit the ground at the same time. B A
Marker

2. Example Problem #3.21 y = ? x = 2.1 m y v0x = v0cos600 = 3.2 m/s
v0y = v0sin600 = 5.54 m/s
v0 = 6.4 m/s
y = ?
600 x
x = 2.1 m
(a) Basically means, “Find value of y when x = 2.1 m.”
Find time when x = 2.1 m: x = 2.1 m = x0 + v0xt = 0 + v0xt  t = 2.1 m / 3.2 m/s  t = s
Now substitute this into equation for y: y = y0 + voyt – 0.5gt  y = 0 + (5.54 m/s)(0.656 s) – 0.5(9.8 m/s2)(0.656 s)2 = m
(b) vy = v0y – gt = m/s – (9.8 m/s2)(0.656 s) = –0.89 m/s

3. Example Problem #4.10 Free-body diagram: y N x F = 80 N W
(a) S Fx = max S Fy = may = 0 (no motion in y – direction)
F = max N – W = 0  N = W
All acceleration is in the x – direction , so use
atotal = ax = 2(x – x0) / t2.