Which cannon ball hits the ground first?

ConcepTest 6 FollowupConcepTest 6 Followup

A. Cannon Ball AB. Cannon Ball BC. Both cannon balls hit

the ground at the same time.

A

B

Marker

y 1

2gt 2 v0yt y0 0 = -0.5*9.8t2 + Voyt,

4.9t2 - V0sin()t = 0.t{4.9t - V0sin()}= 0.

Thus, t = 0 or t = V0sin()/4.9

Example Problem #3.21

(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 = 0.656 s

Now substitute this into equation for y: y = y0 + voyt – 0.5gt2

y = 0 + (5.54 m/s)(0.656 s) – 0.5(9.8 m/s2)(0.656 s)2 = 1.53 m

y

x

y = ?

x = 2.1 m

600

v0 = 6.4 m/sv0x = v0cos600 = 3.2 m/s

v0y = v0sin600 = 5.54 m/s

(b) vy = v0y – gt = 5.54 m/s – (9.8 m/s2)(0.656 s) = –0.89 m/s

Example Problem #4.10Free-body diagram:

x

y

N

W

F = 80 N

(a) Fx = max 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.