SPH4UI Lecture 1 Notes - The Burns Home Page

Question:
Solution:
• The pilot of a hovering helicopter
drops a lead brick from a height
of 1000 m. How long does it
take to reach the ground and
how fast is it moving when it gets
there? (neglect air resistance)
1000 m
• First choose coordinate system.
• Origin and y-direction.
• Next write down position equation:
1
y y0 v0yt gt
2
2
1000 m
• Realize that v 0y = 0.
1
y y0
gt
2
2
y
y = 0
Solution:
1D Free Fall
• Solve for time t when y = 0 given
that y 0 = 1000 m.
1 2
y y0
- gt 2
t
2y
21000m
14.3s
g 9.81m s
0
2
y 0 = 1000 m
• Alice and Bob are standing at the top of a cliff of
height H. Both throw a ball with initial speed v 0 , Alice
straight down and Bob straight up. The speed of the
balls when they hit the ground are v A and v B
respectively. Which of the following is true:
• Recall:
• Solve for v y :
vy
v -v 2 a( y - y )
2 2
y 0y
0
2gy
0
140 m/
s
y = 0
y
(a) v A < v B (b) v A = v B (c) v A > v B
v 0
v A
Alice
v 0
v B
Bob
H
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