Free Fall
Free Fall
• Only GRAVITY acting on object
– Air resistance is negligible
• Acceleration is ALWAYS 9.8 m/s/s
downwards
–
–
–
–
Position may be positive, zero or negative
Velocity may be positive, zero or negative
Acceleration is always 9.8 m/s/s downwards
9.8 m/s/s is such well-known, constant that
it is given its own special letter: g
10
Acceleration at the Top
• Velocity at top is
zero
• What is the
acceleration at the
top?
Free Fall Preflight
• A ball is thrown vertically upward. At the
top of its path its acceleration is
Acceleration of gravity is 9.8 m/s squared directed downward at all times.
acceleration is always g downwards. the acceleration of something in free fall is always -9.8 m/s/s
whenever an object is free falling its acceleration is always -9.8m/s2
The ball is no longer moving so it's acceleration is zero
acceleration is always zero at the top of the path because it is not moving.
77% responded -9.8 m/s/s
23% responded 0
Free Fall
• Drop heavy light objects
• Mechanical Universe Clip
– Guinea/feather tube
– Hammer/feather moon
ACT
•
The speed of an object in free fall
(Neglect Air Resistance)
A.
B.
C.
D.
E.
Always increases.
is constant.
Always decreases.
May increase or decrease or be constant.
May increase or decrease but is never
constant.
a = -g. Velocity becomes more and more
negative. If v > 0, speed decreases. If v <=
0 speed increases.
Free Fall ACTS
Fred throws a ball 30 mph vertically upward. Which of the
following statements are true about the ball’s velocity and
acceleration. (Let up be the positive direction)
On the way up?
A) v < 0
B) v = 0
C) v > 0
A) a < 0
B) a = 0
C) a > 0
A) v < 0
B) v = 0
C) v > 0
A) a < 0
B) a = 0
C) a > 0
On the way down?
How Far
ACT
Fred throws a ball 30 mph vertically upward and then
catches it again at the same height he threw it from. What
is the speed of the ball when he catches it? (Neglect air
resistance)
1) v < 30 mph
2) v = 30 mph
vy2
= vy02 - 2g(y-y0)
vy2
= vy02
3) v > 30 mph
Free Fall Example
Fred throws a ball 30 m/s vertically upward. What
is the maximum height the ball reaches? How long
does it take to reach this height?
v = v0 + a t
t = (v-v0) / a
= (0 – 30 m/s )/(-9.8 m/s2)
= 3.1 seconds
Dy = vo t + 0.5 a t2
Dy = (30 m/s)(3.1 s) + 0.5(-9.8 m/s2)(3.1s)2
= 46 m
ACT
Dennis and Carmen are standing on the edge of a cliff.
Dennis throws a basketball vertically upward, and at the
same time Carmen throws a basketball vertically downward
with the same initial speed. You are standing below the
cliff observing this strange behavior. Whose ball hits the
ground first?
A. Dennis' ball
B. Carmen's ball
C. Same
y = yo+vot + ½ a t2
Carmen v0
v0
H
Dennis: 0 = H + vot - ½ g t2
Carmen: 0 = H - vot - ½ g
t2
Dennis
vA
vB
32
ACT
Dennis and Carmen are standing on the edge of a cliff.
Dennis throws a basketball vertically upward, and at the
same time Carmen throws a basketball vertically downward
with the same initial speed. You are standing below the
cliff observing this strange behavior. Whose ball is moving
fastest when it hits the ground?
A. Dennis' ball
B. Carmen's ball
Correct:
C. Same
v2 = v02 -2gDy
Carmen v0
v0
Dennis
H
vA
vB
35