Physics
Name: ________________________
FREE FALL NOTES
Free Fall – the motion of an object falling under the influence of gravity
**ignore air resistance and the acceleration is constant
acceleration due to gravity = g = - 9.8 m/s2 = - 32 ft/s2
•
When objects are in Free Fall
a = g - 9.8 m/s = - 32 ft/s2
•
displacement = Δy and is in the negative direction
•
the velocity with which the object hits the ground is NOT 0 m/s
•
if the object “falls” or is “dropped” vi = 0 m/s
•
vi is negative (≠ 0 m/s) if the object is thrown downward
2
Use the kinematic equations to solve Free Fall problems.
v f = vi + a!t
!y = 12 (vi + v f )!t
!y = vi !t + 12 a!t 2
v 2f = vi2 + 2a!y
Example:
A flower pot falls out of a window 25 m above the ground. (A) What is its velocity when
it hits the ground? (B) How long is the pot in the air?
v 2f = (0m / s)2 + 2(!9.81m / s 2 )(!25m)
!y = vi !t + 12 a!t 2
v f = 490.5 ms2 = ±22.15 ms
2!y
2("25m)
= !t =
a
"9.81m / s 2
!t = 2.26s
2
v f = !22.15m / s
Throwing an Object Upward
•
The velocity decreases on the way up and is positive
•
The velocity increases on the way down and is negative
•
At the peak, v = 0 m/s
•
The acceleration is constant and equal to – 9.81 m/s2 the entire trip.
•
If the object is caught at the same height:
vf = -vi
∆tup = ∆tdown = ½ ∆ttotal
To find maximum height, look at either the 1st half or the 2nd
half. Remember, v = 0 m/s at the peak.
Example:
A ball is thrown upward at 8 m/s. (A) What is the velocity of the ball when it returns to
the thrower’s hands? (B) How long is the ball in the air? (C) How high did the ball go?
(A)
(B)
(C)
v f = vi + a!t
v f = !vi = !8
m
s
v f " vi
= !t
a
"8 ms " 8 ms
!t =
"9.81 sm2
!t = 1.63s
!y = 12 (vi + v f )!t
!y = 12 (+8 ms + 0)(0.815s)
!y = 3.26m