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Free Fall

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Free Fall
Up and Down
пЃќ
Two balls are thrown at the same initial speed, one
upward and one downward with negligible air
resistance. When the balls strike the ground the ball
thrown upward has a speed equal to
A) four times the speed of the ball thrown downward.
B) two times the speed of the ball thrown downward.
C) the same speed of the ball thrown downward.
D) one half the speed of the ball thrown downward.
E) one quarter the speed of the ball thrown
downward.
Falling on the Earth
пЃќ
Objects fall to the earth.
пЃќ
Items that start by going up, stop and then fall again,
reversing direction.
пЃќ
Items that fall from greater heights end up with more
speed.
пЃќ
The velocity changes, there must be an acceleration.
Surface Gravity
пЃќ
At the surface of the Earth, gravity creates a nearly
constant acceleration.
пЃќ
This gravitational acceleration is called g.
• g = 9.8 m/s2 and is directed downward.
пЃќ
Note that g is an acceleration.
• Don’t be fooled by phrases like g-force.
• g can be used as a unit of acceleration.
Up and Down
пЃќ
пЃќ
The constant acceleration
equations apply for a = -g.
The position is now y, in the
vertical direction.
v
v0
Initial
upwards
velocity
t
Negative slope = -g
yпЂЅпЂ­
1
2
yпЂЅпЂ­
gt пЂ« v 0 t пЂ« y 0
пЂЁv пЂ­ v 0 пЂ©2
2g
2
y
пЂ­ v0
пЂЁv пЂ­ v 0 пЂ©
g
пЂ« y0
y0
t
Velocity
here is –v0
Initial Velocity
пЃќ
A person throws a ball up in
the air releasing the ball at
1.5 m. If it lands on the
ground after 2.0 s, find the
initial velocity.
yпЂЅпЂ­
2
1
2
пЃќ
Simplify the time equation,
with
• y0 = 1.5 m
• y=0m
• t = 2.0 s.
1
gt пЂ« v 0 t пЂ« y 0
2
gt пЂ­ y 0 пЂЅ v 0 t
v0 пЂЅ
2
1
2
gt пЂ­
y0
t
v0 = 9.1 m/s
Exceptions to Free Fall
пЃќ
пЃќ
On Earth’s surface downward acceleration is
constant.
Why do we observe that different objects accelerate
differently?
• Friction reduces the total acceleration.
• We think about velocity instead of acceleration.
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