Freely Falling Object
Movie
• Object falling near Earth’s
surface is one of the most
common examples of
motion with constant
acceleration
•
Near the surface of the
Earth, all objects experience
approximately the same
acceleration due to gravity!
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Demos
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coffee filters
paper and ball
Another Movie!
Freely Falling Object
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A freely falling object is any object moving
freely under the influence of gravity alone.
The acceleration of an object in free fall is
directed downward, regardless of the initial
motion
The magnitude of free fall acceleration is g =
9.80 m/s2
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g decreases with increasing altitude
g varies with latitude
9.80 m/s2 is the average at the Earth’s surface
1
ConcepTest
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You are throwing a ball straight up in
the air. A the highest point, the ball’s
1. velocity and acceleration are zero.
2. velocity is nonzero but its
acceleration is zero.
3. acceleration is nonzero, but its
velocity is zero.
4. velocity and acceleration are both
nonzero.
Free Fall Example
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Initial velocity at A is upward (+) and
acceleration is g (-9.8 m/s2)
At B, the velocity is 0 and the
acceleration is g (-9.8 m/s2)
At C, the velocity has the same
magnitude as at A, but is in the
opposite direction
The displacement is –50.0 m (it
ends up 50.0 m below its starting
point)
ConcepTest
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If you drop an object in the absence of
air resistance, it accelerates downward
at 9.8 m/s2. If instead you throw it
downward, its downward acceleration
after release is
1. less than 9.8 m/s2.
2. 9.8 m/s2.
3. more than 9.8 m/s2.
Think-Pair-Share
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Change the initial velocity in the previous
problem to 30.0 m/s.
(a) When will the ball hit the ground?
(b) How fast is the ball moving when it hits the
ground?
(c) When the ball reach its maximum height?
(d) How fast is the ball moving at its
maximum height?
(e) What is the ball’s acceleration when it is at
its maximum height?
(f) Graph the ball’s position, velocity and
acceleration as a function of time.
2
Pair Activity
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Measure your reaction time using a
meter stick.
Ch 2: Problem 44
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Emily challenges her friend David to
catch a dollar bill as follows. She holds
the bill vertically, with the center of the
bill between David’s index finger and
thumb. David must catch the bill after
Emily releases it without moving his
hand downward. If his reaction time is
0.2 s, will he succeed? Explain your
reasoning.
3