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Dr. Jie Zou PHY 1151G Department of Physics
2
Outline Acceleration
Average acceleration Instantaneous acceleration
Kinematics equation for one-dimensional motion under constant acceleration
Free Fall
Dr. Jie Zou PHY 1151G Department of Physics
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Acceleration Average acceleration: the rate of change
of velocity. aav = v/t = (vf – vi )/(tf – ti). SI units: meter per second squared, (m/s2).
Instantaneous acceleration: acceleration at any instant of time.
When acceleration is constant, the instantaneous and average accelerations are the same. Constant acceleration: Both the magnitude
and direction do not change with time.
Dr. Jie Zou PHY 1151G Department of Physics
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Example: Average Acceleration Sabb advertises a car that goes
from 0 to 60.0 mi/h in 6.2 s. What is the average acceleration of this car?
An airplane has an average acceleration of 5.6 m/s2 during takeoff. How long does it take for the plane to reach a speed of 150 mi/h? (1 mi = 1609 m)
Dr. Jie Zou PHY 1151G Department of Physics
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More about the Directions of Velocity and Acceleration When the velocity and acceleration of an
object have the same signs, the velocity and acceleration point in the same direction and the object speeds up.
When the velocity and acceleration of an object have opposite signs, the velocity and acceleration point in opposite directions and the object slows down.
Dr. Jie Zou PHY 1151G Department of Physics
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(a) And (d): Car speeds up
(b) And (c): Car slows down
Dr. Jie Zou PHY 1151G Department of Physics
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Kinematics Equation for 1D Motion under Constant Acceleration
vav = (vi + v)/2.
Table 2-4
Dr. Jie Zou PHY 1151G Department of Physics
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Example-Full Speed Ahead A boat leaves at an
initial speed of 1.50 m/s. It speeds up with an acceleration at 2.40 m/s2. (a) How fast is the
boat moving after accelerating for 5.00 s.
(b) How far has the boat traveled in this time?
Dr. Jie Zou PHY 1151G Department of Physics
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Free Falling Objects A freely falling object is any object
moving freely under the influence of gravity alone. In real situations, when air resistance can be
neglected, the motion can be approximated as a free fall.
Free fall Dropped (released) from rest Thrown downward Thrown upward
Dr. Jie Zou PHY 1151G Department of Physics
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Acceleration of Freely Falling Object Direction of the free fall acceleration:
The acceleration of an object in free fall is directed downward.
Magnitude of free fall acceleration, g g decreases with increasing altitude and g varies
with latitude 9.80 m/s2 is the average at the Earth’s surface
Assuming the constant value of g=9.80 m/s2 near the Earth surface, free fall is a 1D motion with constant acceleration Both the direction and magnitude do not change.
Dr. Jie Zou PHY 1151G Department of Physics
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Example: Free Fall 1. A person steps off the end of a
3.00-m-high diving board and drops to the water below. (a) How long does it take for the
person to reach the water? (b) What is the person’s speed on
entering the water?
Dr. Jie Zou PHY 1151G Department of Physics
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Example: Free Fall 2. A volcano shoots out blobs of molten
lava, called lava bombs, from ground level. A geologist observing the eruption uses a stopwatch to time the flight of a particular lava bomb that is projected straight upward. If the time for it to rise and fall back to the ground is 4.75 s, and its acceleration is 9.81 m/s2 downward, what is its initial speed?