Chapter 2
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Dr. Jie Zou PHY 1151G Department of Physics 1 Chapter 2 One-Dimensional Kinematics (Cont.)
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Chapter 2. One-Dimensional Kinematics (Cont.). Outline. Acceleration Average acceleration Instantaneous acceleration Kinematics equation for one-dimensional motion under constant acceleration Free Fall. Acceleration. Average acceleration : the rate of change of velocity. - PowerPoint PPT Presentation
Transcript of Chapter 2
Dr. Jie Zou PHY 1151G Department of Physics
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Chapter 2
One-Dimensional Kinematics (Cont.)
Dr. Jie Zou PHY 1151G Department of Physics
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Outline Acceleration
Average acceleration Instantaneous acceleration
Kinematics equation for one-dimensional motion under constant acceleration
Free Fall
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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.
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Example: Average Acceleration
The winner of this race was traveling at a speed of 313.91 mi/h at the end of the quarter-mile course. The winning time was just 4.607 s, what was the average acceleration during the race? (Answer: ~ 3g)
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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.
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(a) And (d): Car speeds up
(b) And (c): Car slows down
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Kinematics Equation for 1D Motion under Constant Acceleration
vav = (vi + vf)/2.
Table 2-4
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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? (Answer: (a) 13.5 m/s (b) 37.5 m)
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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
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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.
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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? (Answer: 0.782 s)
(b) What is the person’s speed on entering the water? (Answer: 7.67 m/s)
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Example: Free Fall 2. A volcano shoots out blobs of
molten lava, called lava bombs, from its summit. 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 its launch height is 4.75 s, and its acceleration is 9.81 m/s2 downward, what is its initial speed? (Answer: 23.3 m/s)
Real-World Physics!
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Example: Free Fall 3. A rocket, initially at rest on the
ground, accelerates straight upward from rest with constant acceleration 35.0 m/s2. The acceleration period lasts for time 4.00 s until the fuel is exhausted. After that, the rocket is in free fall. Find the maximum height reached
by the rocket (ignore air resistance). For answer, see class discussion.
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Homework See online homework assignment
at www.masteringphysics.com Hand-written homework
assignment: Chapter 2, Page 51, Problems: #48.
