1. Fundamentals of Physics School of Physical Science and Technology
Mechanics
(Bilingual Teaching)
张昆实
School of Physical Science and Technology
Yangtze University

2. Chapter 2 Motion Along a Straight Line
2-2 Position and Displacement
2-3 Average Velocity and average Speed
2-4 Instantaneous Velocity and Speed
2-5 Acceleration
2-6 Constant Acceleration: A Special Case
2-7 Another Look at Constant Acceleration
2-8 Free-Fall Acceleration

3. 2-1 Motion
★ The motion is along a straight line only (in one dimension).
★The moving object is either a particle or an object that moves like a particle.
★ Forces will not be discussed.
★ a reference frame (参考系) must be chosen and a coordinate system (坐标系) must be constructed on it.

4. 2-2 Position and Displacement
★ Position in axis
reference point: Origin
● negative direction
● Positive direction
Negative Positive
Origin * *
x = -3m 1
Displacement vector
magnitude：the distance
between the original and
final positions.
direction: from an original
position to final position.
Can be represented by a
plus sign or a minus sign.
x = 3m 2
★ Displacement

5. 2-3 Average Velocity and Average Speed
If a particle moves through a displacement in a time Interval ,Then its ●
★ Average Speed:
average change in displacement
velocity time interval * * * * * *
(2-2) *
Run
400m
In 50s * * * *
average total distance
speed time interval = * * * * s =
total distance
avg * *
(2-3)

6. 2-4 Instantaneous Velocity and Speed
★ Instantaneous Velocity As
● at a given instant:
(1) is the derivative of x with respect to t
(P16 sample problem 2-3).
(2) is the slope of the position-time curve at the point at that instant (P16. sample problem 2-2).
★ Speed is the magnitude of velocity.
(2-4)

7. 2-5 Acceleration ★ Average Acceleration
When a particle’s velocity changes, the particle is said to undergo acceleration.
Average Acceleration:
★ Instantaneous acceleration
(2-7)
(2-8)
(2-9)

8. 2-6 Constant Acceleration: A Special Case
The average velocity
Substituting 2-11 for :
Substituting 2-14 into 2-12:
Eq and 2-15: basic equations for constant acceleration. They can be
combined in three ways to
Yield additional equations.
★ In many cases the acceleration is constant ●
Recast this equation:
●Similarly
(2-13)
(2-14)
(2-11)
(2-15)
(2-12)

9. 2-6 Constant Acceleration: A Special Case
The average velocity
Substituting 2-11 for :
Substituting 2-14 into 2-12:
Eq and 2-15: basic equations for constant acceleration. They can be
combined in three ways to
yield additional equations.
★ In many cases the acceleration is constant ●
Recast this equation:
●Similarly
(2-13)
(2-14)
(2-11)
(2-15)
(2-12)

10. 2-6 Constant Acceleration: A Special Case
(2-11)
(2-16)
Eliminate t :
(2-12)
(2-17)
Eliminate a :
(2-13)
(2-18)
Eliminate v0 :
(2-14)
(2-15)

11. 2-7 Another Look at Constant Acceleration
Eq and 2-15 can be ontained by integration
Indefinite integral
Indefinite integral
To find constant C, let
To find constant C’, let
(4-15)
(4-11)

12. 2-8 Free-Fall Acceleration
★ An important example of straight-line motiom with constant acceleration.
Eliminate the air resistance,
toss an object either up or down,
the object accelerates downward at a certain
constant rate: free-fall acceleration
At sea level in Earth’s midlatitudes,
The value of varies slightly with latitude and
with elevation *** Note: For free fall ***
(1) The directions of motion are now along a verticle axis with the positive direction of upward.
(2) The free-fall acceleration, being downward, is now negative!

13. 2-8 Free-Fall Acceleration
Horizontal motion
( free fall )
Vertical motion
( constant acceleration )
(2-11)
(2-11)
(2-15)
(2-15)
(2-16)
(2-16)
P24. Sample Problem 2-7