1. Motion in One Dimension
Chapter 2
Motion in One Dimension

2. Kinematics
Describes motion while ignoring the agents that caused the motion
For now, will consider motion in one dimension
Along a straight line
Will use the particle model
A particle is a point-like object, has mass but infinitesimal size

3. Position Defined in terms of a frame of reference
One dimensional, so generally the x- or y-axis
The object’s position is its location with respect to the frame of reference

4. Position-Time Graph
The position-time graph shows the motion of the particle (car)
The smooth curve is a guess as to what happened between the data points

5. Displacement
Defined as the change in position during some time interval
Represented as x
x = xf - xi
SI units are meters (m) x can be positive or negative
Different than distance – the length of a path followed by a particle

6. Vectors and Scalars
Vector quantities need both magnitude (size or numerical value) and direction to completely describe them
Will use + and – signs to indicate vector directions
Scalar quantities are completely described by magnitude only

7. Average Velocity
The average velocity is rate at which the displacement occurs
The dimensions are length / time [L/T]
The SI units are m/s
Is also the slope of the line in the position – time graph

8. Average Speed Speed is a scalar quantity
same units as velocity
total distance / total time
The average speed is not (necessarily) the magnitude of the average velocity

9. Instantaneous Velocity
The limit of the average velocity as the time interval becomes infinitesimally short, or as the time interval approaches zero
The instantaneous velocity indicates what is happening at every point of time

10. Instantaneous Velocity
The general equation for instantaneous velocity is
The instantaneous velocity can be positive, negative, or zero

11. Instantaneous Velocity
The instantaneous velocity is the slope of the line tangent to the x vs. t curve
This would be the green line
The blue lines show that as t gets smaller, they approach the green line

12. Instantaneous Speed
The instantaneous speed is the magnitude of the instantaneous velocity
Remember that the average speed is not the magnitude of the average velocity

13. Average Acceleration
Acceleration is the rate of change of the velocity
Dimensions are L/T2
SI units are m/s²

14. Instantaneous Acceleration
The instantaneous acceleration is the limit of the average acceleration as t approaches 0

15. Instantaneous Acceleration
The slope of the velocity vs. time graph is the acceleration
The green line represents the instantaneous acceleration
The blue line is the average acceleration

16. Acceleration and Velocity
When an object’s velocity and acceleration are in the same direction, the object is speeding up
When an object’s velocity and acceleration are in the opposite direction, the object is slowing down

17. Acceleration and Velocity
The car is moving with constant positive velocity (shown by red arrows maintaining the same size)
Acceleration equals zero

18. Acceleration and Velocity
Velocity and acceleration are in the same direction
Acceleration is uniform (blue arrows maintain the same length)
Velocity is increasing (red arrows are getting longer)
This shows positive acceleration and positive velocity

19. Acceleration and Velocity
Acceleration and velocity are in opposite directions
Acceleration is uniform (blue arrows maintain the same length)
Velocity is decreasing (red arrows are getting shorter)
Positive velocity and negative acceleration

20. 1D motion with constant acceleration
tf – ti = t

21. 1D motion with constant acceleration
In a similar manner we can rewrite equation for average velocity:
and than solve it for xf
Rearranging, and assuming

22. 1D motion with constant acceleration
(1)
Using
and than substituting into equation for final position yields
(1)
(2)
(2)
Equations (1) and (2) are the basic kinematics equations

23. 1D motion with constant acceleration
These two equations can be combined to yield additional equations.
We can eliminate t to obtain
Second, we can eliminate the acceleration a to produce an equation in which acceleration does not appear:

24. Kinematics with constant acceleration - Summary

25. Kinematics - Example 1
How long does it take for a train to come to rest if it decelerates at 2.0m/s2 from an initial velocity of 60 km/h?

26. Kinematics - Example 1
How long does it take for a train to come to rest if it decelerates at 2.0m/s2 from an initial velocity of 60 km/h?
Using we rearrange to solve for t:
Vf = 0.0 km/h, vi=60 km/h and a= -2.0 m/s2.

27. Kinematic Equations -- summary

28. Kinematic Equations
The kinematic equations may be used to solve any problem involving one-dimensional motion with a constant acceleration
You may need to use two of the equations to solve one problem
Many times there is more than one way to solve a problem

29. A car is approaching a hill at 30
A car is approaching a hill at 30.0 m/s when its engine suddenly fails just at the bottom of the hill. The car moves with a constant acceleration of –2.00 m/s2 while coasting up the hill. (a) Write equations for the position along the slope and for the velocity as functions of time, taking x = 0 at the bottom of the hill, where vi = 30.0 m/s. (b) Determine the maximum distance the car rolls up the hill.

30. (a). Take at the bottom of the hill where xi=0, vi=30m/s, a=-2m/s2
(a) Take at the bottom of the hill where xi=0, vi=30m/s, a=-2m/s2. Use these values in the general equation

31. (a). Take at the bottom of the hill where xi=0, vi=30m/s, a=-2m/s2
(a) Take at the bottom of the hill where xi=0, vi=30m/s, a=-2m/s2. Use these values in the general equation

32. The distance of travel, xf,
becomes a maximum,
when
(turning point in the motion).
Use the expressions found in part (a) for
when t=15sec.

33. Graphical Look at Motion: displacement-time curve
The slope of the curve is the velocity
The curved line indicates the velocity is changing
Therefore, there is an acceleration

34. Graphical Look at Motion: velocity-time curve
The slope gives the acceleration
The straight line indicates a constant acceleration

35. Graphical Look at Motion: acceleration-time curve
The zero slope indicates a constant acceleration

36. Freely Falling Objects
A freely falling object is any object moving freely under the influence of gravity alone.
It does not depend upon the initial motion of the object
Dropped – released from rest
Thrown downward
Thrown upward

37. Acceleration of Freely Falling Object
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
g decreases with increasing altitude
g varies with latitude
9.80 m/s2 is the average at the Earth’s surface

38. Acceleration of Free Fall
We will neglect air resistance
Free fall motion is constantly accelerated motion in one dimension
Let upward be positive
Use the kinematic equations with ay = g = m/s2

39. Free Fall Example
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

40. A student throws a set of keys vertically upward to her sorority sister, who is in a window 4.00 m above. The keys are caught 1.50 s later by the sister's outstretched hand. (a) With what initial velocity were the keys thrown? (b) What was the velocity of the keys just before they were caught?

41. A student throws a set of keys vertically upward to her sorority sister, who is in a window 4.00 m above. The keys are caught 1.50 s later by the sister's outstretched hand. (a) With what initial velocity were the keys thrown? (b) What was the velocity of the keys just before they were caught?
(a)

42. A student throws a set of keys vertically upward to her sorority sister, who is in a window 4.00 m above. The keys are caught 1.50 s later by the sister's outstretched hand. (a) With what initial velocity were the keys thrown? (b) What was the velocity of the keys just before they were caught?
(b)

43. A ball is dropped from rest from a height h above the ground
A ball is dropped from rest from a height h above the ground. Another ball is thrown vertically upwards from the ground at the instant the first ball is released. Determine the speed of the second ball if the two balls are to meet at a height h/2 above the ground.

44. A ball is dropped from rest from a height h above the ground
A ball is dropped from rest from a height h above the ground. Another ball is thrown vertically upwards from the ground at the instant the first ball is released. Determine the speed of the second ball if the two balls are to meet at a height h/2 above the ground.
1-st ball:
2-nd ball:

45. A freely falling object requires 1. 50 s to travel the last 30
A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?

46. A freely falling object requires 1. 50 s to travel the last 30
A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?
Consider the last 30 m of fall. We find its speed 30 m above the ground:

47. A freely falling object requires 1. 50 s to travel the last 30
A freely falling object requires 1.50 s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?
Now consider the portion of its fall above the 30 m point. We assume it starts from rest
Its original height was then:

48. Motion Equations from Calculus
Displacement equals the area under the velocity – time curve
The limit of the sum is a definite integral

49. Kinematic Equations – General Calculus Form

50. Kinematic Equations – Calculus Form with Constant Acceleration
The integration form of vf – vi gives
The integration form of xf – xi gives

51. The height of a helicopter above the ground is given by h = 3
The height of a helicopter above the ground is given by h = 3.00t3, where h is in meters and t is in seconds. After 2.00 s, the helicopter releases a small mailbag. How long after its release does the mailbag reach the ground?

52. The height of a helicopter above the ground is given by h = 3
The height of a helicopter above the ground is given by h = 3.00t3, where h is in meters and t is in seconds. After 2.00 s, the helicopter releases a small mailbag. How long after its release does the mailbag reach the ground?

53. The height of a helicopter above the ground is given by h = 3
The height of a helicopter above the ground is given by h = 3.00t3, where h is in meters and t is in seconds. After 2.00 s, the helicopter releases a small mailbag. How long after its release does the mailbag reach the ground?
The equation of motion of the mailbag is:

54. Automotive engineers refer to the time rate of change of acceleration as the "jerk." If an object moves in one dimension such that its jerk J is constant, (a) determine expressions for its acceleration ax(t), velocity vx(t), and position x(t), given that its initial acceleration, speed, and position are axi , vxi, and xi , respectively. (b) Show that

55. Automotive engineers refer to the time rate of change of acceleration as the "jerk." If an object moves in one dimension such that its jerk J is constant, (a) determine expressions for its acceleration ax(t), velocity vx(t), and position x(t), given that its initial acceleration, speed, and position are axi , vxi, and xi , respectively. (b) Show that
(a)
constant
when

56. Automotive engineers refer to the time rate of change of acceleration as the "jerk." If an object moves in one dimension such that its jerk J is constant, (a) determine expressions for its acceleration ax(t), velocity vx(t), and position x(t), given that its initial acceleration, speed, and position are axi , vxi, and xi , respectively. (b) Show that
(a)
when

57. Automotive engineers refer to the time rate of change of acceleration as the "jerk." If an object moves in one dimension such that its jerk J is constant, (a) determine expressions for its acceleration ax(t), velocity vx(t), and position x(t), given that its initial acceleration, speed, and position are axi , vxi, and xi , respectively. (b) Show that
(a)
when

58. (b)
Recall the expression for v :

59. The acceleration of a marble in a certain fluid is proportional to the speed of the marble squared, and is given (in SI units) by a = –3.00 v2 for v > 0. If the marble enters this fluid with a speed of 1.50 m/s, how long will it take before the marble's speed is reduced to half of its initial value?

60. The acceleration of a marble in a certain fluid is proportional to the speed of the marble squared, and is given (in SI units) by a = –3.00 v2 for v > 0. If the marble enters this fluid with a speed of 1.50 m/s, how long will it take before the marble's speed is reduced to half of its initial value?

61. The acceleration of a marble in a certain fluid is proportional to the speed of the marble squared, and is given (in SI units) by a = –3.00 v2 for v > 0. If the marble enters this fluid with a speed of 1.50 m/s, how long will it take before the marble's speed is reduced to half of its initial value?