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Erratic Motion-Basic Mecanical Engineering-Lecture Slides, Slides of Mechanical Engineering

Prof. Dasmaya Sidhu delivered this lecture at National Institute of Industrial Engineering for Basic Mechanical Engineering course. It includes: Rectilinear, Kinematics, Erratic, Motion, Erratic, Equations, Relationship, Velocity, Slope

Typology: Slides

2011/2012

Uploaded on 07/31/2012

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Rectilinear Kinematics: Erratic Motion
When particle’s motion is erratic, it is best described
graphically using a series of curves that can be generated
experimentally from computer output.
A graph can be established describing the relationship with
any two of the variables,
a, v, s, t
2
any two of the variables,
a, v, s, t
using the kinematics equations
a = dv/dt,
v = ds/dt,
a ds = v dv
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Rectilinear Kinematics: Erratic Motion

  • When particle’s motion is erratic, it is best describedgraphically using a series of curves that can be generatedexperimentally from computer output. • A graph can be established describing the relationship with any two of the variables,

a, v, s, t

2

any two of the variables,

a, v, s, t

  • using the kinematics equations

a = dv/dt,

v = ds/dt,a ds = v dv

Given the

s-t

Graph, construct the

v-t

Graph

•The

s-t

graph can be plotted if the position of the particle

can be

determined experimentally

during a period of time

t.

•To determine the particle’s velocity as a function of time, the

v

-

t

Graph, use

Velocity = Slope of s –t graph at time t

3

the

v

-

t

Graph, use

v = ds/dt

•Velocity as any instant is determined by measuring theslope of the

s-t

graph

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Acceleration = Slope of v –t graph at time t

Given the

v-t

Graph, construct the

a-t

Graph

•When the particle’s

v-t

graph is known, the

acceleration as a function of time,

the

a

t

graph can be determined using

5

the

a

t

graph can be determined using

a = dv/dt

•Acceleration as any instant is determined bymeasuring the slope of the v-t graph

a

dt

dv

=

Slope of

v-t

graph = acceleration

Acceleration = Slope of v –t graph at time t

6

Given the

a-t

Graph, construct the

v-t

Graph

  • When the

a-t

graph is known, the

v-t

graph may be

constructed using

a = dv/dt

=

dt

a

v

Change in velocity from a –t graph

8

=

dt

a

v

Change in

velocity

Area under

a-t

graph

=

Change in velocity from a –t graph

9

Given the

v-t

Graph, construct the

s-t

Graph

  • When the

v-t

graph is known, the

s-t

graph may

be constructed using

v = ds/dt

Displacement from v –t graph

11

dt

v

s

Displacement

Area under v-t

graph

=

knowing the initial position

s

0

, and add to this area

increments

s

determined from

v-t

graph

describe each of there segments of the

v-t

graph by

a series of eqns, each of these eqns may be

integrated

to yield

eqns

that describe corresponding segments of

Displacement from v –t graph

12

to yield

eqns

that describe corresponding segments of

the

s-t

graph

Given the

a-s

Graph, construct the

v-s

Graph

v-s

graph can be determined by using

v dv = a ds

integrating this

eqn

between the limit

v = v

at

s =

v-s graph from a –s graph

14

integrating this

eqn

between the limit

v = v

0

at

s =

s

0

and

v = v

1

at

s = s

1

(

)

=

1

0

2

0

2

1

1 2

s

s

ds

a

v

v

Area undera-s graph

  • determine the eqns which define the segments of the

a-

s

graph

  • corresponding eqns defining the segments of the

v-s

graph can be obtained from integration, using

vdv = a ds

a-s graph from v –s graph

15

  • At any point (

s,v

), the slope

dv/ds

of the

v-s

graph is measured

  • Since

v

and

dv/ds

are known, the value of

a

can be calculated

a-s graph from v –s graph

17

A bicycle moves along a straight road such that it position isdescribed by the graph as shown.

Construct the

v-t

and

a-t

graphs for 0

t

30s.

Example 12.

18

a-t

Graph.

The

a-t

graph can be determined by differentiating the eqns

defining the lines of the

v-t

graph.

0

6

;

30

10

  1. 0 6. 0 ;

10

0

=

=

=

<

=

=

=

dt

dv

a

v

s

t

dt

dv

a

t

v

s

t

20

0

6

;

30

10

=

=

=

<

dt

a

v

s

t

The results are plotted.

A test car starts from rest and travels along a straight tracksuch that it accelerates at a constant rate for 10 s and thendecelerates at a constant rate. Draw the v-t and s-t graphs and determine the time t’needed to stop the car. How far has the car traveled?

Example 12.

21