JJ205 ENGINEERING MECHANICS CHAPTER 3 EQUILIBRIUM OF PARTICLE PREPARED BY: AMRAN BIN AWANG @ MUDA DEPARTMENT OF MECHANICAL ENGINEERING
JJ205 ENGINEERING MECHANICS
CHAPTER 3
EQUILIBRIUM OF PARTICLE
PREPARED BY:
AMRAN BIN AWANG @ MUDA
DEPARTMENT OF MECHANICAL ENGINEERING
CHAPTER OBJECTIVES
To introduce the concept of the free-body diagram for a particle.
To show how to solve particle equilibrium problems using the equations of equilibrium.
CONDITION FOR THE EQUILIBRIUM OF A PARTICLE
Particle at equilibrium if- At rest
- Moving at constant a constant velocity
Newton’s first law of motionΣF = 0
where ΣF is the vector sum of all theforces acting on the particle
Newton’s second law of motion
ΣF = ma
When the force fulfill Newton's first law of motion,
ma = 0
a = 0
therefore, the particle is moving in constant velocity or at rest
The Free-Body Diagram
Best representation of all the unknownforces (ΣF) which acts on a body
A sketch showing the particle “free” from the surroundings with all the forces acting on it
Consider two common connections in this subject Spring
Cables and Pulleys
Spring
Linear elastic spring: change in length is directly proportional to the force acting on it
- spring constant or stiffness k: defines the elasticity of the spring
- Magnitude of force when spring is elongated or compressed F = ks
where s is determined from the difference in spring’s deformed length l and its undeformed length lo
s = l - lo
- If s is positive, F “pull”
onto the spring
- If s is negative, F “push”
onto the spring
EXAMPLE
Given lo = 0.4m and k = 500N/m
To stretch it until l = 0.6m, A force, F = ks
=(500N/m)(0.6m – 0.4m) = 100N is needed
To compress it until l = 0.2m,
A force, F = ks
=(500N/m)(0.2m – 0.4m)
= -100N is needed
- Cables (or cords) are assumed to havenegligible weight and they cannot stretch
-A cable only support tension or pulling force
- Tension always acts in thedirection of the cable
- Tension force in a continuouscable must have a constantmagnitude for equilibrium
Cables and Pulley
1. Draw outlined shape • Isolate particle from its surroundings
2. Show all the forces• Indicate all the forces• active forces: set the particle in motion• Reactive forces: result of constraints and
supports that tend to prevent motion
Procedure for Drawing a FBD
3. Identify each forces
- Known forces should be labeled with proper magnitude and direction
- Letters are used to represent magnitude and directions of unknown forces
A spool is having a weight W which is suspended from the crane bottom
Consider FBD at A since these forces act on the ring
Cables AD exert a resultant force of W on the ring
Condition of equilibrium is used to obtained TB and TC
Each term must be expressed in the same units Eg: s = vt + ½ at2 where s is
position in meters (m), t is time in seconds (s), v is velocity in m/s and a is acceleration in m/s2
Regardless of how the equation is evaluated, it maintains its dimensional homogeneity
All the terms of an equation can be replaced by a consistent set of units, that can be used as a partial check for algebraic manipulations of an equation
The bucket is held in
equilibrium by the cable
Force in the cable =
weight of the bucket
Isolate the bucket for
FBD
Two forces acting on
the bucket, weight W
and force T of the cable
Resultant of forces = 0
W = T
Example 3.1
The sphere has a mass of 6kg and issupported. Draw a free-body diagram ofthesphere, the cordCE and the knot at C.
SolutionCord CE Two forces acting, forceof the sphere and forceof the knot Newton’s Third Law: FCEis equal but opposite FCE and FEC pull the cordin tension For equilibrium, FCE =FEC
FBD at Knot
Three forces acting, force by cord CBA, cord CE
and spring CD
Important to know that
the weight of the sphere
does not act directly on
the knot but subjected to
by the cord CE
Coplanar Systems
A particle is subjected to coplanar forces in
the x-y plane
Resolve into i and j components for
equilibrium
ΣFx = 0
ΣFy = 0
Scalar equations of equilibrium
require that the algebraic sum
of the x and y components to
equal o zero
Scalar Notation
Sense of direction = an algebraic sign that corresponds to the arrowhead direction of the component along each axis
For unknown magnitude, assume arrowhead sense of the force
Since magnitude of the force is always positive, if the scalar is negative, the force is acting in the opposite direction
Consider the free-body diagram of the particle subjected to two forces
Assume unknown force F acts to the right for equilibriumΣFx = 0 ; + F + 10N = 0F = -10N
Force F acts towards the left for equilibrium
The chain exerts three forces on the ring at A.
The ring will not move, or will move with constant velocity, provided the summation of the forces along the y axis is zero
With any force known, the magnitude of other two forces are found by equations of equilibrium
Solution
+→ ΣFx = 0;
+↑ ΣFy = 0;
Solving,
TB = 4.90kN
TD = 4.25kN
*Note: Neglect the weights of the cables since they
are small compared to the weight of the engine
Example 3.3If the sack at A has a weight of 20N (≈ 2kg), determine the weight of the sack at B and the force in each cord needed to hold the system in the equilibrium position shown.
Example 3.4Determine the required length of the cord AC so that the 8kg lamp is suspended. The undeformed length of the spring AB is l’AB = 0.4m, and the spring has a stiffness of kAB = 300N/m.