WORK AND ENERGY

PRINCIPLE

Work and Kinetic Energy

In the previous article we applied Newton’s second

law to various problems of particle motion to

establish the instantaneous relationship between

the net force acting on a particle and the resulting

acceleration of the particle. When we needed to

determine the change in velocity or the

corresponding displacement of the particle, we

integrated the computed acceleration by using the

appropriate kinematic equations.

amF

There are two general classes of problems in which

the cumulative effects of unbalanced forces acting

on a particle are of interest. These cases involve:

(1) integration of forces with respect to the

displacement of the particle, and

(2) integration of forces with respect to the time

they are applied.

We may incorporate the results of these

integrations directly into the governing equations of

motion so that it becomes unnecessary to solve

directly for the acceleration.

Integration of equation of motion with respect to

displacement leads to the equations of work and

energy.

Definition of Work Let’s now develop the quantitative meaning of “work”. The figure shows a force acting on a particle at A which moves along the path shown. The position vector measured from some convenient origin O locates the particle as it passes A and is the differential displacement associated with an infinitesimal movement from A to A'.

The work done by the force during the displacement is defined as

F

r

rd

F

rdFdU

rd

F

The magnitude of this dot product is dU=F ds cosa, where a is the angle between and and where ds is the magnitude of . This expression may be interpreted as the displacement multiplied by the force component Ft=Fcosa in the direction of the displacement.

F

rd

rd

Alternatively, the work is dU may be interpreted as the force multiplied by the displacement component ds cosa in the direction of the force.

acos dsFdUordsFdU t

t

n

With this definition of work, it should be noted that the component Fn=Fsina normal to the displacement does no work. Thus, the work dU may be written as

dsFdU t

So dU may be computed as

aa cos cos dsFrdFdU

Work is positive if the working component Ft is in the direction of the displacement and negative if it is in the opposite direction. Forces which do work are termed as active forces. Constraint forces which do no work are termed as reactive forces.

Unit of Work

The SI units of work are those of force (N) times

displacement (m) or N·m. This unit is given the special

name joule (J), which is defined as the work done by

a force of 1 N acting through a distance of 1 m in the

direction of the force.

Calculation of Work

During a finite movement of the point of application of

a force, the force does an amount of work equal to

2

1

2

1

cos

,

2

1

s

st

s

s

zzyyxx

dsFdsFU

ordFdFdFUrdFdU

a

ds

In order to carry out this integration, it is necessary to know the relations between the force components and their respective coordinates or the relation between Ft and s. If the functional relationship is not known as a mathematical expression which can be integrated but is specified in the form of approximate or experimental data, then we can compute the work by carrying out a numerical or graphical integration as represented by the area under the curve of Ft versus s.

ds

Examples of Work When work must be calculated, we must always begin with the definition of work , insert appropriate vector expressions for the force and the differential displacement vector , and carry out the required integration. Among the most frequently met forces will be constant forces, spring forces and weights.

rdFU

F

rd

a

b

Work Associated with a Constant External Force Consider the constant force applied to the body as it moves from position 1 to position 2. The work done by this force is

kPjPiPP zyx

As can be seen, the work done depends only on the coordinates of positions and , there is no reference to the path travelled between these positions. The same work will be done whether the particle has followed path a or path b.

121212

2

1

2

1

2

1

zzPyyPxxP

dzPdyPdxPU

zyx

z

zz

y

yy

x

xx

Work Associated with a Spring Force

We consider here the common linear spring of

stiffness k where the force required to stretch or

compress the spring is proportional to the

deformation x. We wish to determine the work done

on the body by the spring force as the body

undergoes an arbitrary displacement from an initial

position x1 to a final position x2.

The force exerted by the spring on the body is .From the definition of work

ikxF

2

2

2

1

2

1

2

121

2

1

2

1

xxkkxdx

idxikxrdFU

x

x

If the initial position is the position of zero spring deformation so that x1 = 0, then the work is negative for any final position x2 ≠ 0.This is verified by recognizing that if the body begins at undeformed spring position and then moves to the right, the spring force is to the left; if the body begins at x1 = 0 and moves to the left, the spring force is to the right. On the other hand, if we move from an arbitrary initial position x1 ≠ 0 to the undeformed final position x2 = 0, we see that the work is positive. In any movement toward the undeformed spring position, the spring force and the displacement are in the same direction.

In the general case, neither x1 nor x2 is zero. The magnitude of the work will be equal to the shaded trapezoidal area under the line. In calculating the work done on a body by a spring force, care must be taken to ensure that the units of k and x are consistent.

Work Associated with Weight We assume that g is constant, that is the altitude variation is sufficiently small so that the acceleration of gravity may be considered constant.

The work done by the weight W is

yWyyWyyWU

WyWydyW

dzFdyFdxFU

y

y

zyx

122121

12

2

1

0000

If y < 0 work will be positive, if y > 0 work will be negative.

W1

W1

A1

A2

y1

y2

y

y

x

dy

g

Work and Curvilinear Motion We now consider the work done on a particle of mass m, moving along a curved path under the action of the force , which stands for the resultant of all forces acting on the particle. The position of m is specified by the position vector , and its displacement along its path during the time dt is represented by the change in its position vector.

F

F

r

rd

The work done by during a finite movement of the particle from point 1 to point 2 is

F

2

1

2

121

s

stdsFrdFU

where the limits specify the initial and final end points of the motion. From Newton’s second law and

amF

2

1

2

121

s

srdamrdFU

Since , in the relationship ads = vdv, a=at (at = acosq).

dsarda t

2

1

2

2

2

121

2

1

2

1

2

1

2

1

vvmmvdv

dsmardamrdFU

v

v

s

st

s

s

where the integration is carried out between points 1 and 2 along the curve, at which points the velocities have the magnitudes v1 and v2, respectively.

Principle of Work and Kinetic Energy The kinetic energy T of the particle is defined as And is the total work which must be done on the particle to bring it from a state of rest to a velocity v. Kinetic energy T is a scalar quantity with the units of N·m or joules (J) in SI units. Kinetic energy is always positive, regardless of the direction of the velocity. Work – energy equation can now be stated as

2

2

1mvT

TTTU 1221

The equation states that the total work done by all forces acting on a particle as it moves from point 1 to point 2 equals the corresponding change in kinetic energy of the particle. Although T is always positive, the change T may be positive, negative or zero. When written in this concise form, the equation tells us that the work always results in a change of kinetic energy. Alternatively, the work – energy relation may be expressed as the initial kinetic energy T1 plus the work done U1-2 equals the final kinetic energy T2, or When written in this form, the terms correspond to the natural sequence of events.

2211 TUT

Advantages of the Work – Energy Method A major advantage of the method of work and energy is that it avoids the necessity of computing the acceleration and leads directly to the velocity changes as functions of the forces which do work. Further, the work energy equation involves only those forces which do work and thus give rise to changes in the magnitude of the velocities.

If we consider a system of two particles joined together by a connection which is frictionless and incapable of any deformation, the forces in the connection are equal and opposite, and their points of application have identical displacement components in the direction of forces. Therefore, the net work done by these internal forces is zero during any movement of the system. Thus, the equation of work – energy is applicable to the entire system, where U1-2 is the total or net work done on the system by forces external to it and T is the change, T2 – T1, in the total kinetic energy of the system.

The total kinetic energy is the sum of the kinetic energies of both elements of the system. So, another advantage of the work – energy method is that it enables to analyze a system of particles joined without dismembering the system. Application of the work – energy method requires isolation of the particle or system under consideration. Therefore, correct drawing of the free body diagram is of vital importance in the solution of any problem.

Power The capacity of a machine is measured by the time rate at which it can do work or deliver energy. The total work or energy output is not a measure of this capacity since a motor, no matter how small, can deliver a large amount of energy if given sufficient time. Thus, the capacity of a machine is rated by its power, which is defined as the time rate of doing work.

When a force does work an amount U, the power P will be Power is a scalar quantity. In SI system it has the units of N·m/s = Joule /s = Watt (W).

vFP

vdt

rd

dt

rdF

dt

dUP

F

Potential Energy The work done on a particle must be calculated separately for each force. The work done on a particle by forces named as “conservative forces” can be determined by a potential energy function which depends only on the position of the particle. Spring forces and forces of gravitational attraction are named as conservative forces and when the work by these are calculated using the potential energy concept, they provide simplification in the analysis of many problems.

A) Gravitational Potential Energy

Let’s consider the motion of a particle in close proximity to the surface of the earth, where the gravitational attraction (weight) mg is essentially constant. The gravitational potential energy Vg of the particle is defined as the work mgh done against the gravitational field to elevate a particle a distance h above some arbitrary reference plane (called a datum), where Vg is taken to be zero. Thus, the potential energy is written as

Vg=mgh

This work is called potential energy because it may be converted into energy if the particle is allowed to do work on a supporting body while it returns to its lower original datum plane. In going from one level at h=h1 to a higher level at h=h2, the change in potential energy becomes

Vg = mg (h2 – h1)=mgh

The corresponding work done by the gravitational force on the particle is –mgh. Thus, the work done by the gravitational force is the negative of the change in potential energy.

B) Elastic Potential Energy

The second example of potential energy occurs in the deformation of an elastic body, such as a spring. The work which is done on the spring to deform it is stored in the spring and is called the elastic potential energy Ve. This energy is recoverable in the form of work done by the spring on the body attached to its movable end during the release of the deformation of the spring. The force supported by a linear spring at any deformation x, tensile or compressive, from its undeformed position is F=kx.

Thus the elastic potential energy of the spring as the work done on it to deform it an amount x is defined as If the deformation, either tensile or compressive, of a spring increases from x1 to x2, during the motion, then the change in potential energy of the spring is which is positive.

2

0 2

1kxkxdxV

x

e

2

1

2

22

1xxkVe

Conversely, if the deformation of a spring decreases during the motion interval, then the change in potential energy of the spring becomes negative. Because the force exerted on the spring by the moving body is equal and opposite to the force F exerted by the spring on the body, it follows that the work done on the spring is the negative of the work done on the body. Therefore, we may replace the work U done by the spring on the body by –Ve, the negative of the potential energy change for the spring, provided that the spring is included in the system.

Work – Energy Equation With the elastic member included in the system, we can modify the work – energy equation to account for the potential energy terms. If Uʹ1-2 stands for the work of all external forces other than gravitational forces and spring forces, than we can write or where V is the change in total potential energy. Alternatively, we may write or

eg VVTU 21

22221111 egeg VVTUVVT

eg VVTU

21

VTU 21

For problems where the only forces are gravitational, elastic and nonworking constraint forces, the Uʹ term is zero and the equation becomes or Where E=T + V is the total mechanical energy of the particle and its attached spring. When E is constant The transfers of energy between kinetic and potential may take place as long as the total mechanical energy T + V does not change. This equation expresses “the law of conservation of dynamical energy”.

2211 VTVT 21 EE

0E