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Chapter: Mechanical : Dynamics of Machines : Force Analysis

Important Short Question and Answers: Force Analysis

Mechanical - Dynamics of Machines - Force Analysis



1.     Write D’Alembert’s principle. What is the use of it?

D'Alembert's principle states that the inertia forces and torques, and the external forces and torques acting on a body together result in statical equilibrium.

Use (or) Application (or) significance:

By applying D’Alembert’s principle to a dynamic analysis problem, we can reduce it into an equivalent problem of static equilibrium.


2.     Distinguish between static force and inertia force.

·     While analyzing the mechanism, if mass of the body and inertia force are not considered, then it is called static force.

·     The inertia force is an imaginary force, which when acts upon a rigid body, brings it in an equilibrium position.

Inertia force = - Acceleration  force = - m. a


3. What are the conditions for a body to be in equilibrium under the action of (a) two forces, (b) two forces and torque

a)    Condition for two forces:

·     The forces are of the same magnitude.

·     The forces act along the same line.

·     The forces are in opposite direction.


b)    Condition for two forces & Torque:

·     The forces are equal in magnitude, parallel in direction and opposite in sense.

·     The forces from a couple, which is equal and opposite to the applied torque.


4. What is engine shaking force?

The force produces in an engine due to the mass of piston, and mass of the connecting rod is called engine shaking force.


5.     Differentiate between static & dynamic equilibrium. (or) What are the conditions for a body to be in static and dynamic equilibrium?

Necessary and sufficient conditions for static and dynamic equilibrium are: 1.Vector sum of all the forces acting on a body is zero.

2. The vector sum of all the moments of all the forces acting about any arbitrary point or axis is zero.

First conditions are sufficient conditions for static equilibrium together with second condition is necessary for dynamic equilibrium.


6. What is free body diagram?


A free body diagram is a sketch of the isolated or free body which shows all the pertinent weight force, the externally applied loads, and the reaction from its supports &connections acting upon it by the removed elements.

7. Define piston effort and crank effort.

·     Piston effort is defined as the net or effective force applied on the piston, along the line of stroke. It is also known as effective driving force (or) net load on the gudgeon pin.

·     Crank effort is the net effort (force) applied at the crank pin perpendicular to the crank, which gives the required turning moment on the crankshaft.


8. Define crank pin effort.

The component of force acting along connecting rod perpendicular to the crank is known as crank-pin effort.


9.     What are the requirements of an equivalent dynamical system? (or) Write the conditions for any disturbed mass have the same dynamical properties.

·     The mass of the rigid body must be equal to the sum of masses of two concentrated masses.

i.e. m1 + m2 = m

·     The centre of gravity of the two masses must coincide with the centre of gravity of the rigid body.

 i.e. m1l1 = m2l2

·     The sum of mass moment of inertia of two masses about their centre of gravity is equal to the mass moment of inertia of the rigid body.

i.e. l1 . l2 = (kG)2

10. What is the function of a flywheel? how does it differ from that of a governor?

·     The function of flywheel is to reduce the fluctuations of speed during a cycle above and below the mean value for constant load from prime mover. The function of governor is to control the mean speed over a period for output load variations.

·     Flywheel works continuously from cycle to cycle. Governor works intermittently, i.e. only when there is change in the load.

·     Flywheel has no influence on mean speed of the prime mover. Governor has no influence over cyclic speed fluctuations.


11.Differentiate between the usage of flywheel in engines and punching presses with turning moment diagrams.   

·     In the engines, the output of the flywheel was constant and input torque was varying during each cycle.

In case of punching press, the input of the flywheel is constant and output torque is varying cyclically.

12. Define coefficient of fluctuation of energy.

It is the ratio of maximum fluctuation of energy to the work done per cycle.

CE=Maximum fluctuation of energy/Work done per cycle.


13.  Why flywheels are needed in forging and pressing operations?

In both forging and pressing operations, fly wheels are required to control the variations in speed during each cycle of an engine.


14. Define unbalance and spring surge?

Unbalance: A disc cam produces unbalance because its mass is not symmetrical with the axis of rotation.

Spring surge: Spring surge means vibration of the retaining spring.


15.Define windup. What is the remedy for camshaft windup?

·     Twisting effect produced in the camshaft during the raise of heavy load follower is called as windup

·     Camshaft windup can be prevented to a large extend by mounting t he flywheel as close as possible to the cam.


1. The stroke of a steam engine is 600 mm and the length of connecting rod is 1.5 m. The crank rotates


at 180 r.p.m. Determine: 1. velocity and acceleration of the piston when crank has travelled through an angle of 40° from inner dead centre, and 2. the position of the crank for zero acceleration of the piston. [Ans. 4.2 m/s, 85.4 m/s2; 79.3° from I.D.C]


2. The following data refer to a steam engine :


Diameter of piston = 240 mm; stroke = 600 mm; length of connecting rod = 1.5 m; mass of reciprocat-ing parts = 300 kg; speed = 125 r.p.m.


Determine the magnitude and direction of the inertia force on the crankshaft when the crank has turned through 30° from inner dead centre. [Ans. 14.92 kN]

3. A vertical petrol engine 150 mm diameter and 200 mm stroke has a connecting rod 350 mm long. The


mass of the piston is 1.6 kg and the engine speed is 1800 r.p.m. On the expansion stroke with crank angle 30° from top dead centre, the gas pressure is 750 kN/m2. Determine the net thrust on the piston. [Ans. 7535 N]


4. A certain machine tool does work intermittently. The machine is fitted with a flywheel of mass 200 kg and radius of gyration of 0.4 m. It runs at a speed of 400 r.p.m. between the operations. The machine is driven continuously by a motor and each operation takes 8 seconds. When the machine is doing its work, the speed drops from 400 to 250 r.p.m. Find 1. minimum power of the motor, when there are 5 operations performed per minute, and 2. energy expanded in performing each operation.


[Ans. 4.278 kW; 51.33 kN-m]


5. A constant torque 4 kW motor drives a riveting machine. A flywheel of mass 130 kg and radius of gyration 0.5 m is fitted to the riveting machine. Each riveting operation takes 1 second and requires 9000 N-m of energy. If the speed of the flywheel is 420 r.p.m. before riveting, find: 1. the fall in speed of the flywheel after riveting; and 2. the number of rivets fitted per hour.

[Ans. 385.15 r.p.m.; 1600]

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