UNIT 4:

Turning effects of Forces

Q.  Define the following:

o   Resultant vector

o   Torque

o   Centre of mass

o   Centre of gravity

Ans.

Resultant vector:

                A force that has the same effect as the combine effect of all the forces to be added is called resultant force. The resultant force is represented by a vector which is known as resultant vector.

Torque:

                The turning effect of a force is called torque or moment of the force.

Centre of mass:

                A point in a system where an applied force causes the system to move without rotation is called centre of mass.

Centre of gravity:

                A point where the whole weight of the body appears to act vertically downward is called centre of gravity of a body.

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Q.  Differentiate the following:

o   Like an d unlike forces

o   Toque and couple

o   Stable and neutral equilibrium

Ans.

Like and unlike forces:

            Like forces are parallel and have same direction where as the unlike forces are also parallel but have opposite direction.

Torque and couple:

            In torque, single force can produce torque but in couple at least two forces are required.

Stable and neutral equilibrium:

            In stable equilibrium, the body after a slight tilt returns to its previous position but in neutral equilibrium, when the body is disturbed it remains its new position and does not return to its previous position.

Q.  How head to tail rule helps to find the resultant of forces?

Ans.  The forces are added by using the head to tail rule. According to this rule all vectors are drawn according to some suitable scale and named by uppercase and bold alphabets ( A, B, etc). Then the first vector is drawn and the second vector is placed such that tail of second vector coincides with the head of first vector. Now the resultant vector is drawn such that its tail is at the tail of its first vector while its head is at the head of vector of second vector.

Q.  How can a force be resolved into its rectangular components?

Ans.  Consider a force making an angle with x-axis. To resolve the force into its rectangular components, a perpendicular vector is drawn from x-axis to the head of force which is called y-component of the force .  Another vector is drawn along x-axis from the tail of force to the tail of y-component which x-component of the force . These two components are perpendicular to each other.

Q.  When a body is said to be in equilibrium?

Ans.  A body is said to be in equilibrium if no net force acts on it.

Q.  Explain the first condition for equilibrium.

Ans. According to the first condition for equilibrium, the resultant of all the forces acting on the body should be zero.

Or,

Q.  Why there is a need of second condition for equilibrium if a body satisfies first condition for equilibrium?

Ans. A body is said to be in equilibrium under the first condition for equilibrium when two equal and opposite forces act on it. But when the forces are shifted the body will not be in equilibrium although it satisfies the first condition for equilibrium. This situation demands another condition for equilibrium. This is called second condition for equilibrium.

Q.  What is second condition for equilibrium?

Ans.  According to the second condition for equilibrium the resultant of all the torques acting on the body should be zero.

Q.  Give an example of a moving body which is in equilibrium.

Ans. When a body moves with uniform velocity having zero acceleration is an example of moving body in equilibrium.

Q.  Think of a body which at rest but not in equilibrium.

Ans. When simple pendulum reaches its extreme position it comes to rest for a while but gravitational force remains acting on it due to which it is not in equilibrium.

Q.  Why a body cannot be in equilibrium due to single force acting on it?

Ans. When a single force acts on the body, the body moves in the direction of force and the body is not being balanced by another force. Hence the body under the action of single force is not in equilibrium.

Q.  Why the height of vehicles is kept as low as possible?

Ans. The height of vehicle is kept as low as possible for its stability. The stability of vehicle depends upon its centre of mass. By keeping the centre of mass of vehicle as low as possible the stability of the vehicle is achieved.

Q.  Explain what is meant by stable, unstable and neural equilibrium? Give one example in each case.

Ans.

Stable equilibrium:

            If a body is said to be in stable equilibrium if after a slight tilt it returns to its previous position.

Unstable equilibrium:

            If a body does not return to its previous position when set free after a slightest tilt is said to be in unstable equilibrium.

Neutral equilibrium:

            If a body remains in when disturbed from its previous position, it is said to be in a state of neutral equilibrium.