Chapter 1: Force
Moment of force and equilibrium
Q 1.
(i)Name the two kinds of motion a rigid body can have when acted upon by a force.
(ii)What do you mean by linear or translational motion? Give example.
(iii)What is rotational motion? Give example.
ANSWERS:
(i)Two kinds of motion a rigid body can have when acted upon a force are:
Translational motion and Rotational motion
(ii)When a force acts on a stationary rigid body which is free to move, the body starts moving in a
straight path in the direction of force. This is called linear or translational motion. For example, on
pushing a ball lying on the floor, it begins to move.
(iii)If a body is pivoted at a point and a force is applied on the body at a suitable point, it rotates the
body about the axis passing through the pivoted point. This is the turning effect of the force and the
motion of the body is called rotational motion. For example, when a force is applied normally on the
handle of a door, the door begins to rotate about an axis passing through the hinges on which the
door rests.
Q 2.
(i)What are the factors on which turning of a body depends?
(ii)What do you mean by moment of force or Torque?
(iii)Name the C.G.S and S.I units of moment of force.
(iv)What do you mean by positive and negative moment?
(v)Examples of turning effect of force or moment of force?
ANSWERS:
(i)The turning effect of force depends on the following two factors:
(a)the magnitude of the force applied
(b)the perpendicular distance of the line of action on force from the axis of rotation.
, (ii)The moment of force is equal to the product of the magnitude of the force and the perpendicular
distance of the line of action of force from axis of rotation and the turning effect on the body about
an axis is due to the moment of force applied on the body.
(iii)The C.G.S unit of moment of force is dyne X cm. The S.I unit of moment of force is newton X
meter.
(iv)If the effect on the body is to turn it anticlockwise, the moment of force is called anticlockwise
movement and is taken positive.
If the effect on the body is to turn it clockwise, the moment of force is called clockwise movement
and it is taken negative.
(v)Examples of moment of force:
Larger the perpendicular distance from the axis of rotation, less is the force needed to produce
the same turning effect.
NOTE: For producing the maximum turning effect on the body by a given force, the force is applied
on the body at a point for which the perpendicular distance of the line of action of the force from
the axis of rotation is maximum. In this situation, the given force provides the maximum torque to
turn the body.
Q 3.
(i)Define couple.
(ii)Give two examples of couple in action.
(iii)Define couple arm or arm of couple.
(iv)Moment of couple.
(v)Name the C.G.S and S.I units of moment of couple.
ANSWERS:
(i)Two equal and opposite parallel forces, not acting along the same line, form a couple. A couple is
always needed to produce a rotation.
(ii)Examples of couple in action are:
(a)Rotation of a door about its hinge.
(b)Turning of a water tap.
(iii)The perpendicular distance between the two forces is called the couple arm or arm of couple.
(iv) F
D B
Moment of force and equilibrium
Q 1.
(i)Name the two kinds of motion a rigid body can have when acted upon by a force.
(ii)What do you mean by linear or translational motion? Give example.
(iii)What is rotational motion? Give example.
ANSWERS:
(i)Two kinds of motion a rigid body can have when acted upon a force are:
Translational motion and Rotational motion
(ii)When a force acts on a stationary rigid body which is free to move, the body starts moving in a
straight path in the direction of force. This is called linear or translational motion. For example, on
pushing a ball lying on the floor, it begins to move.
(iii)If a body is pivoted at a point and a force is applied on the body at a suitable point, it rotates the
body about the axis passing through the pivoted point. This is the turning effect of the force and the
motion of the body is called rotational motion. For example, when a force is applied normally on the
handle of a door, the door begins to rotate about an axis passing through the hinges on which the
door rests.
Q 2.
(i)What are the factors on which turning of a body depends?
(ii)What do you mean by moment of force or Torque?
(iii)Name the C.G.S and S.I units of moment of force.
(iv)What do you mean by positive and negative moment?
(v)Examples of turning effect of force or moment of force?
ANSWERS:
(i)The turning effect of force depends on the following two factors:
(a)the magnitude of the force applied
(b)the perpendicular distance of the line of action on force from the axis of rotation.
, (ii)The moment of force is equal to the product of the magnitude of the force and the perpendicular
distance of the line of action of force from axis of rotation and the turning effect on the body about
an axis is due to the moment of force applied on the body.
(iii)The C.G.S unit of moment of force is dyne X cm. The S.I unit of moment of force is newton X
meter.
(iv)If the effect on the body is to turn it anticlockwise, the moment of force is called anticlockwise
movement and is taken positive.
If the effect on the body is to turn it clockwise, the moment of force is called clockwise movement
and it is taken negative.
(v)Examples of moment of force:
Larger the perpendicular distance from the axis of rotation, less is the force needed to produce
the same turning effect.
NOTE: For producing the maximum turning effect on the body by a given force, the force is applied
on the body at a point for which the perpendicular distance of the line of action of the force from
the axis of rotation is maximum. In this situation, the given force provides the maximum torque to
turn the body.
Q 3.
(i)Define couple.
(ii)Give two examples of couple in action.
(iii)Define couple arm or arm of couple.
(iv)Moment of couple.
(v)Name the C.G.S and S.I units of moment of couple.
ANSWERS:
(i)Two equal and opposite parallel forces, not acting along the same line, form a couple. A couple is
always needed to produce a rotation.
(ii)Examples of couple in action are:
(a)Rotation of a door about its hinge.
(b)Turning of a water tap.
(iii)The perpendicular distance between the two forces is called the couple arm or arm of couple.
(iv) F
D B
