Two kinds of motion:
- Translational/Linear motion
- Rotational motion
Translational/linear motion - 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 the applied force
Rotational motion - when a pivoted body is not free to move and a force is applied on the body at a suitable point, it rotates the body about the axis passing through the pivoted point
The turning effect on the body about an axis is due to the moment of force =(or torque) applied on a body
The moment of force/torque is equal to the product of the magnitude of the force ad the perpendicular distance of the line of action of the force from the axis of rotation
Factors affecting turning of body:
- Magnitude of force applied
- Perpendicular distance of line of action from axis of rotation(pivoted point)
Moment of force = force x perpendicular distance of force from point of pivot
Unit of moment of force: N m
CGS unit of moment of force = dyne cm
If force is measured in gravitational unit: kgf m
CGS if force is measured in gravitational unit: gf cm
Anticlockwise moment - positive
Clockwise moment - negative
Couple: two equal and opposite parallel forces not acting along the same line
A coupe is always needed to produce rotation
Couple arm - perpendicular distance between two forces
Moment of force = Either force X perpendicular distance between two forces(couple arm)
When a number of forces acting on a body produce no change in its state of rest or of linear or rotational motion, the body is said to be in a state of equilibrium
Two conditions to be satisfied:
(i) The resultant of all the forces is zero, so they do not change either the state of rest or of linear motion
(ii) the algebraic sum of all the forces about a fixed point is zero, so they do not change the rotational state of the body
Static equilibrium - when a body remains in a state of rest under the influence of several forces
Dynamic equilibrium - when a body remains in the same state of motion(translational or rotational) under the influence of several forces
Conditions for equilibrium:
- Resultant of all forces acting on the body should be zero
- The algebraic sum of the moments if all the forces acting on the point of rotation should be zero
Principle of moments: In equilibrium, the sum of the anticlockwise moments is equal to the sum of clockwise moments