Statement (A): The direction of the contact force acting on a body is not necessarily perpendicular to
the contact surface.
Statement (B): When a body moves on the rough surface, the normal reaction on a body and frictional
force are always perpendicular to each other.
Answers
Answer:
Explanation:
NORMAL REACTION - DEFINITION
When any two surfaces are in contact, a contact force is exerted between the surfaces. Normal reaction is the component of the contact force normal to the contact surface.
For example, for a block placed on ground, the ground exerts an upward normal reaction force on the block.
ANGLE OF FRICTION - DEFINITION
concept
It is the angle ( α ), measured between the normal force (N) and resultant force (R).
A body rests on a rough horizontal plane. A force is applied to the body directed towards the plane at an angle α with the vertical.
The body can be moved along the plane:
Consider a block placed on a rough horizontal plane. Now, the reaction force
R
is because it is equal and opposite to the
weight
W
. If the force
F
is applied to the block towards the plane at an angle α, the resolved forces will act along vertical and horizontal direction.
The horizontal component of force Fsinα have to overcome the frictional force so that the block just begins to slide. Frictional force is equal to limiting friction F
limiting
, when this condition is satisfied the angle of applied force α will be greater than angle of friction α and block can move along the plane.
tanα=μ
LAWS OF FRICTION AND EXAMPLE - EXAMPLE
concept
Laws of Friction are:
When an object is moving, the friction is proportional and perpendicular to the normal force (N)
Friction is independent of the area of contact as long as there is an area of contact.
The coefficient of static friction is slightly greater than the coefficient of kinetic friction.
Within rather large limits, kinetic friction is independent of velocity.
Friction depends upon the nature of the surfaces in contact.
Example: A body of mass M is placed on a rough inclined plane of inclination θ and coefficient friction μ
k
. A force of (mgsinθ+μ
k
mgcosθ) is applied in the upward direction, the acceleration of the body is:
Equating forces along X and Y axes,
N=mgcosθ
Also,
−mgsinθ−μ
k
mgcosθ−ma+mgsinθ+μ
k
mgcosθ=0
∴ma=0
LIMITING FRICTION - DEFINITION
The maximum static friction that a body can exert on the other body in contact with it is called limiting friction. This limiting friction is proportional to the normal contact force between the two bodies, It is given by:
(f
s
)
max
=μ
s
N
SELF-ADJUSTING NATURE OF FRICTION - DEFINITION
When a moving body is suddenly stopped, then frictional force arises when there is a relative motion between two surfaces. When body is stopped means there is no relative motion and no resultant force is there hence frictional force acting becomes zero.
RELATION BETWEEN NORMAL REACTION AND FRICTION FORCE - FORMULA
concept
When two bodies are kept in contact, electromagnetic forces act between the charged particles at the surfaces of the bodies. Thus, each body exerts a contact force of the other. The magnitudes of the contact forces acting on the two bodies are equal but their directions are opposite and therefore the contact forces obye Newton's third law.
The direction of the contact forces acting on a body is not necessarily perpendicular to the contact surface. The resolution of contact forces in two components i.e. perpendicular to contact surface and along surface.
Perpendicular component is normal force and parallel component is friction.
∴ R =
(
f
2
+N
2
)
R - Contact force
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Answer:
A is true b is false ..............