Mechanics (Part – 4)

The contact force that causes a change in the state of motion of objects is friction.

• Friction acts between surfaces in contact and opposes the relative motion or the tendency of such motion.
• It can slow down or stop moving objects, and it also helps objects start moving when a force is applied.
• However, any contact force (like applied force, normal force, tension, and friction) can cause a change in motion depending on the situation.

Hence all the above statements are correct.

Thus option (d) is the correct answer.

Friction is maximum in the stationary state of the object, slightly less in sliding, and least in rolling.

Static friction > Sliding friction > Rolling friction.

• This force is more effective between rough surfaces.
• There is always a thin layer of water present on the surface of the ice, which makes it smooth.
• Therefore, it is difficult to walk on a smooth surface of ice as the friction is almost zero.

The magnitude of the torque is equal to the product of the force and the perpendicular distance of the line of action of the force from the axis of rotation, while the direction of the torque is along the axis of rotation.

If the force tends to rotate the body counter clockwise, then the torque is said to be positive and if it tends to rotate the body clockwise, then the torque is said to be negative.

Unit of Torque: Newton-meter (N·m)

Dimensional formula of Torque: [ML²T⁻²]

• It is based on the principle that a wire or thread resists twisting with a force that is proportional to the stress.
• The torsion balance consists essentially of a wire or thread attached at one end and arranged so that a force applied at the other, or free, end tends to twist it out of shape.

As per option (a), if displacement is zero, then work
w = f x d
w = f x 0 = 0
That is, the work will also be zero. Work can be both positive and negative. Its unit is joule.

• (Work Force) x (Displacement) in the direction of force . The S.I. unit of work is the Newton meter or joule.