Force and Motion PYQ Solution

Exam PrabhuPhysics
Q. Why is the Centrifugal Force Called a Pseudo Force?

The centrifugal force is called a pseudo force (or fictitious force) because it does not arise from any real physical interaction between two bodies. It appears only when the motion of an object is observed from a rotating or accelerating frame of reference, i.e., a non-inertial frame.

The main reasons are:

  1. No Real Physical Interaction: Centrifugal force is not produced by any physical interaction such as gravity, tension, friction, or contact.
  2. Exists Only in a Non-Inertial Frame: Centrifugal force appears only when motion is described from a rotating frame of reference. It is introduced because the rotating frame itself is accelerating. For example, an observer sitting inside a car making a sharp turn feels pushed outward away from the centre of the curve.
  3. Frame-Dependent Nature: To a stationary outside observer standing on the ground (inertial frame), there is no outward force acting on the passenger. In reality, the passenger tends to continue moving in a straight line due to inertia, while the vehicle changes its direction.
  4. Real Force in Circular Motion is Centripetal Force: The circular motion is maintained by a real centripetal force which is directed towards the centre, such as tension, friction, or gravitational force.
Centrifugal force is not observed as a real force in an inertial frame and disappears when the motion is described from an inertial frame.
Therefore: Centrifugal force is called a pseudo force because it has no real physical source and arises only due to the acceleration of the reference frame. It is a mathematical way used to describe motion conveniently from a non-inertial rotating frame.

Q. Why does a cyclist bend inwards while riding along a curved path?

When a cyclist rides along a curved path of radius r at speed v, he feels a pseudo force (centrifugal force) directed outward. So basically, a net inward (centripetal) force is required to keep moving along the circular path and to continuously change the direction of velocity.

This force is supplied by the horizontal component of the frictional force between the tyres and the road.

If the cyclist remains upright:

  • Friction acts at the ground-level contact point of the tyre.
  • The weight acts vertically downward through the higher centre of mass.

The resultant effect of these forces produces a turning effect (torque) that tends to topple the bicycle outward.

By leaning inward at an angle θ, where

tan θ = v2 rg

the resultant of the normal reaction and frictional force is inclined towards the centre of the curve and passes through the centre of mass.

The vertical component of this resultant force balances the weight, while its horizontal component provides the necessary centripetal force. Thus, the cyclist remains stable without toppling by balancing the turning effect due to gravity and the horizontal frictional force.

Remember: A cyclist bends inward to provide the required centripetal force and to prevent the bicycle from toppling outward. The faster the cyclist moves, or the smaller the radius of the curve, the greater the required angle of inclination.

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