NCERT Class 11 Physics Solutions: Chapter 5 – Laws of Motion Part 14

Doorsteptutor material for ISAT Class-5 is prepared by world's top subject experts: Get full length tests using official NTA interface: all topics with exact weightage, real exam experience, detailed analytics, comparison and rankings, & questions with full solutions.

Question 5.37:

A disc revolves with a speed of and has a radius of . Two coins are placed at and away from the centre of the record. If the coefficient of friction between the coins and the record is , which of the coins will revolve with the record?


Coin placed atfrom the centre

Mass of each coin

Radius of the disc,

Frequency of revolution,

Coefficient of friction,

In the given situation, the coin having a force of friction greater than or equal to the centripetal force provided by the rotation of the disc will revolve with the disc. If this is not the case, then the coin will slip from the disc.

Coin placed at:

Radius of revolution,

Angular frequency,

Frictional force,

Centripetal force on the coin:

Since , the coin will revolve along with the record.

Coin placed at


Angular frequency,

Frictional force,

Centripetal force is given as:

Since , the coin will slip from the surface of the record.

Question 5.38:

You may have seen in a circus a motorcyclist driving in vertical loops inside a ‘death-well’ (a hollow spherical chamber with holes, so the spectators can watch from outside). Explain clearly, why the motorcyclist does not drop down when he is at the uppermost point, with no support from below. What is the minimum speed required at the uppermost position to perform a vertical loop if the radius of the chamber is ?


In a death-well, a motorcyclist does not fall at the top point of a vertical loop because both the force of normal reaction and the weight of the motorcyclist act downward and are balanced by the centripetal force. This situation is shown in the following figure:

because both the force of normal reaction and the weight of …

Death-Well, a Motorcyclist Does Not Fall at the Top Point

Loading image

The net force acting on the motorcyclist is the sum of the normal force and the force due to gravity

The equation of motion for the centripetal acceleration can be written as:

Normal reaction is provided by the speed of the motorcyclist. At the minimum speed ,