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Wednesday, 13 November 2013

F.Sc Notes: Physics XI: Chapter 05 Circular Motion Exercise Short Questions:

 FSc Notes: Physics XI: Chapter 05 Circular Motion Exercise SQ:



Question 5.1 Explain the difference between tangential velocity and the angular velocity. If one of these is given for a wheel of known radius, how will you find the other?
Answer 5.1
Linear velocity of a particle moving along a curve or circle directed along the tangent at any point on the curve is known as tangential velocity .
Rate of angular displacement to unit time is known as angular velocity .                                      


Question 5.2 Explain what is meant by centripetal force and why it must be furnished to an object if the object is to follow a circular path?
Answer 5.2
The force which compels the body to move in a circle is known as centripetal force.When a body follows a circular path its direction changes every moment. To change the direction of motion continuously a continuous perpendicular force is required which is known as centripetal force it is directed towards the center of the circle.


Question 5.3 What is meant by moment of inertia? Explain its significance.
Answer 5.3
A property of a body to resist its uniform circular motion. Product of mass and perpendicular distance of the body from the axis of rotation is known as moment of inertia.


Question 5.4 What is meant by angular momentum? Explain the law of conservation of angular momentum?
Answer 5.4
Cross product of linear momentum P and position vector r is known as angular momentum.
Law of conservation of Angular Momentum. The angular momentum of an isolated system remains the same if no external torque is applied on it.
L=I1w1=I2w2
If I decrease its w increases so product remains the same.


Question 5.5 Show that orbital angular momentum Lo = mvr?
Answer 5.5
We know that  and its magnitude is given by
L=r*p = rpsinθ = mvrsinθ
where θ  is the angle between r and p .
In case of circular orbital motion θ =90 degree between r and p .
Hence                       Lo = mvrsin90 = mvr
Lo = mvr                               ( sin90=1)


Question 5.6 Describe what should be the minimum velocity, for a satellite, to orbit close to the Earth around it?
Answer 5.6
We that minimum to put a satellite in orbit vo=√gR .
As low flying satellite has g = 9.8m\s(2)
And    R= 6400m.
putting the values vo=√9.8*6400  =7.9km\s .
This is the minimum velocity for a satellite to orbit close to earth.


Question 5.7 State the direction of the following vectors in the simple situations angular momentum and angular velocity?
Answer 5.7
In simple situations angular momentum and angular velocity both are parallel to the axis of rotation.


Question 5.8 Explain why an object, orbiting the Earth, is said to be free falling. Use your explanation to point out why object appear weightless under certain circumstances.
Answer 5.8
If the object is thrown fast enough parallel to the earth, the curvature of its path match the curvature of the earth the object in this case will simply circle around the earth this is known as free fall.
Weightlessness is a sensation experienced by any individual when there are no external objects touching one's body and exerting a push or pull upon it. In free fall, the only force acting upon your body is the force of gravity  a non-contact force. So the object appears weightless.


Question 5.9 When mud flies off the tire of a moving bicycle, in what direction does it fly explain?
Answer 5.9
When mud flies off the tire of a moving bicycle it flies along the tangent to the tire. When tire is moving every part of it has tangential velocity, angular momentum and centripetal force. The adhesive forces of mud provides the required centripetal force when speed of tire increases. When required centripetal force increases the adhesive force the mud flies off the tire.


Question 5.10 A disc and a hoop start moving from the top of an inclined plane at the same time. Which one will be  moving faster on reaching the bottom?
Answer 5.10
The velocity of disc on reaching the bottom of the inclined plane,
v=√4gh/3=√4/3√gh  and for hoop  velocity at the bottom of the inclined plane v1 =√gh
So  v=√4/3v1=1.15v1
Hence Disc will reach the bottom earlier.


Question 5.11 Why does a diver change his body positions before and after diving in the pool?
Answer 5.11
To follow law of conservation of momentum.


Question 5.12 A student holds two dumb-bells with stretched arms while sitting on a turn table. He is given a push until he is rotating at certain angular velocity. The student then pulls the dumb-bells towards his chest. What will be the effect on rate of rotation?
Answer 5.12
Using Law of conservation of momentum: I1wi=I2wf .
With stretched arms moment of inertia I1and angular velocity wi .The product I1wi is the angular momentum remains the same when he pulls the dumb bells towards his chest his moment of inertia I2 decrease and angular velocity wf increases to keep angular momentum I2wf  constant.


Question 5.13 Explain how many minimum number of Geo-stationary satellites are required for global coverage of TV transmission?
Answer 5.13
A Geo-stationary satellite covers an angle of 120 degree so whole of the earth’s surface can be covered by three correctly poisoned Geo-stationary satellites for global coverage of TV transmission.

Written By: Asad Hussain

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