In This Post we are providing Chapter-6 WORK, ENERGY AND POWER NCERT MOST IMPORTANT QUESTIONS for Class 11 PHYSICS which will be beneficial for students. These solutions are updated according to 2021-22 syllabus. These MCQS can be really helpful in the preparation of Board exams and will provide you with a brief knowledge of the chapter.
NCERT MOST IMPORTANT QUESTIONS ON WORK, ENERGY AND POWER
1. What is the speed of an aircraft if the pilot remains in contact with the seat, even while looping in vertical plane?
Ans. For the pilot to stay at rest, centripetal force should
be provided by his weight, i.e., mv2r=mg
∴v=rg
Q.2 The kinetic energy of a particle moving in a horizontal circle may remain the same everywhere. Is it true for the motion in a vertical circle.?
Ans. No. For the motion in a vertical circle, the speed of the particle at the bottom is 5gr and at the top is gr. Therefore, K.E. of the particle in a vertical circle is maximum at the bottom and minimum at the top.
Q.3 A stone of mass m tied to the ended of a string of length l is whirled in a vertical circle. What is the net force acting on the stone at the bottom and at the top of the vertical circle, if tension in the string at the bottom at the top is Tb and Tt respectively
?
Ans. Net force acting on the stone at the bottom of
the circle =Tb−mg
Net force at the top =Tt+mg
Q.4 Can magnitude of kinetic energy be negative?
Ans. No, kinetic energy can be zero or positive only
because KE=12mv2, because m and v2 cannot
be negative.
Q.5 What is the significant of negative mechanical energy?
Ans. Negative value of mechanical energy indicates a bound state electron in an atom and a satellite revolving around a planet both are in bound state having negative mechanical energy.
Q.6 If rain drop move decreasing acceleration, does work done by gravitational force on the drop change.
Ans. It should be clearly understood that whether the drop move with decreasing acceleration or with uniform speed work done by the gravitational force on the drop remains the same.
Q.7 Can a body have momentum without energy? Ans. Yes, when E=k+u=0 either both are zero or k=−u. thus K.E. may or may not be zero As P=2mk
∴P=0 only when k=0
and P≠0 when k=−u
Q.8 Can a body have momentum when its energy is negative?
Ans. Yes, when k<u, total energy E=k+u is negative, the body has the momentum (∵k≠0). For example in an atom, electron has momentum though its energy is negative.
Q.9 Can work be defined at a position or at instant?
Ans. No, work is not defined at a position or at an instant. Work is defined during the time interval within which the force has undergone displacement unlike velocity and acceleration, momentum and kinetic energy it is not the characteristic of moving body it is something which happen an a body which produces or destroys motion of the body.
Q.10 A man rowing boat upstream is at rest with respect to the shore is he doing work.
Ans. The boat is at rest with respect to the shore but itis moving up stream with respect to water. The man is doing work relative to the stream because he is applying force to produce relative motionbetween the boat and stream, but he does no work relative to the shore as displacement relative to shore is zero.
Q.11 Why a metal ball rebounds better than a rubber ball?
Ans. When rubber ball hits a massive object say earth the ball is distorted. A large amount of heat is generated in the ball by the rubbing of the rubber molecules against each other this effect is essentially absent in a band material so metal ball would often loseless energy upon collision than would a rubber ball.
Q.12 The outer casing of a rocket is burnt due to friction between casing and air, who supplies the energy necessary for burning. The rocket or the atmosphere?
Ans. Due to friction with the air the speed hence the kinetic energy of the rocket decreases. This decrease in the kinetic energy appears in the form of heat energy, thus energy necessary for burning is supplied by the rocket itself.
Q.13 An artificial satellite orbiting the earth in very thin atmosphere loses its energy gradually due to dissipation against atmospheric resistance- however small, why then does its speed increase progressively as it comes closer and closer to the earth.
Ans. As a satellite comes closer to the earth its potential energy decrease. According to the law of conservation of energy the total energy of the system must remain constant. Now total energy =KE+PE+ heat energy. Because the heat loss due to friction is very small a decrease in potential energy result in a net increase in kinetic energy. Hence speed of a satellite progressively increase as it comes closer to the earth.
Q.14 In a tug-of-war one team is slowly giving way to the other is work being done on the losing team. How about the wining team.
Ans. Work is done on the losing team because that team is being dragged in the direction of the force applied by the wining team. This work done is said to be positive work, work is also done on the wining team because the wining team is being displaced opposite to the direction of the force applied by the losing team. This work done is said to be negative work.
Q.15 In a circus, the diameter of globe of death is 30 m. From what minimum height must a cyclist start in order to roll down the inclined and go round the globe successfully?
Ans. Diameter of globe =30m
Radius of globe, r=15m
Let h be the minimum height from which the
cyclist after rolling down an incline will acquire velocity =2gh
For looping the loop, the minimum velocity at the lowest point should be 5gr.
∴5gr=2gh
⇒h=5r2=5×152=37.5m
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