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The weight of an object is the force that attracts the object towards the surface of the earth. Weightlessness is the absence of sensation of the weight of an object. We mostly experience weightlessness during a roller coaster ride.
The topic Weightlessness comes under Unit 6 Chapter 8 Gravitation of NCERT Class 11 Physics. The understanding of this topic is important for the CBSE Class 12 Board Examination. Unit 4, 5 and 6 carries a combined weightage of 17 Marks in the CBSE Class 11 Physics Examination.
Weightlessness
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Weight is the force with which the earth attracts an object towards it. Weightlessness is the term used to describe the sensation of complete or almost complete dearth of weight. It can also be defined as the absence of gravity.
It is a condition of free fall, in which the effect of gravity is cancelled by the centrifugal force growing from an orbital flight, thus there is no force of gravity acting on the objects. It can be accurately described as the condition in which the body does not feel its weight at all.
Example- When an apple falls from a tree, it won’t feel its weight. This condition which is experienced by anybody during a free fall is called weightlessness.
Also Read: Weightlessness Experiment
Why do we feel weightless?
An individual feels weightless when no external objects are touching its body. This happens when all forces which may come in contact with the body are removed. These sensations are common in the state of free-fall.
During the fall, the only primary force which acts on the body/object is gravity. This is why an individual would feel weightless during a free-fall. However, weightlessness is only a sensation and should not be altered with reality pertaining to an individual who has lost weight. The concept of weightlessness has much to do with the presence and absence of contact forces rather than with weight.
Weightlessness in space
Astronauts orbiting in space get the feeling of weightlessness as there is no external contact force in space when it comes to pushing or pulling upon their bodies. Gravity is the only force acting upon their body. Gravity being action at a distance force can’t be felt and therefore won’t provide any sensation of weight. That is why astronauts feel weightless in space.
Crew members of spacecraft are often subjected to the problems of weightlessness. It came into the limelight from U.S. and early Soviet manned missions that there is a decrease in heart and respiration rates along with gradual loss of bone calcium and weight during short flights. These are temporary and a reversal of these effects occur as soon as one returns back to earth.
Weightlessness in the orbital motion of satellites
In the case of a satellite orbiting around the earth, there is an acceleration which is acting towards the center of the earth. This is known as the centripetal acceleration represented by ac. There is also the acceleration due to gravity of the earth which is balancing this centripetal acceleration [g=ac] ; they are thus equal in magnitude and are balancing each other. Inside the satellites, there is no acceleration due to gravity which indicates that everything is moving with a uniform velocity. Therefore, inside an orbiting satellite weightlessness is experienced.
Is gravity on space lesser than gravity on earth?
The force of gravity acting on the astronaut in space is certainly lesser than gravity present on the earth’s surface. But the gravity in space is not small enough to bring about a drastic change in weight. Suppose the space station to orbit is at an altitude of approximately 400km above the earth’s surface, then the value of g at that present location will be reduced from 9.8m/s2 to 8.7m/s2. Even though it definitely reduces weight, it cannot feel the absolute weightless sensations that astronauts experience. This happens because of the absence of a surface to offer them support as they are free fallings towards the earth.
Otis L. Evader’z Elevator Experiment
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Otis L.Evader’z conducted his famous elevator experiment which involved him riding the elevator up and down while standing on a bathroom scale. He noticed that the scale reading was different when he accelerated up and down when compared to him being at rest or travelling at a different speed.
The scale reading can be classified as a measure of normal upward force, the value of which can be predicted during different stages of motion. The value of the normal force acting on Otis’s 80kg body could be predicted provided the acceleration was known. This prediction is made by Newton's second law of motion.
In the diagram, Otis weighing 80 kgs is travelling with a constant speed stage A, thereby accelerating upwards at stage B, accelerating downwards at stage C and free falling at stage D post which the elevator cable snaps.
The normal force is greater than the force of gravity when the elevator accelerates upwards at stage B and it is less than the force of gravity when the elevator accelerates downwards at stage C & D. The same then becomes equal to the force of gravity when there is zero acceleration.
The elevator rider would feel his normal weight at stage A, a little heavier at stage B, less than the normal weight at stage C and weightless at stage D. The rider weighs the same in all the cases, yet the sensation of weight felt by him is different.
Important Questions Based on Weightlessness
Ques.1: Why an astronaut in an orbiting spacecraft is not in zero gravity although weight less? (1 Mark)
Answer: The astronaut in an orbiting spacecraft is in the gravitational field of the earth and experiences gravity. However, gravity is used to provide necessary centripetal force, so the body is in a state of free fall towards the earth.
Ques.2: Which of the following symptoms is likely to affect an astronaut in space (a) swollen feet, (b) swollen face, (c) headache, (d) orientational problem? (3 Marks)
Answer: (a) Legs hold the entire mass of a body in standing position due to gravitational pull. In space, an astronaut feels weightlessness because of the absence of gravity. Therefore, swollen feet of an astronaut do not affect him/her in space.
(b) A swollen face is caused generally because of apparent weightlessness in space. Sense organs such as eyes, ears, nose, and mouth get swollen in space. This symptom can affect an astronaut in space.
(c) Headaches are caused because of mental strain. It can affect the working of an astronaut in space.
(d) Space has different orientations. Therefore, orientational problems can affect an astronaut in space.
Ques.3: (a) A body is weightless at the centre of earth. Why?
(b)Why does one feel giddy while moving on a merry round? (3 Marks)
Answer: (a) At the centre of the earth, the gravity is zero. The weight of a body can be depicted by the product of mass and gravity. So, the weight of the body will be zero at the centre of earth.
(b) When mowing in a merry-go-round, our weight appears to decrease when We move down and increase when we move up. This change in Weight makes us feel giddy.
Ques.4: Is it appropriate to describe the condition of weightlessness as the condition of masslessness? Why? (1 Mark)
Answer: No. Weight and mass are different physical quantities. A body may have zero weight but a body cannot have zero mass. So, it is not appropriate to describe the condition of weightlessness as a condition of masslessness.
Ques.5: Is weightlessness implied when there is no gravity? (1 Mark)
Answer: No, weightlessness is the zero gravity or as we say that it can't be felt. Weightlessness elaborates the absence of sensation of weight, it is often known as that state of zero gravity because it is well against the gravitational force.
Ques.6: What is weightlessness? Give two examples. (2 Marks)
Answer: The force exerted by the earth on a body is called the weight of the body.
However the mass of the earth can be determined by acceleration due to gravity near the surface of earth.
g=GMR²
g=9.8m/s2,R=6400km, G=6.67×10−11
We experience weightlessness during the situation of free fall. It happens when all contact forces are removed. Free fall is the situation when a body is moving only owing to the power of gravity since gravity is a non-contact force.
Example 1- Astronauts who orbit the earth often experience a sensation of weightlessness. This is because there is no outer contact force pulling or pushing upon their body, therein gravity is the only force acting upon their body.
Example 2- Weightlessness of a satellite. On satellites we tend to feel weightlessness because the forces on the body are the contact forces and gravity.
The surface does not exert any force on the body of the individual and the apparent weight is zero. Therefore, no support is needed to hold a body on a satellite.
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