Case Study:

During a live broadcast from the International Space Station (ISS), astronauts were seen floating freely inside the spacecraft, pushing gently off walls and moving in all directions.

Before diving into the complex physics of orbit, it helps to revisit the basics of why objects fall toward the Earth's center in the first place.

This observation often creates confusion because the ISS orbits Earth at an altitude of about 400 km, where Earth’s gravitational pull is still nearly 90% of its surface value. If gravity is still acting, why do astronauts experience weightlessness?

This calculation is based on the Universal Law of Gravitation, which dictates that gravity never truly hits zero, even in the far reaches of space.

Scientists explain that both the spacecraft and the astronauts inside it are in a state of continuous free fall toward Earth. If the term 'free fall' sounds a bit scary, check out our fun guide to acceleration due to gravity to see how it works in everyday life. However, due to their high tangential velocity, they keep missing the Earth’s surface, resulting in an orbit. As a result, there is no normal reaction force acting on the astronauts from the spacecraft floor. Instruments inside the ISS show that objects released inside do not fall to the floor but remain suspended, demonstrating apparent weightlessness.

This phenomenon is not due to the absence of gravity but due to the condition of free fall, where all objects accelerate equally under gravity, making their relative motion zero.

This concept is a common exam favorite; you can explore a deeper breakdown on whether astronauts are truly 'weightless' here.

CASE-BASED QUESTIONS

MCQ

Q1. Why do astronauts float inside a spacecraft?
A. There is no gravity in space
B. The spacecraft moves very fast
C. They are in a state of free fall
D. Air pressure is very low

Q2. At the altitude of the ISS, gravity is:
A. Zero
B. Very weak
C. Nearly equal to Earth’s surface gravity
D. Opposite in direction

Assertion - Reason

Q3. Assertion (A): Astronauts inside a spacecraft experience weightlessness.
Reason (R): There is no gravitational force acting on them in space.

Options:
A. Both A and R are true, and R is the correct explanation
B. Both A and R are true, but R is not the correct explanation
C. A is true, R is false
D. A is false, R is true

Application-Based Question

Q4. If a ball is released inside the spacecraft, what will happen and why?

Q5. Explain why astronauts do not fall toward Earth despite gravity acting on them.

Data/Logic-Based Question

Q6. If gravitational acceleration at Earth’s surface is 9.8 m/s², and at ISS altitude it is about 8.7 m/s², explain why weightlessness is still observed.

ANSWER KEY WITH EXPLANATION

A1. C - Explanation: Astronauts are in continuous free fall along with the spacecraft, so no normal force acts on them, causing weightlessness.

A2. C - Explanation: Gravity at ISS altitude is still strong (~90% of surface gravity), as per NCERT gravitation concept.

A3. C - Explanation: Assertion is true. Reason is false because gravity still acts; weightlessness is due to free fall.

A4. The ball will float alongside astronauts because both are in free fall with the same acceleration, resulting in no relative motion.

A5. Astronauts continuously fall toward Earth but move forward with high speed, causing them to orbit instead of hitting the surface.

A6. Even though gravity is slightly reduced, both spacecraft and objects inside accelerate equally, so no normal force exists—leading to apparent weightlessness.

HOTS EXTENSION QUESTIONS

1. If the speed of a spacecraft decreases slightly, predict how the condition of weightlessness inside it will change.

2. Could weightlessness be experienced inside a freely falling lift on Earth? Justify your answer with reasoning.

Still have a nagging question about orbital velocity? Post it on our student discussion forum or test your speed with our interactive physics quizzes.

Need a bit of extra help with these concepts? Request a personal tutor for one-on-one guidance, or reach out to us for any other academic support.