5.8
Weightlessness
When an object is dropped, it accelerates toward the center of the Earth. If the net external force on the object is its weight, it is said to be in f…
Recall that if the frictional effects of air are ignored, all bodies irrespective of their masses, fall with a constant acceleration known as acceleration due to gravity.
Such bodies are called freely falling bodies and the net force acting on them is only due to their weight. They reach the ground at the same time.
Hence, in the absence of contact forces, any freely falling body experiences weightlessness.
For example, in a roller coaster ride, the rider is acted upon by the normal force and the gravitational force. When the car reaches the top of the loop, the rider is momentarily lifted off the seat, thus removing the normal force.
The only force acting on the person at this position is his weight, which makes him feel "weightless". Weightlessness is merely a sensation as the weight of one's body is always acting when on Earth.
Q1: What is weightlessness and why do astronauts experience it in space?
Weightlessness is a sensation felt when all contact forces acting on a body disappear, leaving only gravity. Astronauts in a spacecraft accelerating downward at gravitational acceleration g experience weightlessness because no reaction force acts on them. Although gravity exists everywhere in the universe, astronauts do not feel its effects during free fall, making their apparent weight zero.
Q2: Why do all freely falling bodies experience weightlessness regardless of mass?
All bodies fall with constant acceleration due to gravity when air resistance is ignored. In free fall, the only force acting on a body is its weight, with no contact forces present. Since all objects accelerate at the same rate regardless of mass, they all experience weightlessness simultaneously, reaching the ground at the same time.
Q3: How does a roller coaster loop demonstrate the concept of weightlessness?
At the top of a roller coaster loop, the rider is momentarily lifted off the seat, removing the normal force. With only gravitational force acting on the rider, they feel weightless despite gravity still pulling them downward. This sensation occurs because the absence of contact forces creates the perception of zero weight, even though the rider's actual weight remains unchanged.
Q4: What would a weighing scale read if a person stood on it in a freely falling elevator?
A weighing scale would read zero because no reaction force acts on the person during free fall. The scale measures the normal force between the person and the platform, which disappears when both fall together at gravitational acceleration. This demonstrates that weightlessness is a sensation caused by the absence of contact forces, not the absence of gravity itself.
Q5: Is weightlessness the same as having no weight or no gravity?
No. Weightlessness is merely a sensation caused by the absence of contact forces, not the absence of actual weight or gravity. An object's weight always acts when on Earth because gravity continuously attracts it toward the center. Weightlessness occurs when internal and external forces balance such that no normal force opposes gravitational acceleration.
Q6: How does air resistance affect whether an object experiences true free fall?
When air resistance is ignored, all bodies fall with constant gravitational acceleration and experience weightlessness. However, objects falling on Earth never experience true free fall because air always exerts an upward resistance force. This air resistance reduces the net downward force, preventing the object from accelerating purely under gravity alone.
Q7: What is the relationship between free fall and the sensation of weightlessness?
Free fall occurs when the only force acting on an object is gravity, with no contact forces present. During free fall, an object experiences weightlessness because the absence of normal forces creates zero apparent weight. Whether in an aircraft or orbiting spacecraft, the sensation of weightlessness is identical when free fall occurs, demonstrating that weightlessness depends on force conditions, not location.