Science · X-Factor Notes
MOTION UNDER GRAVITY revision notes for UPPCS Prelims.
High-Yield Revision| Concept | What to remember | Asked in UPPCS |
|---|---|---|
| Early Declaration of Gravity | The great 7th century AD astronomer and mathematician Brahmagupta declared that all things gravitate to the Earth, anticipating Newton. | |
| Centripetal Force for Satellite | The centripetal force necessary for a satellite to orbit the Earth is provided by the gravitational force on the satellite by the earth. | UPPCS (Pre) 2006 |
| Effect of Distance on Gravity | If the distance between the Earth and the Sun were doubled, the gravitational force between them would be one-fourth () of the original force. | |
| Planetary Satellites (Note) | Mercury and Venus have no natural satellites. Earth has one, and Mars has two little satellites. | |
| Escape Velocity (Definition) | The minimum speed required for a free object to escape from the gravitational influence of a celestial body (like Earth) and move outward into space. | |
| Earth's Escape Velocity | The escape velocity of Earth is 11.2 km/sec. |
| Concept | What to remember | Asked in UPPCS |
|---|---|---|
| Free Fall in Vacuum | If objects (like a feather, rubber ball, and wooden ball) fall freely and simultaneously from the same height in a vacuum, they will reach the ground together (due to the absence of air resistance and equal gravitational acceleration on all objects). | |
| Weight of Falling Object | In ideal conditions (negligible air resistance), if an object is falling from above, its weight remains the same because mass and local gravity are unchanged. | |
| Mass vs. Weight |
| Concept | What to remember | Asked in UPPCS |
|---|---|---|
| Pendulum Clock Speed | A pendulum clock runs faster in winter because the change in temperature causes the pendulum length to contract, making the period smaller. | |
| Absence of Lunar Atmosphere | The Moon has no atmosphere because the escape velocity of gas molecules is less than their RMS velocity (meaning gas molecules easily escape the Moon's gravitational pull). | |
| Moon's Gravity vs. Earth's |
| The mass of a body does not change with respect to gravity. If a body has a mass of 100 kg on Earth, its mass on the Moon (where gravity is (g/6) is still 100 kg. |
| Weightlessness | The term used to describe the condition where there is no force to support the body, often referred to as Zero gravity. |
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| Earth's Rotation and Weight | If the speed of rotation of the Earth increases, the weight of a body at the equator will decrease (because the centrifugal force opposing gravity increases at the equator). |
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| Displacement vs. Time (Free Fall) | For an object falling freely from rest, the variation of displacement () with time () is represented by a parabolic graph. |
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| Catastrophic Earthquake Acceleration | The acceleration due to gravity of a catastrophic earthquake will be greater than . |
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