# Does the force of gravity change?

In ordinary life, the magnitude of something's gravitational pull does not alter considerably as it rises above the Earth. (For significant variations to occur, an item must travel far higher than a jumbo jet.) At a height of 200 kilometers, the gravitational force is still roughly 94 percent of what it was at sea level. But because there are less particles per unit area closer together, the strength of the field increases as you go up.

However, on a cosmic scale, things are very different. On large scales, the universe is described by general relativity, which says that the magnitude of the gravitational force depends on how much matter is close by. So if the amount of matter in the immediate vicinity increases or decreases, then so too will the force of gravity.

On a galactic scale, where stars and planets are involved, gravity can be thought of as a property of space itself. It is a product of how much mass is located nearby and also depends on how far away other mass is. If there is more mass outside of a given region than inside, then it will experience greater gravitational forces from within. This is why galaxies have larger masses per unit area farther out than ones with less massive neighbors.

On a solar system scale, where only the Sun and the planets are involved, gravity remains constant unless something changes about either the size of the objects involved or their distance from one another.

## What is the gravitational force acting on the weight?

The weight W, or gravitational force, is thus just the object's mass multiplied by the gravitational acceleration. Because the gravitational constant (g) is proportional to the square of the distance from the earth's center, the weight of an item falls with altitude. For example, if we assume that the object is a mountain and it has a mass of 10,000 kg, then its weight will be 100 N at the surface of the earth and 1 N when it reaches orbit.

Thus, the force of gravity decreases as an object gets farther away from the earth. If a satellite in orbit experiences no other forces, then it will fall toward the planet over time due to this effect.

This video by NASA explains it well: https://www.youtube.com/watch?v=JWK_bB6O1G0

For objects near the surface of the earth, such as mountains, the force of gravity does not change but the area over which it acts increases. So the net force on these objects is still downward because there's more mass closer to the earth than far away. But for objects high in the atmosphere, such as clouds, the force of gravity decreases with height.

Thus, the net force on objects within Earth's atmosphere is always downwards regardless of their location above sea level or not.

## Is gravity increasing or decreasing?

The force of gravity is proportional to the masses of the two objects and inversely proportional to the square of the distance between them. This indicates that the force of gravity increases with mass but decreases as the distance between things increases. Therefore, gravity can be said to be increasing over time.

In reality, gravity is always falling toward Earth at a constant rate of 9.8 m/s2. But because objects near you are getting heavier, they're pulling on the earth's surface harder, which causes gravity near you to increase.

As far as I know, this effect isn't detectable by any instrument. But it is visible in the orbits of planets around the sun. The closer a planet is to the sun, the more energy it receives from the sun. The more energy it receives, the faster it travels in its orbit around the sun. So over time, planets get pulled closer into the sun as well as farther away.

This process keeps planets from being pulled into the sun until about 5 million years ago when Earth was almost destroyed by fire. By analyzing the layers of rock exposed on different continents, we have learned that 5 million years ago, there was a dramatic change in climate that caused all the water on Earth to evaporate.

## Does the force of gravity decrease with altitude?

Gravitational acceleration diminishes with altitude, as demonstrated by the solid line in Figure 1 (left). G = 9.5 m/s2 at 100 km altitude, which is 9% larger than 8.7 m/s2. However, at 500 km altitude, g is close to 8.45 m/s2, which is over 3% less than 8.7 m/s2. So, although gravitational acceleration decreases with altitude, it does not do so uniformly but rather in large jumps.

The dashed line in Figure 1 (left) shows how gravitational acceleration varies with distance from the earth's center if dm/dr was constant at 10 km-1. You can see that this variation is very different from the actual variation observed in nature. The reason is that the density of the atmosphere increases with altitude, so dm/dr gets smaller as you go up. At any given point above Earth's surface, the magnitude of the gravitational field points downward, but over time the field becomes weaker and weaker until it reaches zero at infinity.

Figure 1. Left: Variation in magnitude of the force of gravity with altitude. The solid line shows the expected variation if gravitational acceleration decreased with altitude like other forces; the dashed line shows what would happen if it kept constant value of 10 km-1. Right: A schematic diagram of a satellite in orbit about the planet. All objects in orbit around a central body follow ellipses with the center of the circle being the center of mass of the system.

## How does the gravitational force of the Earth vary?

The radius of the Earth at the equator is 6,378 kilometers, therefore imagine you were atop a 5-kilometer-high mountain at the equator (around 16,400 feet). You'd be 6,383 kilometers from the center of the Earth, and the gravitational pull would have diminished by a factor of (6,378/6,383) 2 = 0.9984. At the poles, the radius of the Earth is 6,371 kilometers, so you'd be 6,375 kilometers from the center of the Earth and the gravitational force would have decreased by a factor of (6,371/6,375) 2 = 1.0004.

Thus the gravitational force of the Earth decreases as you go from the equator to the pole. However, because the mass of the Earth increases as you go from the equator to the pole, the strength of the gravitational force increases as well. So overall, the gravitational force of the Earth is always decreasing but becoming weaker as it approaches the poles.

However, this doesn't mean that you could walk across the Earth in equal amounts of time if you started at either pole. For example, it would take you nearly two months to reach the other pole if you started at the North Pole, but only one month if you started at the South Pole. The reason for this is that the distance between the poles is 12,786 miles (20,000 km), which is much farther than the 6,373 miles (10,000 km) between the equators.

## How is gravity affected?

Gravity has a greater effect on an item with a high mass than it has on an object with a little mass. The same method may be used to calculate the gravitational force acting on things on other planets and moons, however the gravitational acceleration varies each planet and moon. On Earth, it is 9.80ms-2.

On Mars, the gravitational acceleration would only be about 0.38ms-2 because the mass of the planet is less than that of Earth. On Jupiter or Saturn, the gravitational acceleration would be over 100ms-2 because these planets are very massive.

The effect of gravity can be used to explain many phenomena related to astronomy and physics. For example, objects at a distance from the center of a galaxy will orbit around it faster than objects closer in because the latter experience a stronger gravitational pull. This explains why stars move across our sky in patterns known as star clusters - they're not moving in a random way but rather along with their closest neighbors. Star clusters also appear to rotate around the galactic center much like a wheel spinning around its axis because stars near the edge rotate more slowly than those close in.

Stars are formed from gas and dust that collapse under its own weight to form a nebula. As this cloud of gas and dust gets thicker, it begins to take on more and more mass, which increases the strength of the force of gravity within it.

##### Alma Dacosta

Alma Dacosta is a teacher who loves to teach and help her students grow. She has been teaching for six years now, and she enjoys all the new things that come with every year. Alma likes to use different methods of teaching so that no two lessons are ever the same. She loves watching her students learn and grow as they progress through their schooling, because it's rewarding to see them succeed after countless hours of hard work.

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