Gravity is the pull that every object exerts on other objects. The more massive an object, the greater the gravitational pull it can exert on another object. The further the distance between two objects, however, the smaller the gravitational pull. It is gravity that holds planets and stars together and gives them their nearly spherical shape, and it is the gravitational pull of the Earth that keeps things – such as ourselves – from floating off into space. It is the Sun’s gravitational attraction that holds the planets in their orbits.
The mass of an object is fundamental to that object and does not change when the gravitational pull changes (for example, on a different planet or the Moon). However, when we weigh an object, we are measuring the Earth’s gravitational pull on it. Therefore, on a different planet, the same object could have a different weight, because the planet’s mass and radius (i.e. gravitational pull) are different than the Earth’s. On Mercury, the gravitational force on an object – its weight – would be only 37% as large as on Earth. For example, a person with a mass of 61 kg would weigh 135 lbs on Earth but would weigh only 50 lbs on Mercury.
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