Mass and weight are not equal in science

     

1   Mass is not easy to understand.

1. Gravity is the force by which a planet or other body draws objects toward its centre. 
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3. The force of gravity keeps all of the planets in orbit around the sun.
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5. Mass is a     measure     of how much matter is in an object, while weight is a measure of  the pull of gravity on you, your weight.  You'll be lighter on the moon. 
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7. Mass is the amount of matter in an object. Mass is measured in kilograms (kg). Weight is a force due to the pull of gravity on an object. 

2   Mass and weight are not equal in science: 

1. Mass
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3. The amount of matter in an object, measured in kilograms (kg) or grams (g). 
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5. It doesn't matter where it is, mass remains the same.
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9. Weight
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11. The force of gravity acting on an object, measured in newtons (N). 
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13. Weight changes depending on the strength of the gravitational field acting on the object. 
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15. For example, you weigh less on the moon than on Earth because the moon's mass is different from Earth's. 
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17. In everyday life, mass and weight are often used one for the other, but in physics they have different meanings. 

Based on this source.

3   Acceleration

1. Gravity force is the force that causes things to go faster or accelerate.  
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3. When you place a marble on the side of a hill, you'll see the marble begin to accelerate as it makes its way down to the bottom. That's gravity at work.
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5. Gravity is the force by which a planet or other body draws objects toward its centre. 
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7. The force of gravity keeps all of the planets in orbit around the sun.

Based on this source.

The gravity of the Sun keeps the planets in their orbits. They stay in their orbits because there is no other force in the Solar System which can stop them.