Define the term acceleration due to gravity: The acceleration of an object which is solely caused due to the gravitation field of any heavenly body, say Earth, is known as acceleration due to gravity. Acceleration due to gravity of Earth is 9.81 ms-2, Mercury is 3.7 ms-2, Venus is 8.87 ms-2, and Moon is 1.62ms-2
So here's the definition. But what does it really mean when someone says "acceleration due to gravity"?
Let's imagine that you're on an airplane and you've an object in your hand, say a guitar. Then, you throw the object out of the plane. Now the object is in open air, can you tell me what forces are acting on the guitar after you threw it? Obviously, the force of gravity. (But keep in mind, that the earth has atmosphere due to which any object moving in open air also experiences a drag force, called air resistance or air drag. For the sake of simplicity, we are going to assume that there's no air resisting the guitar's motion.) Now the object moves towards the center of the earth with increasing velocity. Since the velocity is changing, the object has acceleration. This acceleration is caused by gravitational force of the earth, hence this acceleration is called "acceleration due to gravity". For earth, it's fixed and its value is 9.81 ms-2.
Notice why i told you to ignore the presence of air resistance? This is because for any acceleration to be qualified as acceleration due to gravity, it must not be affected by any other force except the gravitational force between the object and heavenly body. This is why the definition itself says "solely caused due to gravitational field of heavenly body".
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