Question

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Why is acceleration 'minus' when something is thrown up?
(please explain this in simple words as for 9th grade level, make sure it's understandable) ​

Answer 1

Answer:

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Acceleration is negative when a body is thrown up as when a body is thrown upwards, it has some initial velocity in the upwards direction, but the acceleration on it i.e. acceleration due to gravity acts downwards, thus as velocity and acceleration are in the opposite direction, and also generally we take upwards direction as positive( it has something to do with the number line in maths if you know about it), thus acceleration is negative(minus) when something is thrown up.

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Answer 2

The acceleration is 'minus' or negative due to the gravitational force of attraction.

When we throw something upward then it is in a above motion and at the same time the force of gravitation acts on it in its opposite direction. As a result the velocity of the object gradually decreases and leads to negative acceleration or retardation.

The opposite case happens when we throw something from the above to the downward direction. In this case the object is already in downward motion and the force of gravitation also acts on it. As a result the velocity gradually increases and leads to acceleration or positive acceleration.

Now, you may have query, that why a gas balloon always goes upward?

⇒ It is because, a gas balloon generally contains hydrogen gas and it is lighter than the other gases. And its velocity also decreases with the increase in height and after reaching to a certain height due to pressure of hydrogen gas inside the balloon it gets burst.

Now, you may also have query that why it doesn't happen similar to the gas balloon with a spaceship or rocket or aeroplane?

⇒ In this case we get the example of the newton's third law or the action-reaction force. During this time the rocket or spaceship or the aeroplane provides the action force to the steam and to the downward surface such as soil, air etc. and those materials provide reaction force on the spaceship or rocket or aeroplane and as a result it an move and float.