Question

when is the value of g negative
pls explain

Answer 1

Velocity is a measure of how fast you are going in a particular direction and acceleration is a measure of how your velocity is changing. For an object thrown upwards, it has an upward (positive) velocity that immediately begins to change as it goes up. It slows down, stops for an instant, and then falls, speeding up downwards (negative).

While the velocity changes from positive to negative, the acceleration is always the same. The acceleration simply describes that change in velocity from positive to negative. The acceleration is always negative because negative means moving to the left on a number line (going from the positive toward the negative)

You can better understand acceleration if you think about a tiny string attached to the object. On the way up, imagine which way the string must pull to slow the object down as it rises. Obviously, it must pull against the motion, or downwards. Now imagine which way it would pull as the object falls back down, getting faster and faster. Again, it must pull downwards. This is what it means to accelerate downwards. Slowing down upwards and speeding up downwards result from the same downward pull on the string!

In fact, the tiny string is really just a force and accelerations are caused by forces, in this case, the force of gravity.