Solving Newtonian Problems: Deciding When to Use + or - 9.8

[7tongc5]

ex) you throw a baseball straght up. I returns after 3.5 sec. How fast did you throw it?

i could then use x = x(initial) + v(initial)*3.5s + 1/2(-9.8)(3.5^2) and solve for initial velocity which comes out to be 17.5m/s.

in this problem, 9.8 was negative and in some other problems, 9.8 was positive. how do i decide when to set 9.8 + or -?

 

[Integral]

It depends on how you define your coordinate system for the problem. It really does not make any difference as long as you are consistent.

 

[quicknote]

In Newtonian physics, the acceleration due to gravity on Earth is typically represented as a constant value of -9.8 m/s^2. This means that all objects, regardless of their mass, will experience the same acceleration towards the ground when dropped or thrown.

In the problem given, the baseball was thrown straight up and then returned after 3.5 seconds. In this case, the acceleration due to gravity acted in the opposite direction of the initial velocity, resulting in a negative value of 9.8 in the equation.

However, in some other problems, the acceleration due to gravity may act in the same direction as the initial velocity, resulting in a positive value of 9.8 in the equation. For example, if the object was thrown straight down, the acceleration due to gravity and the initial velocity would both act in the same direction, resulting in a positive value for 9.8.

In general, when solving Newtonian problems, it is important to consider the direction of the acceleration due to gravity in relation to the initial velocity of the object. If they act in opposite directions, the value for 9.8 should be negative, and if they act in the same direction, the value for 9.8 should be positive. This will ensure that the correct calculations are made and the problem is solved accurately.