Force Problem: force direction and choice of sign for g

[unknown physicist]

Homework Statement

The Force applied to an object is equal its mass times its acceleration Assume that the ball has a mass of 40g. What is the force due to gravity acting on the ball?

Homework Equations

F=ma

The Attempt at a Solution

F=(.04)(-9.81)

I get a negative answer, does that mean that the force is negative? or should I change -9.81 to 9.81?

 

[PeroK]

I get a negative answer, does that mean that the force is negative? or should I change -9.81 to 9.81?

Why do you think it matters?

 

[unknown physicist]

Why do you think it matters?

Because acceleration in the y dimension, is negative. It can't possibly be negative. So I think it should stay.

 

[gneill]

The choice of reference frame determines the signs of vector quantities measured in that reference frame. It's up to you to choose and specify a reference frame before beginning your calculations.

It's perfectly valid to choose "down" as being positive and "up" as negative if it is convenient for describing the problem that way. Of course, once you've made that choice you have to stick with the convention you've imposed.

Generally g is taken to be a positive constant and you tack on the required sign to suit your choice of reference frame when you write equations involving g.

 

[CWatters]

+1

You choose up as positive. If you get a negative answer for the force that means it's acting downwards. Which way did you expect the force of gravity to act :-)